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http://www.astronomy.com/Content/Dynamic/Articles/000/000/001/604djcxv.asp
XMM-Newton studies the X-ray universe from Earth orbit.
ESA
A mystery that has been haunting the fields of physics and cosmology has just grown deeper. Dark energy, that stealthy ghost that lurks in the shadows of the universe, is now believed by most scientists to be a strange but significant occupant of the cosmos, an unidentified antigravity that is stretching the very fabric of space. In fact, all the evidence — beginning in 1998 with the discovery of the universe’s accelerating expansion — has added up to an unsettling cosmic recipe: 4 percent ordinary matter, 23 percent dark matter, and 73 percent dark energy. But now, a recent X-ray survey of distant galaxy clusters suggests that perhaps dark energy is not the secret ingredient after all
http://www.esa.int/sci_mediacentre/release2003.html?release=54
Such a result indicates that the Universe must be a high-density environment, in clear contradiction to the 'concordance model,' which postulates a Universe with up to 70% dark energy and a very low density of matter. Blanchard knows that this conclusion will be highly controversial, saying, "To account for these results you have to have a lot of matter in the Universe and that leaves little room for dark energy."
To reconcile the new XMM-Newton observations with the concordance models, astronomers would have to admit a fundamental gap in their knowledge about the behaviour of the clusters and, possibly, of the galaxies within them. For instance, galaxies in the faraway clusters would have to be injecting more energy into their surrounding gas than is currently understood. That process should then gradually taper off as the cluster and the galaxies within it grow older.
Alain Blanchard must be the most important dissident to the
"concordance" cosmology picture. the leader of the opposition.
his most recent preprint in arxiv is
http://arxiv.org/astro-ph/0402297
I'm not sure but I think
the article you pointed to in Astronomy magazine
by Amanda Jefter (dated 23 December 2003) refers
to earlier articles of Blanchard
mainly this one
http://arxiv.org/astro-ph/0304237
but also this 3-pager
http://arxiv.org/astro-ph/0311626
Nereid may have responded to the gist of what
is in these articles in another thread. I forget which.
Or I may have. I dont think Blanchard's case is strong
enough yet to start bringing more people over to his side
but he certainly bears watching. If he continues to
assemble evidence of much more dark matter then
he could start a shift of opinion.
I think the argument here is between dark matter and dark energy.
the concordance model says that familiar types of matter total around 4 or 5 percent of the average density in space
but that leaves 96 percent to account for
the "concordance" estimate is that it is split 73 d.energy plus
23 d.matter
Blanchard's main message, if I understand it, is to give much more importance to dark matter and less to dark energy.
he may also favor a lower value than 71 for the hubble parameter
and consequently a lower overall density (so that the observed amount of ordinary matter would play a larger role)
too bad everybody is so sure about Wendy Freedman's figure of 71 for the hubble parameter----Blanchard would have a hard job getting people to listen to a much lower estimate for that.
i will try to explain better later when theres more time
MARCUS.
when two observations disagee with one another there can
only be so many reasons, if one rules out equiptment used
and changes in the object observed, then that leaves
little more than the interpretation of the data, or
the numbers used in the interpretation of the data ,
i think latter is the crux of the problem, what standard
numbers can be used?
I have no interest in dismissing Blanchard because I like
to have possibilities for change in the picture.
I much appreciate your assembling challenges to the prevailing view.
but realistically, if you take a hardnose look at it,
Blanchard has little ground to stand on. He can always
talk to uncritical journalists and say "maybe the
accepted model is wrong" and let them amplify it because
it makes a news story to say an accepted picture is wrong
(versus no news story to say it is right)
As for his wanting to adjust the Hubble parameter---the base of data supporting WendyFreedman's 71 is big and solid.
the HST satellite was put in orbit partly so Freedman's study could be
done, it was called the "hubble key" project
the confidence in that 71 +/- 3 is very high
Blanchard has no measurements to prove that the hubble parameter is, like he surmises, 40-50. He just adjusts it down like that to get some slack to help his other numbers work out.
the fact that he sometimes adjusts the hubble parameter to be different from what it has been MEASURED is a sign that his picture is probably wrong
Blanchard's XMM-Omega only looked at a small patch of sky.
Blanchard's critics have pointed out that the patch he studied might be atypical.
He himself says his results are not conclusive and a much wider
survey needs to be done. And Sean Carroll a bigtime cosmologist
says that there are some other ways to explain
Blanchard's statistics besides the explanation Blanchard proposes.
So only a journalist like Amanda Jefter of "Astronomy" magazine would make it look as if the prevailing model had been effectively called into doubt. It is premature to think of this as any more than a preliminary challenge.
For my part, I'll keep a part of my brain open and ready for surprises. No matter how insigificant Blanchard looks right now (claiming there is more dark matter than the majority think, and less dark energy) he could be right. I want to be prepared to learn someday that the prevailing model was wrong, so I'll keep an eye on Blanchard. You never know from what direction change is going to come.
i can see a great deal depends on the Hubble parameter
over the years its had its ups and downs, but maybe now
it is stable enough to use as a yard stick, but to base
everything on it alone is asking for skepticism.
measuring the Hubble is the only way we have of gauging the overall density of the universe. without confidence in estimates of this parameter not much quantitative can be said about the universe
the reason for that (as you wolram probably know but just for explicitness I'll say it) is that
the critical density for flatness is
3 H2/8piG
this is the most basic formula in cosmology IMO
and when they measure H (by comparing distances with redshifts)
they are actually measuring the average overall density of energy in the universe
because if you square H and multiply by 3
and then divide by 8 pi and then divide by G
you get the critical density
which turns out to be about 0.83 joules per cubic kilometer of space.
you can scoff all you want and be as skeptical as you please
but that is the bedrock of today's cosmology
and when Blanchard and the others argue about
how much of this and how much of that---darkmatter, ordinary matter, pretzels, cosmic chickenpoop or whatever
the basic thing that governs all their arguments is that
their numbers for amounts of this and that have to add up
and they have to add up to 0.83 joules per cubic km.
if you start fudging with H so that you can get a different
overall sum to add up to, different from that 0.83 joules,
then you arent really playing cosmology according to
today's rules, which is all right with me
(vigorous science should have a healthy fringe)
Furthermore I agree that historically the hubble parameter has had a very bumpy ride----estimates jittering around between 40 and 140, or even wider. But now for the time being it is 71.
would it be all right to include in this thread reports of new research that SUPPORT the commonplace dark energy estimates?
or should we have two separate threads and keep this one only for
evidence that challenges dark energy?
I will conform with what you think best and move this to a separate thread if that seems good.
I just saw this. I think it is from middle of 2003 and relates to
this article:
http://arxiv.org./abs/astro-ph/0305008
It may also relate to this article which reports record-breaking high redshift observations of type Ia supernovae:
http://arxiv.org./abs/astro-ph/0308185
That would tend to date this newsletter article sometime after August of 2003, but I dont know when the article I quote here was written:
-------from "Hubble newsletter--------
....
..."Right now we're about twice as confident than before that Einstein's cosmological constant is real, or at least dark energy does not appear to be changing fast enough (if at all) to cause an end to the universe anytime soon," says Adam Riess of the Space Telescope Science Institute, Baltimore. ...
Riess and his team joined efforts with the Great Observatories Origins Deep Survey (GOODS) program, the largest deep galaxy survey attempted by Hubble to date, to turn the Space Telescope into a supernova search engine on an unprecedented scale. In the process, they discovered 42 new supernovae in the GOODS area, including 6 of the 7 most distant known.
Cosmologists understand almost nothing about dark energy even though it appears to comprise about 70 percent of the universe. They are desperately seeking to uncover its two most fundamental properties: its strength and its permanence.
In a paper to be published in the Astrophysical Journal, Riess and his collaborators have made the first meaningful measurement of the second property, its permanence. Currently, there are two leading interpretations for the dark energy as well as many more exotic possibilities. It could be an energy percolating from empty space as Einstein's theorized "cosmological constant," an interpretation which predicts that dark energy is unchanging and of a prescribed strength.
An alternative possibility is that dark energy is associated with a changing energy field dubbed "quintessence." This field would be causing the current acceleration — a milder version of the inflationary episode from which the early universe emerged. When astronomers first realized the universe was accelerating, the conventional wisdom was that it would expand forever. However, until we better understand the nature of dark energy—its properties—other scenarios for the fate of the universe are possible. If the repulsion from dark energy is or becomes stronger than Einstein's prediction, the universe may be torn apart by a future "Big Rip," during which the universe expands so violently that first the galaxies, then the stars, then planets, and finally atoms come unglued in a catastrophic end of time. Currently this idea is very speculative, but being pursued by theorists.
At the other extreme, a variable dark energy might fade away and then flip in force such that it pulls the universe together rather then pushing it apart. This would lead to a "big crunch" where the universe ultimately implodes. "This looks like the least likely scenario at present," says Riess.
Understanding dark energy and determining the universe's ultimate fate will require further observations. Hubble and future space telescopes capable of looking more than halfway across the universe will be needed to achieve the necessary precision. The determination of the properties of dark energy has become the key goal of astronomy and physics today.
--------------end of quote-------------
If anyone can date this and provide a link please do.
I've been meaning to get back to the topics we were discussing a while ago (particularly dark matter); I think we covered dark matter at 50,000', but didn't discuss dark energy.
The diagram I really wanted to show PF members and guests is one I can't now find [:(] [*(] (it kinda combined the ones I discuss below, together with estimates of what SNAP would do to the error zones).
However, there's a good paper, by Spergel at al (http://lambda.gsfc.nasa.gov/product/map/pub_papers/firstyear/parameters/wmap_parameters.pdf), from WMAP's first year of data, which may illustrate some of what I want to convey. First, the paper lays out in some detail one way to get from WMAP observations to estimates of various cosmological parameters, and how estimates of those parameters are tightened by using other astronomical data.
The main thing I want to draw attention to is Figure 12, on page 44 (there's a typo in the last line of the text: it should read "calculations for this figure assumed a priori that w > -1.")*
The left hand figures show the w-\Omega_{m} and w-h 68% and 95% confidence regions of these three parameters, for four independent sets of observations (WMAP, the 2dF galaxy redshift survey, distant supernovae, and Hubble Space Telescope on the Hubble constant; \Omega_{m} is the density of matter, and h the Hubble constant); the right hand figures show the same regions with the data sets combined. (Figure 11 on the previous page shows the same thing, for different assumed properties of dark energy).
Look at how big the coloured regions are (you have to imagine the SN and HST regions; the latter covers just about ALL the bottom left-hand figure!). What does this mean? Well, that just about any point on either left-hand plot is consistent with at least one of the observations. Some of the points would have marginal consistency with the observations (e.g. ruled out at the 95% confidence limit), but there've been examples in physics which are just like that.
Further, these are only the formal CL regions; as the paper itself makes clear, different analyses of the data will give different CL regions.
Interestingly, this paper also mentions some of the points raised by Blanchard.
Note that quite a number of details and possibly confounding effects remain to be run to ground. For example, how much 'foreground contamination' is there in the WMAP angular power spectrum? What as yet unrecognised systematic effects might there be in the distant supernovae data?
We've come some way since Snowmass 2001, but Resource Book on Dark Energy (http://supernova.lbl.gov/~evlinder/sci.html) retains its value.
Conclusions?
a) a non-zero cosmological constant is consistent with the data
b) so are at least some quintessence models
c) HST (and other) determinations of h almost, but not quite, rule out the kinds of Einstein-de Sitter model which Blanchard mentions
d) "the Hubble diagram of distant Type Ia supernovae remains the only direct evidence for a non-zero cosmological constant" (Blanchard) - it's worth looking at these observations in more detail
... and so there's lots of work for astronomers!
*w: "In [...] quintessence models, the dark energy properties are quantified by the equation of state of the dark energy: w = p/\rho where p and \rho are the pressure and the density of the dark energy. A cosmological constant has an equation of state, w = -1."
marcus: That would tend to date this newsletter article sometime after August of 2003, but I don't know when the article I quote here was written: *SNIP If anyone can date this and provide a link please do. It's from the STScI-2004-12 press release (http://hubblesite.org/newscenter/newsdesk/archive/releases/2004/12/text/), dated 20 Feb, 2004
http://aps.arxiv.org/abs/astro-ph/0305559
Phantom energy which violates the dominant-energy condition and is not excluded by current constraints on the equation of state may be dominating the evolution of the universe now. It has been pointed out that in such a case the fate of the universe may be a big rip where the expansion is so violent that all galaxies, planet and even atomic nuclei will be successively ripped apart in finite time. Here we show however that there are certain unified models for dark energy which are stable to perturbations in matter density where the presence of phantom energy does not lead to such a cosmic doomsday.
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MARCUS, i have no objections to use this thread for any
dark energy, dark matter topic, maybe someone can start
the continuance of the thread by explaining the properties
of "dark energy", and how it fits in with the known energy
spectrum, or if it is a purely gravitational energy, if it
is why it has no effect on say, deep space probe trajectories.
maybe the force is canceled in the galaxies, but that
would suggest a "boundary", or gravity gradient around
galaxies, I'm babbling.
Hi NERIED
I found the journal article that goes with that news item.
It is
http://arxiv.org./abs/astro-ph/0402512
Here is a bit from the news item, to provide context
Originally posted by marcus
-------from "Hubble newsletter--------
....
..."Right now we're about twice as confident than before that Einstein's cosmological constant is real, or at least dark energy does not appear to be changing fast enough (if at all) to cause an end to the universe anytime soon," says Adam Riess of the Space Telescope Science Institute, Baltimore. ...
Riess and his team joined efforts with the Great Observatories Origins Deep Survey (GOODS) program, the largest deep galaxy survey attempted by Hubble to date, to turn the Space Telescope into a supernova search engine on an unprecedented scale. In the process, they discovered 42 new supernovae in the GOODS area, including 6 of the 7 most distant known.
.....
--------------end of quote-------------
the abstract of the journal article also talks about finding 6 of the 7 most distant supernovae known
I guess they were able to look back to a time before the expansion of the universe started accelerating----if the dark energy density is constant and its equation of state is steady at w = -1 then such
a time of decelerating expansion should have occurred. Just guessing. have to read the article to be sure.
Wolram started this thread about the question of dark energy,
what is the evidence pro and con.
Ted Bunn one of the moderators at SPR has given a kind of brief overview covering the highlights of this issue. I just saw his post a few minutes ago. It seemed so good I thought we could use it here either to start a "dark energy" thread or to add to wolram's thread.
It was prompted by a poll someone put up about what do you think dark energy is: cosmological constant, or quintessence, or a delusion (the expansion is not really accelerating and there is no dark energy), or various other things.
------------Ted Bunn post--------
In article <1b7c3dda.0402281450.4c1ffa6b@posting.google.com>,
Melroy <melroysoares@hotmail.com> wrote:
>here are the choices:
>
>1) cosmological constant
>2) quintessence or some sort of rolling scalar field
Personally, I think one of these two is the most likely.
>3) phantom energy (with w <-1)
This is theoretically ill-motivated and doesn't have any
observational support either, so I'd be surprised if it turns out to
be right.
>4) general relativity is incorrect and we need another theory of
>gravity to descrbe universe at cosmological scales which >automatically predicts accelerated expansion
>5) Same as (4) and this theory will also solve
>the dark matter problem
I think these are very unlikely, partly because GR is such a terrific
theory, and I don't believe it's wrong, but mostly because of the CMB
observations, which are astonishingly consistent with GR / dark matter
models. More on this below.
>6) universe is not accelerating and we are misinterpreting supernova
>as well as CMB and large scale structure data.
If all we had was the supernova data, then I'd be tempted by this one.
But the interesting thing right now is that numerous different lines
of evidence are all pointing in the same direction, towards a Universe
with a lowish matter density (Omega_matter = 0.2-0.3 or so) with
something cosmological-constant-like bringing Omega_total up to 1.
To be specific, aside from the supernova data, you've got
A. CMB data. Even without input from any other observations,
the power spectra from WMAP and other CMB experiments strongly
say that the Universe is flat, and also fit a low-matter-density
Universe much better than a high-density one. In other words,
they strongly suggest something lambda-like all by themselves.
The theoretical CMB power spectra have bumps and wiggles in them, and
the data match those wiggles remarkably well. It would be astonishing
if that fit were by chance. That means that the CMB data alone pretty
strongly disfavor various no-dark-matter or
general-relativity-is-wrong scenarios, since the models that generated
those theoretical power spectra are based on GR + weakly-interacting
cold dark matter.
B. Over a decade of observations of large-scale structure observations
(galaxy power spectrum, peculiar velocities, ...) strongly suggesting
Omega_matter in the 0.2 range. Theorists spent years pooh-poohing
these results, but the data have been remarkably consistent for a long
time.
C. Galaxy clusters. You can take inventories of galaxy clusters a bunch
of different ways. You can measure how much of various forms of visible
matter they contain. You can get the overall mass distribution
from the temperature profile of the hot gas they contain, from
applying the virial theorem to the motions of the galaxies, and from
gravitational lensing of background galaxies. These all
agree on a matter density in the 0.2-0.3 range, with only a small
fraction of that being baryonic.
By themselves, B and C don't say anything about Lambda, but combined
with the CMB data that strongly favor flatness, they do.
D. Supernovae. Back when the supernova data just showed that distant
supernovae were fainter than expected, it was pretty easy to imagine
that some systematic error was the cause. But cosmological-constant
models predict that supernovae should be fainter than expected at
moderate redshift and then turn over and become brighter than
expected at high redshift. Recent observations seem to see that
turnover. I think it's much harder to explain an effect like that,
which changes sign at the expected point, with a systematic error.
Still, if D were all we had to go on, I'd be very skeptical. The
thing that impresses me is that A-C completely independently
point to the same model of the Universe, probing completely different
physics at different epochs.
>7) something other than first (6)
This one is always possible! It'd be the most fun of all, of course.
Personally, I think that the considerable amount of consistency among
different sorts of cosmological observations suggests that we're on
the right track, and that the near future in cosmology doesn't have
any huge surprises like this (although no doubt it'll have lots of fun
little surprises). But I could be wrong.
-Ted
-------------end quote----------------
for me the most exquisite bit of evidence in the whole picture is the turnaround
from "fainter than expected" to "brighter than expected" past a certain z-limit
it has to do with an inflection-point in a certain breast-shape curve
assuming a positive cosmological constant (the simplest dark energy explanation) the growth curve of the universe is
convex, with decelerating expansion, for some ten billion years and then inflects and becomes concave, with accelerating expansion
what this means is that supernovae with MODERATE redshift, because they are recent, are in the recent accelerating phase, and they will be fainter than would be expected without dark energy
but also that supernovae with larger redshift, because they happened very long ago during the decelerating phase, will be brighter than expected
----quote from Bunn-----
D. Supernovae. Back when the supernova data just showed that distant
supernovae were fainter than expected, it was pretty easy to imagine
that some systematic error was the cause. But cosmological-constant
models predict that supernovae should be fainter than expected at
moderate redshift and then turn over and become brighter than
expected at high redshift. Recent observations seem to see that
turnover. I think it's much harder to explain an effect like that,
which changes sign at the expected point, with a systematic error.
----end quote-----
the whole thing is unexpected, nobody was planning for a cosmological constant or a dark energy in 1998 when they noticed this deviation in the supernovae that indicated acceleration, but the turnover is a surprise inside a surprise---twisting the knife of the unexpected.
it makes it a lot harder to fit an alternative explanation to the data or to question the data as some kind of coincidence
fewer and fewer keys will fit the lock
I will get the Lineweaver figure 14 showing the growth curve with its inflection point
http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Figures/figure14.jpg
the why of that inflection point is not hard to understand, maybe someone will explain why a constant vacuum energy density produces that changeover from decel to accel
by MARCUS.
the why of that inflection point is not hard to understand, maybe someone will explain why a constant vacuum energy density produces that changeover from decel to accel
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now if they can do that with "clean", math it will deserve an
N P, i can see that evidence for dark energy is mounting up, the
paper MARCUS posted would be a good starting point for a
review of the evidence for it, it would be nice if the argument
is not all one sided lets see what alternatives hold water.
http://arxiv.org/abs/astro-ph/0304325
We compare the WMAP temperature power spectrum and SNIa data to models with a generalized Chaplygin gas as dark energy. The generalized Chaplygin gas is a component with an exotic equation of state, p_X=-A/\rho^\alpha_X (a polytropic gas with negative constant and exponent). Our main result is that, restricting to a flat universe and to adiabatic pressure perturbations for the generalized Chaplygin gas, the constraints at 95% CL to the present equation of state w_X = p_X / \rho_X and to the parameter \alpha are -1\leq w_X < -0.8, 0 \leq \alpha <0.2, respectively. Moreover, we show that a Chaplygin gas (\alpha =1) as a candidate for dark energy is ruled out by our analysis at more than the 99.99% CL. A generalized Chaplygin gas as a unified dark matter candidate (\Omega_{CDM}=0) appears much less likely than as a dark energy model, although its \chi^2 is only two sigma away from the expected value.
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http://arxiv.org/PS_cache/astro-ph/pdf/0402/0402228.pdf
Unified dark energy models : a phenomenological approach
V.F. Cardone, * A. Troisi, and S. Capozziello
Dipartimento di Fisica “E.R. Caianiello”, Universit`
a di Salerno and INFN, Sez. di Napoli,
Gruppo Coll. di Salerno, via S. Allende, 84081 -Baronissi (Salerno), Italy
A phenomenological approach is proposed to the problem of universe accelerated expansion and of the dark energy nature. A general class of models is introduced whose energy density depends
on the redshift z in such a way that a smooth transition among the three main phases of the universe evolution (radiation era, matter domination, asymptotical de Sitter state) is naturally
achieved. We use the estimated age of the universe, the Hubble diagram of Type Ia Supernovae and the angular size -redshift relation for compact and ultracompact radio structures to test whether
the model is in agreement with astrophysical observation and to constrain its main parameters.
Although phenomenologically motivated, the model may be straightforwardly interpreted as a two fluids scenario in which the quintessence is generated by a suitably chosen scalar field potential. On the other hand, the same model may also be read in the context of unified dark energy models or in the framework of modified Friedmann equation theories.
PACS numbers: 98.80.-k, 98.80.Es, 97.60.Bw, 98.70.Dk
I. INTRODUCTION
In the last few years, an increasing bulk of data has
been accumulated leading to the emergence of a new
cosmological scenario. The Hubble diagram of type Ia
Supernovae (SNeIa) first indicated that the universe expansion
is today accelerating [1, 2]. The precise determination
of first and second peaks in the anisotropy
spectrum of cosmic microwave background radiation
(CMBR) by the BOOMERanG and MAXIMA collaborations
[3] strongly suggested that the geometry of the
universe is spatially flat. When combined with the data
on the matter density parameter
M, these results lead to the conclusion that the contribution
X of dark energy is the dominant one, being M . 0.3,
X . 0.7.
This picture has been strenghtened by the recent determination
of CMBR spectrum measured by the WMAP team
According to the standard recipe, pressureless cold
darkmatter anda homogenously distributedcosmic fluid
with negative pressure, referred to as dark energy, fill the
universe making up of order 95% of its energy budget.
What is the nature of this dark energy still remains an
open and fascinating problem. The simplest explanation
claims for the cosmological constant thus leading to
the so called CDM model Although being the best
fit to most of the available astrophysical data the
CDM model is also plagued by many problems on different
scales. If interpreted as vacuum energy, is up
to 120 orders of magnitudes smaller than the predicted
value. Furthermore, one should also solve the coincidece
problem, i.e. the nearly equivalence of the matter and
contribution to the total energy density.
As a response to these problems, much interest has
been devoted to models with dynamical vacuum energy,
*
Corresponding author, email: winny@na.infn.it
dubbed quintessence [6]. These models typically involve
scalar fields with a particular class of potentials, allowing
the vacuum energy to become dominant only recently
(see [7, 8] for comprehensive reviews). Altough
quintessence by a scalar field is the most studied candidate
for dark energy, it generally does not avoid ad
hoc fine tuning to solve the coincidence problem. On
the other hand, a quintessential behaviour may also be
recovered without the need of scalar fields, but simply
by taking into account the effective contribution to cosmology of some (usually neglected aspects)of fundamental
physics A first tentative were undertaken showing
that a universe with a non vanishing torsion field
is consistent with SNeIa Hubble diagram and Sunyaev -
Zel’dovich data on clusters of galaxies [10]. The same
quintessential framework can be obtained with the extension
of Einstein gravity to higher order curvature invariants
leading to a model which is in good agreement
with the SNeIa Hubble diagram and the estimated age
of the universe [11]. It is worth noting that these alternative
schemes provide naturally a cosmological component with negative
pressure whose origin is simply related
to the geometry of the universe itself thus overcoming
the problems linked to the physical significance of scalar
fields.
Despite the broad interest in darkmatter and darkenergy,
their physical properties are still poorly understood
at a fundamental level and, indeed, it has never been
shown that the two are in fact two different ingredients.
MARCUS.
one thing i am not clear on, did this decel to accel
have a starting point or was it universal, for if it
was universal, started everywhere at the same time, the
trigger would have to be extremely "pure".
I very much hope Nereid will provide some confirmation or correction here.
What interests me is the observational data for this "turnaround" from decel to accel. How much data do they have already and how much do they need, of supernovas in what z range.
I am guessing that the evidence for acceleration is mostly from supernovas with z less than 0.5
and that to be sure about a changeover from an earlier deceleration era to current accelerataion they need a lot of datapoints
of supernovas in the range z = 0.5 to 1.5 or 1.7
and they only have a few datapoints so far in the range z bigger than 0.5, because it is hard to find and observe supernovas that far away.
But it can be done. I seem to remember reading of some observations of Type Ia SNe as far back in time as z = 1.7. Am I misremembering?
this seems very far away to observe an individual star event. Would appreciate clarification
http://www.detnews.com/2004/nation/0402/22/nation-70295.htm
In the new work, led by Adam Riess of the Space Telescope Science Institute, researchers used the orbiting Hubble telescope to measure various properties of light emitted by 16 exploding stars, known as supernovas. Because the stars are at various distances from Earth, they yield information about what was happening at different points in the past. The supernovas included six of the seven most distant supernovas ever studied, dating two-thirds of the way back to the Big Bang.
http://arxiv.org/abs/astro-ph/0401207
The recent observations support that our universe is flat and expanding with acceleration. A quintessence model with a general relation between the quintessence potential and the quintessence kinetic energy was proposed to explain the phenomenon. The dark energy potential includes both the hyperbolic and the double exponential potentials. We analyze this model in detail by using the recent supernova and the first year Wilkinson Microwave Anisotropy Probe (WMAP) observations. For a flat universe with vacuum energy which is a special case of the general model, we find that $\Omega_{\rm m0}=0.295^{+0.082}_{-0.075}$ or $\Omega_{\rm \Lambda}=0.705_{-0.082}^{+0.075}$ and the transition redshift $z_{\rm T}$ when the universe switched from the deceleration phase to the acceleration phase is $z_{q=0}=0.68$. For the general model, we find that $\Omega_{\rm m0}\sim 0.3$, $\omega_{\rm Q0}\sim -0.9$, $\beta\sim 0.5$ and $z_{\rm T}=0.5045$.
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this is to heavy for me i will have to read it several times
to gleen what i can.
Originally posted by wolram
MARCUS.
one thing i am not clear on, did this decel to accel
have a starting point or was it universal, for if it
was universal, started everywhere at the same time, the
trigger would have to be extremely "pure".
the trigger is very interesting and we can discuss it because this
concerns the model
even tho so far the data is to sparse to be sure that the turnaround happened, we can still describe how it happens in the model
the trigger is the thinning out of the average density of matter in the universe
the expansion of space is only observable over very large distances and on a large scale the universe appears to have matter distributed uniformly with some average density which can (at least in a rough sense) be estimated. Current estimates are around 0.2 joule per cubic kilometer. Sorry about the metric units, if some prefer customary. To put more precision on it: 0.22.
Back when Roser Pello's galaxy emitted the light we are now seeing, the density of matter was 11 times bigger. Something like 2.2 joules per cubic kilometer. Or to be more accurate 2.4.
Now we assume a cosmological constant of 0.6 joule per cubic km. That is what they estimate the dark energy density to be. So that is some energy that is intrinsic to space itself, not attached to any substance floating around in space. It has an expansive effect.
But back in the time of z = 10 (when Pello's galaxy emitted the light) the acceleration effect of the Lamdda 0.6 joules was overshadowed by the deceleration effect of the matter 2.2 or 2.4 joules.
So there was net deceleration.
Since that time space has expanded 11-fold
so the density of matter has gotten less by 11-fold
and is now down to 0.2 (or more precisely 0.22) joules
But while the matter density is being thinned out by expansion, the Lamda stays constant and is forever and always 0.6.
So now the Lamda dominates the matter and its acceleration effect prevails.
So the trigger is that the average density of matter masks the dark energy until the matter has thinned out enough.
And local unevenness means that locally the story can deviate and there can be no one welldefined time of changeover. But these local effects are imperceptible because the whole thing is only perceptible over very large scale anyway. So there is a welldefined time when average deceleration changed over to average acceleration.
I have seen estimates like the changeover was when the age of the universe was 8 billion to 12 billion years old but this is very tenuous because the supernova data is still, AFAIK, scanty.
Maybe we can estimate when, according to the model, it should have happened. I have not done that yet. But that would in any case be only a theoretical prediction and what matters is for them to get more datapoints of supernovas far back in time.
I guess what is critical is getting a lot of supernovas with redshift
bigger than 1. They may be hard to see and they may not have found
very many yet. I am unsure about the z-range of supernova data that is needed. Maybe someone else can help out.
I did a rough calculation of when (at what redshift z) the turnaround should have happened according to the concordance model.
bear in mind this does not say when it really happened or indeed if it did at all---just a theoretical model calculation
I got that it should have happened at around z=4.5
I find this puzzling. I did not think it was so long ago. Unless I made a mistake it seems too long ago for supernova data to go back to.
the decelerating era would be too far away to see supernovas of that era.
For what its worth here is the calculation. the current measure lambda is 0.6 joule per cubic km
it that is constant thru all space and time (the simple "cosmological constant" assumption) then to go back to a deceleration era one must go back so far that matter density is 1.2 joule per cubic km.
(the technical thing coming from the Friedmann equations is that since matter has negligible pressure its density must be twice the dark energy density for its contractive effect to dominate over the expansive accelerating effect of the dark energy----twice 0.6 is 1.2)
But matter (ordinary and dark) is now measured at 0.22.
To get back to an era when this was not piddling 0.22 but a good strong 1.2, one must shrink down space by factor of 5.45 or let us say 5.5
that means z = 4.5
(the z is always one less than the factor space expands by, it is custom in how astronomers talk)
So if we see an object with z = 4.5, then that object is living in an era when the expansion of space was decelerating. If this calculation is OK.
Now I have heard of observing a supernova with z = 1.7
but I never heard of one with z = 4.5
that is so far away that it is hard enough to see a quasar or a galaxy. So I am, for the time being, very confused.
Can anyone help?
How can Ted Bunn say that already we "seem to see" signs of this changeover when the actual predicted time of the changeover is so long ago. (or my calculation is wrong?)
I went to the A&C reference sticky to get a calculator to see how long ago z = 4.5 is.
Ned Wright's
http://www.astro.ucla.edu/~wright/CosmoCalc.html
it says z=4.5 corresponds to 12.3 billion years ago
or age of universe is 1.36 billion years.
so baryonic matter is dissolved into the predominant dark energy
in the final analysis, and gravity will be a "spent" with
no way to recoup, i think i prefer an alternative outcome.
an excellent answer MARCUS.
Originally posted by wolram
so baryonic matter is dissolved into the predominant dark energy
in the final analysis, and gravity will be a "spent" with
no way to recoup, i think i prefer an alternative outcome.
an excellent answer MARCUS.
I must have been drowsy when I wrote my original response.
It was too speculatative and didnt go anywhere, so I scrapped it.
Your hunches about the distant future are at least as good as mine
so I will not try to present any opposing view.
Glad you approve of the earlier post. Thanks for saying so!
i suppose it would be incorrect not to mention the two brane
theory of an oscillating universe that needs no dark energy,
i discarded the papers i downloaded so i have no links, this
theory seemed to exite a few people, i lost interest after
finding that there is no way of testing it.
Originally posted by wolram
i suppose it would be incorrect not to mention the two brane
theory of an oscillating universe that needs no dark energy,
i discarded the papers i downloaded so i have no links, this
theory seemed to exite a few people, i lost interest after
finding that there is no way of testing it.
have to pass on that one.
maybe someone else can reply.
it strikes me as a good idea to mention alternative models and explanations (not incorrect, on the contrary)
where I dont follow you is how any sort of alternative model could fit the data and not need dark energy, unless it took leave of General Relativity. So far I havent heard of any acceptable replacement for vintage 1915 GR and dark energy is really GR's "fault".
Dark energy, so far, is just a question mark. A gap in the balance sheet of GR. GR says space is expanding, as it indeed appears to do at a certain rate. GR says that for space to be flat (so that the sum of angles of a triangle is two right angles) and have the observed rate of expansion it must have an average energy density of
0.83 joules per cubic km.
And astronomers can only detect around 0.22 joules (ordinary plus inferred dark matter). So there is this gap of about 0.6 joules.
And attributing it to constant vacuum energy predicts the observed amount of acceleration so that helps narrow down the estimate of 0.6.
But first and foremost before any acceleration there is simply this gap between what GR says flatness requires and the amount of ordinary and invisible matter that astronomers have been able to measure and infer.
To me it seems like a big blank, a question mark, an open doorway, as if we are at the beginning of a process of discovery, not nearing the end of one.
The alternative would be to trash 1915 GR and find some explanation for gravity which does not have this 73 percent gap----this unexplained 0.6 joules per cubic click.
But much of the beautiful stuff Nereid (for one) talks about---the lensing effect of clusters of galaxies and their clouds of dark matter: making space into a huge magnifying glass. And the fact we can listen to neutron stars spiraling in and speeding up as they radiate energy via the gravitational field---and the delicate corrections of the GPS that actually make it work---all that good stuff represents 1915 GR coming into its own and gaining prominence. As a model I suspect that GR is near the beginning of its life-cycle. And I see no competitors jumping up offering to explain the same things---the lensing, the spiraling-in, the time-corrections, the way energy is released in the cores of galaxies, and so on.
at this point the joke is on us humans: we have an elegant GR theory that matches certain observation with exquisite precision and predicts fascinating things we are gradually starting to observe----but the elegant theory has a 73 percent gap in it
(the actual figure of the size of the gap was arrived at by help of observing acceleration through supernovas but the rough-sized gap was already puzzling people before that)
maybe the biggest challenge is not to get panicked and chuck the theory because of this gaping 73 percent gap
somebody has to think up a model of what space is at the microscopic level such that space (according to that model) has an intrinsic energy to it which is exactly 0.6 joules per cubic kilometer
until that happens, you and I and everybody are just looking at a big blank questionmark and trying not to notice that Nature is laughing at us
http://skyandtelescope.com/news/current/article_592_1.asp
Steinhardt and Turok say their model does all of this just as well as inflation and goes it one better — by naturally incorporating the recently discovered "dark energy," which is making the expansion of the universe speed up. Their theory replaces inflation and dark energy with a single energy field that oscillates in such a way as to sometimes cause expansion and sometimes a recollapse.
---------------------------------------------------------------------
this is a clip from 2002, i dont know if this theory is still
hot, from the outset it seems a cosy theory with the oscillating
energy field replacing DE, but how on earth can 3 branes be
detected?
http://www.sciencedaily.com/releases/2004/02/040223074521.htm
Date:
2004-02-23
Cosmologists understand almost nothing about dark energy even though it appears to comprise about 70 percent of the universe. They are desperately seeking to uncover its two most fundamental properties: its strength and its permanence.
In a paper to be published in the Astrophysical Journal, Riess and his collaborators have made the first meaningful measurement of the second property, its permanence.
well it seems that few are interested in this topic maybe
MARCUS is the only one that can interact and is willing
to open up, for the passive majority i can only ask, what "do you
think"?
Originally posted by marcus
the trigger is the thinning out of the average density of matter in the universe
This seems to say that once conventional matter becomes sparse, accelerated expansion takes over. How then can anything trigger a deceleration and contraction after such thinning out has taken effect?
This would eliminate a contraction phase, which Turok and Steinhardt predict in their model of a Cyclic Universe. (Correct me if I’m wrong) After reading their latest paper it seems they compare a prior contraction, (preceding the BB) to the eventual re-collapse of this Universe, which so far shows no sign of ever doing so.
Beyond Inflation: A Cyclic Universe Scenario (http://uk.arxiv.org/PS_cache/hep-th/pdf/0403/0403020.pdf)
VAST, i hope MARCUS will not mind me quoting his post i dont
want to inflate his ego to much but his explanations are
clear and understandable[t)]
-------------------------------------------------------------------
.
Now we assume a cosmological constant of 0.6 joule per cubic km. That is what they estimate the dark energy density to be. So that is some energy that is intrinsic to space itself, not attached to any substance floating around in space. It has an expansive effect.
But back in the time of z = 10 (when Pello's galaxy emitted the light) the acceleration effect of the Lamdda 0.6 joules was overshadowed by the deceleration effect of the matter 2.2 or 2.4 joules.
So there was net deceleration.
Since that time space has expanded 11-fold
so the density of matter has gotten less by 11-fold
and is now down to 0.2 (or more precisely 0.22) joules
But while the matter density is being thinned out by expansion, the Lamda stays constant and is forever and always 0.6.
So now the Lamda dominates the matter and its acceleration effect prevails.
So the trigger is that the average density of matter masks the dark energy until the matter has thinned out enough.
Thanks wolram. I have to thank both you and marcus because this thread has been really insightful!
If I understand this correctly, the trigger is also referred to as a cosmic coincidence, associated with a certain phase of the Universe.
If theory and observational measurements confirm accelerated expansion, it seems the acceleration is just going to continue getting faster and faster.
So far what this suggests for the long run is an alternative to an eventual re-collapse. If a cosmological constant is to remain at 0.6 joule, then acceleration should always prevail
by VAST.
Thanks wolram. I have to thank both you and marcus because this thread has been really insightful!
--------------------------------------------------------------------
VAST you cant imagine how happy i am that you asked a question
on this thread, i would also like to thank MARCUS.
Originally posted by Vast
This seems to say that once conventional matter becomes sparse, accelerated expansion takes over. How then can anything trigger a deceleration and contraction after such thinning out has taken effect?
This would eliminate a contraction phase, which Turok and Steinhardt predict in their model of a Cyclic Universe. (Correct me if I’m wrong) After reading their latest paper it seems they compare a prior contraction, (preceding the BB) to the eventual re-collapse of this Universe, which so far shows no sign of ever doing so.
Beyond Inflation: A Cyclic Universe Scenario (http://uk.arxiv.org/PS_cache/hep-th/pdf/0403/0403020.pdf)
Actually this scenario was predicted sometime ago, but contary to your thoughts here:This would eliminate a contraction phase,which Turok and Steinhardt predict in their model of a Cyclic Universe.
ranyart.
yes i have read the cyclic universe theories, the most
prominent feature is the avoidance of the singularity
and a new way of accounting for the apparent over abundance
of energy.
http://www.nature.com/nsu/030609/030609-7.html
Alternative proposed to dark energy's cosmic doomsday.
---------------------------------------------------------------
The destruction begins, say Robert Caldwell of Dartmouth College in New Hampshire, USA, and his coworkers2, about a billion years before it ultimately ends in a Big Rip. First, gravity loses its grip at cosmic scales, allowing clusters of galaxies to drift apart.
Sixty million years before doomsday, our own galaxy, the Milky Way, fractures as stars slip from each other's grasp. A few months before the end, planetary systems like the solar system will be dismembered, and 30 minutes before the Big Rip, the planets and stars themselves disintegrate.
In the split-second before the end, atoms and molecules are torn apart, then the particles that constitute them. Finally, space itself flies open.
All of this is driven, the argument goes, by something known as phantom energy, which fills all of space. The density of phantom energy increases with time, like a bomb that grows ever bigger.
Originally posted by wolram
http://www.nature.com/nsu/030609/030609-7.html
...All of this is driven, the argument goes, by something known as phantom energy, which fills all of space. The density of phantom energy increases with time, like a bomb that grows ever bigger.
this is different from the usual picture
what people ordinarily assume for dark energy is
a constant energy density
(often called "cosmological constant" or Lambda)
it does not increase with time
and it does not eventually rip things apart that are gravitationally bound entities (like galaxy or solar system or planet)
nobody knows that the Lambda is actually constant----the evidence has been mounting that it is constant, some reported here at PF, but
nothing conclusive
since no one can say that it is constant, people are free to imagine other scenarios like "phantom energy" which increases over time and has dramatic effects.
I think it's fair to say there is LESS reason to take phantom energy seriously than there is reason to assume the ordinary constant dark energy but that's not to say people should be discouraged from constructing these alternative scenarios
Originally posted by ranyart
Any later developing lifeforms would be looking back at this moment as the dawn of Universe, no contraction needed, its a perceptional 'Bounce'!
The usual cosmic model is that of a universe beginning in a cosmic singularity, therefore it seems reasonable to assume a multi universe scenario each beginning in the same manner.
But I think this is off topic to dark energy.
NEREID, mentioned the SNOWMASS BOOK, this is new to me its at
http://supernova.lbl.gov/~evlinder/sci.html.
i think its interesting and informative.
thanks NEREID.
hellfire
Mar12-04, 06:54 AM
May be I am asking a little bit too late here, but here are some basic questions for my understanding of the observations done by A. Blanchard:
1. I read that this is an X-ray observation of the intergalactic medium in far clusters. In the press release it is written that in old clusters ‘there are more X-rays than today’. What does this mean? Does this mean that old clusters ‘in past’ were more distributed throughout space than today and that they went a gravitational collapse since then (so that some of them grouped into a single cluster and therefore the X-ray emission is received from a narrower region)?
2. It is also argued in the press release, that, if dark energy were existent, it would impede the gravitational collapse of clusters leaving them unchanged from ‘the past’ up to now. But, what is meant with ‘past’? Usually, it is postulated that clusters formed due to gravitational collapse (bottom-up model). Therefore the dark energy (if existent) must be dominating or influencing this process in a later point of time after the actual collapse (otherwise they would not be formed). When is this epoch to be located in time?
3. What is the relation between X-ray intensity and mass? I remember have read somewhere that additionally to mass estimations done according to observations of dynamics and application of the virial theorem, there is the possibiltiy of mass estimations of clusters according to X-ray observations.
Thanks. Regards.
I can't opine about this paper because I haven't read it still
"A model of holografic dark energy"
http://arxiv.org/abs/hep-th/0403127
But it seemed to me that the denomination "holographic dark energy" is at least curious, no?
http://arxiv.org/PS_cache/astro-ph/pdf/0204/0204500.pdf
by HELLFIRE.
1. I read that this is an X-ray observation of the intergalactic medium in far clusters. In the press release it is written that in old clusters ‘there are more X-rays than today’. What does this mean? Does this mean that old clusters ‘in past’ were more distributed throughout space than today and that they went a gravitational collapse since then (so that some of them grouped into a single cluster and therefore the X-ray emission is received from a narrower region)?
---------------------------------------------------------------------
the above url is over 170 pages long and is comprehensive.
i haven't been able to confirm that old clusters are more
x ray active, maybe NEREID or other PF members can answer
your questions, in the meantime i will read the posted article.
the subject of X ray emissions from early-type galaxies seems
to be very wide, from the posted url----------
studies of large sample of early type galaxies are suggestive that
the X-ray structure of these systems is mostly determined by
the gravitational potential well produced by large amounts of
dark matter rather than by thermal mechanisms.
---------------------------------------------------------------------
cD galaxies are galaxies with a nucleus of a very luminous
elliptical embedded in an extended amorphous halo of low surface
brightness, "this galaxy type may have been formed by collision".
---------------------------------------------------------------------
some early type galaxies that have hot (1KeV) interstellar
medium trapped by the galaxy potential well whose emission is
due to thermal processes.
---------------------------------------------------------------------
i havent read all the article yet but i think you will find it
very infomative.
posted by METEOR.
But it seemed to me that the denomination "holographic dark energy" is at least curious, no?
----------------------------------------------------------------
yes it is, if it wasn't for the Hubble scale problem this theory
might be better than others.
hellfire
Mar13-04, 05:05 AM
Thanks wolram, it looks a bit intimidating... but I will try to take a look. Anyway, it would be great if someone could answer shortly my questions.
Originally posted by hellfire
Thanks wolram, it looks a bit intimidating... but I will try to take a look. Anyway, it would be great if someone could answer shortly my questions.
hello hellfire, I remember enjoying your discussions before and I would like to try later today to answer, if no one gets to it earlier. I may not be able to because am supposed to be away for part of the day.
I think we already found the relevant technical paper by Blanchard
and posted, discussing this business of more dark matter and less (or zero) dark energy. IIRC he needs to assume a lower Hubble parameter to make all the numbers add up.
It will take me a while to reconstruct his argument. Maybe you already have and can explain it.
hellfire, it turns out there was some discussion in this very thread!
what I was remembering was on the first page of this thread. this link may be the one to the technical article that corresponds to the
wide-audience account you mentioned.
Originally posted by marcus
Alain Blanchard must be the most important dissident to the
"concordance" cosmology picture. the leader of the opposition.
his most recent preprint in arxiv is
http://arxiv.org/astro-ph/0402297
I'm not sure but I think
the article you pointed to in Astronomy magazine
by Amanda Jefter (dated 23 December 2003) refers
to earlier articles of Blanchard
mainly this one
http://arxiv.org/astro-ph/0304237
but also this 3-pager
http://arxiv.org/astro-ph/0311626
Nereid may have responded to the gist of what
is in these articles in another thread. I forget which.
Or I may have. I dont think Blanchard's case is strong
enough yet to start bringing more people over to his side
but he certainly bears watching. If he continues to
assemble evidence of much more dark matter then
he could start a shift of opinion.
I think the argument here is between dark matter and dark energy.
the concordance model says that familiar types of matter total around 4 or 5 percent of the average density in space
but that leaves 96 percent to account for
the "concordance" estimate is that it is split 73 d.energy plus
23 d.matter
Blanchard's main message, if I understand it, is to give much more importance to dark matter and less to dark energy.
he may also favor a lower value than 71 for the hubble parameter
and consequently a lower overall density (so that the observed amount of ordinary matter would play a larger role)
...
I looked back at an earlier thread and found another blanchard link
http://arxiv.org/abs/astro-ph/0311381
hello again,
I'm copying your post so we have your questions in front of us. If we are lucky we will be able to put your questions together with the links to Blanchard's papers and recall how his arguments go, and respond to your questions. Or you will beat me to it and figure out
Blanchard's reasoning yourself.
Originally posted by hellfire
May be I am asking a little bit too late here, but here are some basic questions for my understanding of the observations done by A. Blanchard:
1. I read that this is an X-ray observation of the intergalactic medium in far clusters. In the press release it is written that in old clusters ‘there are more X-rays than today’. What does this mean? Does this mean that old clusters ‘in past’ were more distributed throughout space than today and that they went a gravitational collapse since then (so that some of them grouped into a single cluster and therefore the X-ray emission is received from a narrower region)?
2. It is also argued in the press release, that, if dark energy were existent, it would impede the gravitational collapse of clusters leaving them unchanged from ‘the past’ up to now. But, what is meant with ‘past’? Usually, it is postulated that clusters formed due to gravitational collapse (bottom-up model). Therefore the dark energy (if existent) must be dominating or influencing this process in a later point of time after the actual collapse (otherwise they would not be formed). When is this epoch to be located in time?
3. What is the relation between X-ray intensity and mass? I remember have read somewhere that additionally to mass estimations done according to observations of dynamics and application of the virial theorem, there is the possibiltiy of mass estimations of clusters according to X-ray observations.
Thanks. Regards.
http://www.astro.ucla.edu/~wright/cosmolog.htm
2 Feb 2004 - A group led by Prof. Tom Shanks of the University of Durham, UK, has suggested that the Sunyaev-Zeldovich effect may have significantly affected the WMAP results on the anisotropy of the cosmic microwave background. However, the S-Z effect goes through zero at 220 GHz and the ARCHEOPS balloon-borne CMB experiment which observes at 143 and 217 GHz sees the same map and angular power spectrum as WMAP. The fact that ARCHEOPS agrees with WMAP within -4.4+/-2.8% in the amplitude of the first acoustic peak means that the S-Z effect has very little influence on the WMAP results. So this was another scientific theory that, like the dodecahedral Universe, was already disproven by the time the authors sent out their press release.
-----------------------------------------------------------------------
the S-Z effect has been used in many arguments in cosmology
NWs view is that it is immaterial regarding WMAP data.
but others are taking advantage of this effect to define
the parameters of galaxies.
http://www.sciencedaily.com/releases/2002/04/020422073037.htm
By comparing the X-ray emission and the Sunyaev-Zeldovich effect, Mohr can study even faint, high-redshift galaxy clusters that are currently inaccessible by other means. Such measurements, correlating galaxy cluster redshift distribution, structure and spatial distribution, should determine the equation of state of dark energy and, therefore, help define the essence of dark energy.
. What is the relation between X-ray intensity and mass? I remember have read somewhere that additionally to mass estimations done according to observations of dynamics and application of the virial theorem, there is the possibiltiy of mass estimations of clusters according to X-ray observations.
---------------------------------------------------------------------
http://www.astro.su.se/~ostlin/ColMag/Source/colMag_clusters.html
The image here shows a view of the core of the Virgo cluster in the X-ray waveband and illustrates the structure of the hot, gravitationally-bound gas in the cluster's potential well. This potential well is sufficiently deep that the gas between the galaxies within the cluster is compressed and heated to high enough temperature that emits radiation at X-ray wavelengths. Images of this X-ray radiation illustrate the very extended potential well of the cluster, which contains two major peaks, associated with sub-groups of galaxies within the cluster. Nevertheless, the X-ray image still appears much smoother than the distribution of the individual galaxies. Moreover, the temperature and the distribution of the X-ray gas can be used to estimate the mass of the cluster (assuming that the hot X-ray gas behaves as an ideal gas) and in all cases this has been found to significantly exceed the mass contained within the galaxies. These observations are one of the strongest pieces of evidence for dark matter on large scales in the Universe. Observing at X-ray wavelengths requires the use satellites to get above the absorption from the Earth's atmosphere, which is opaque in the X-ray band.
2. It is also argued in the press release, that, if dark energy were existent, it would impede the gravitational collapse of clusters leaving them unchanged from ‘the past’ up to now. But, what is meant with ‘past’? Usually, it is postulated that clusters formed due to gravitational collapse (bottom-up model). Therefore the dark energy (if existent) must be dominating or influencing this process in a later point of time after the actual collapse (otherwise they would not be formed). When is this epoch to be located in time?
--------------------------------------------------------------------
according to first paper on this thread, 5 billon YRS ago.
after reading all these papers the main stream view seems
to hold true, we used to have a nice quiet universe until
AE came along and spoilt it, now we have a suicidal one full
of DARK ENERGY that is pushing us to oblivion, it seems that
everything in nature gets recycled except nature itself,
life after the BIG RIP seems implausible without some kind of
divine intervention, so on a cosmological time scale its
BIG BANG count to---------------- its all over by by.
it maybe philosophical but i canot think that our universe
is a use once and throw away item, I'm sure that main stream
science will be found incorrect, and that universes dont
die they just get recycled.
hellfire
Mar13-04, 04:04 PM
Excellent, thanks again wolram for the references and also marcus for your efforts. After reading again the article (I think I misinterpreted something) and the references I consider all my questions answered:
Regarding the epoch of start of domination of the cosmological constant (5 billion years ago), I have to apologize since I did not read the article referred in the first post of this thread.
The third question (x-ray / mass relation) seams to be clear with:
This potential well is sufficiently deep that the gas between the galaxies within the cluster is compressed and heated to high enough temperature that emits radiation at X-ray wavelengths.
And this fits with the claim of Blanchard, that they found fewer x-rays than today: potential wells are postulated to be deeper today, which is an argument against the cosmological constant.
Regards.
hellfire
Mar14-04, 04:03 AM
[*(]
in the press release in the second link of the thread:
http://www.esa.int/sci_mediacentre/...html?release=54
it is in fact written that
They seem to give out more X-rays than today.
According to the quote in the previous post, this would imply that potential wells were more deep in the past, which fits with the cosmological constant hypothesis and contradicts Blanchards own thesis.
Any help?
Regards.
HELLFIRE.
may i refer you to NED Wrights web site ref (Hubble parameter)
http://www.bright.net/~mrf/App9.html
blanchard is arguing that more DARK MATTER exists in the
universe than others theories, which could require an
adjustment to the H-P=71, as the H-P has been refined over
the years it now seems almost unshakable, the BIG problem
with cosmology seems to be that numbers can be manipulated
to fit a pet theory, i guess we will have to wait for the
data from new satellite missions to see what amount of
dark matter, dark energy is out there.
i was hopping that MARCUS or NEREID would jump in as they
are easier to understand and much better qualified than me.
HELLFIRE.
i have found this link in ref to to your question, have a
look ,i have not read it all yet so maybe you can tell me
if it helps.
http://www-xray.ast.cam.ac.uk/~jss/research/mnr_5510.pdf
Originally posted by wolram
... would jump in as they
are easier to understand and much better qualified than me.
the more explaining you do on PF the easier to understand
you get
6 months ago you only asked one-line questions
and the more questions you asked the better they got
now you elucidate and the more you
practice the better it gets
(hellfire and Nereid may have noticed, why should I jump in?)
today is Greg B's 21 birthday
what shall we say to him on this occasion
except that we see that the PF he has built
seems now and then to be good for something
beyond simple recreation
well to start i think GREG Bs built a world leading science
forum that everyone should be proud of, but MARCUS i value
your contributions as you know i am a minnow in this sea
of whales, i am stuck on the variance of x-ray luminosity
in clusters over time, intuitively i would say that it would
decrease, but the clusters may be evolving gravitationally
or could be acquiring mass, the last post in this thread is
the closest i have come to finding an answer.
hellfire
Mar15-04, 04:22 PM
wolram, regarding your last reference: I have been reading some references and I think the process of cooling of the intra cluster gas is not relevant in this case. Cooling refers to the loss of energy due to the x-ray emissions (which are generated due to thermal bremsstrahlung) and its characteristic time is of the scale (or usually longer than) the hubble time for most of the clusters. It is therefore usualy not considered for time evolution of the luminosity. As far as I understood, Blanchard also does not consider the cooling effect for his main argumentation.
My impression now is that x-ray emission depends not only on the potential well, but on a function called density contrast. This is the quotient of the main density of the cluster (density perturbation) and the energy density of the universe at the time where the cluster gets virialized (more or less stability of the gravitationally bound system is reached).
In general, it is the number of clusters of a given virial mass, which depends on the density contrast, but, since there is a postulated relation between luminosity and virial mass, also the emission of x-rays should depend on the density contrast.
Blanchard claims to have found more x-rays than today. I thought this implies deeper potential wells (in contradiction with Blanchards own thesis) but it seams that this is not the case due to the dependence on the density contrast function.
Due to the relation between density and radius the cosmological constant enters the density contrast function making use of the Friedmann equation. I am not able now to make any qualitative statement about the relations. As you see I just think now I have found the right equations, but I am still far from understanding properly the effect of evolution of luminosity.
Anyway, it would be great if somebody could confirm or correct me.
These are the references I used:
http://arxiv.org/astro-ph/9611085
http://nedwww.ipac.caltech.edu/level5/March02/Sarazin/Sarazin_contents.html
http://nedwww.ipac.caltech.edu/level5/Sept02/Padmanabhan/Pad_contents.html
Regards.
Originally posted by marcus
the more explaining you do on PF the easier to understand
you get
6 months ago you only asked one-line questions
and the more questions you asked the better they got
now you elucidate and the more you
practice the better it gets
(hellfire and Nereid may have noticed, why should I jump in?)
today is Greg B's 21 birthday
what shall we say to him on this occasion
except that we see that the PF he has built
seems now and then to be good for something
beyond simple recreation This thread is roaring along so quickly, it's scary [t)] I've started to reply to some post or other here several times, only to find, upon reading more of the thread, that some other aspect needs to be covered (or has already been covered). Whew, what a ride! [:))]
Someone may have commented on this already (yes, I've been in the slow class several times), but Peacock's paper (http://nedwww.ipac.caltech.edu/level5/Sept03/Peacock/Peacock_contents.html) on large scale structure and cosmology may be quite interesting to several readers (see if you can ignore the equations wolfram, and get an idea of the logic; you may also pick up a thing or two about some of those puzzling parameters which you read in other papers, e.g. 'tilt', 'bias', 'tensors').
I've been distracted from this thread by other (local off-web) activity and also swamped by the complexity of the issues. I need to find a suitable tutorial and an up-to-date review article that surveys how things stand at present and how the various investigations into dark energy are going.
Right now I am convinced that several of you other posters (hellfire, wolram, Nereid) are ahead of me and I need to catch up.
This morning over coffee I looked at Sahni's review article
"Dark Matter and Dark Energy"
http://arxiv.org./astro-ph/0403324
You can tell its meant as a review article because it has 190 references in its bibliography
You can tell its meant to be understandable to a wider audience because it doesnt explain very much in detail.
It is the sort of thing that OUGHT to help, but instead of helping integrate, it zaps my understanding by showing me that the jigsaw puzzle has 1000 pieces rather than the 100 or so I expected. I should be grateful to Nature for being so complex? Is this a sign something really interesting will emerge? Or is this just the wrong review article to try to read? Maybe there is a better one
Varun Sahni's Figure 1 is a graphic undeniable demonstration of the existence of dark matter:
the rotation curve for M33 keeps going up!
"It is interesting that the total mass of an individual
galaxy is still somewhat of an unknown quantity since a turnaround to the v ~ r−1/2 law at large radii has not been convincingly observed." (page 4)
Can this be true? They have not even seen where the rotation curve
stops increasing and begins to decline? The situation is more confusing than I thought.
In the most basic entry-level details of the picture, there are unexpected signals. here is some more on page 4:
"An important difference between the distribution of dark matter in galaxies and clusters needs to be emphasised: whereas dark matter appears to increase with distance in galaxies, in clusters exactly the reverse is true, the dark matter distribution actually decreases with distance..."
so clusters of galaxies have a central concentration of DM but individual galaxies do not-----their DM is spread out an order of magnitude more than their visible matter. And he emphasizes this aspect by giving a little evidence:
"...Indeed, for certain dwarfs (such as DD0154) the rotation curve has been measured to almost 15 optical length scales indicating that the dark matter surrounding this object is extremely spread out (see also figure 1).
A foreground cluster, on the other hand, acts as a gravitational lens which focuses the light from background objects such as galaxies and QSO’s thereby allowing us to determine the depth of the cluster potential well. Observations of strong lensing by clusters indicate
that dark matter is strongly concentrated in central regions with a projected mass of 1013 − 1014 solar masses being contained within 0.2 - 0.3 Mpc of the central region. As we shall see later, this observation may prove to be problematic for alternatives to the dark matter hypothesis such as the Modified Newtonian..."
Then there is the business of the Xray-emitting gas. Varun Sahni says that even the visible matter in clusters of galaxies is not what I thought it was. It is not stars, he says, most of the mass of visible matter in a cluster is in the form of Xray-emitting gas!
"The mass-to-light ratio in clusters can be as large as M/L = 300Msol/Lsol. However since most of the mass in clusters is in the form of hot, x-ray emitting intracluster gas, the extent of dark matter in these objects is estimated to be M/Mlum = 20, where Mlum is the total mass in luminous matter including stars and gas."
If this is true it will take a bit of getting used to.
the U has right amount of energy density to make it flat
and most of that energy isnt matter, it is DE
but there is some matter
however most of the matter isnt visible it is DM
on the other hand there is some visible matter
but (before you get too comfortable with that, notice that)
the visible matter is mostly not stars it is visible by
its Xrays because the great bulk of it is Xray-emitting gas.
I had accepted the idea that only 4 percent of the U was
ordinary baryonic matter and I decided to be content with that,
but I thought it at least was stars. Now I find that the
4 percent is mostly Xray-emitting gas. Bah. I shall write the
editors of Nature and threaten to cancel my subscription if
they cannot abide by the rules of ordinary decency
Something almost funny in Varun Sahni's review article.
the speed of the earth relative to galactic center varies, being 7 percent faster in the summer than in the winter
so the earth should bump into more dark matter in the summer
a controversy has arisen because a research group called DAMA
claims to have detected this bumping
with an annual variation just as predicted:
more bumping in the summer
but regretfully it must be said that not a single other research group has been able to reproduce this result
the controversy and lack of confirmation does not deter Sahni from including this in his review and he also has a nice little picture of the earth going around the sun to explain why the speed varies by 7 percent----Figure 3 on page 8
thanks STINGRAY.
it seems that the properties of these early type galaxies
are Dependant on the potential energy of dark matter rather
than any barionic mechanisms, i am catching up your last
post helped lots.
thanks for link NEREID your observation on the speed of
this thread just hit me, i think i spent all my fuel to
early and now have to coast, but that will be enjoyable
as i have lots to read.
by MARCUS.
Something almost funny in Varun Sahni's review article.
the speed of the earth relative to galactic center varies, being 7 percent faster in the summer than in the winter
this is a strange comment.
Originally posted by wolram
by MARCUS.
...this is a strange comment.
Pax wolram. :-)
One research group has detected us plowing through dark matter
ever since 1996
but no other group is able to sense this happening.
Originally posted by marcus I've been distracted from this thread by other (local off-web) activity and also swamped by the complexity of the issues. I need to find a suitable tutorial and an up-to-date review article that surveys how things stand at present and how the various investigations into dark energy are going.
Right now I am convinced that several of you other posters (hellfire, wolram, Nereid) are ahead of me and I need to catch up.
This morning over coffee I looked at Sahni's review article
"Dark Matter and Dark Energy"
http://arxiv.org./astro-ph/0403324
You can tell its meant as a review article because it has 190 references in its bibliography
You can tell its meant to be understandable to a wider audience because it doesnt explain very much in detail.
It is the sort of thing that OUGHT to help, but instead of helping integrate, it zaps my understanding by showing me that the jigsaw puzzle has 1000 pieces rather than the 100 or so I expected. I should be grateful to Nature for being so complex? Is this a sign something really interesting will emerge? Or is this just the wrong review article to try to read? Maybe there is a better one This is, IMHO, a good review! Of course, it could have been longer (there's always more you want to know [:(] ).
AND it has the figure that I was trying to find for a previous post! It's figure 11, on page 33.Originally posted by marcus
Can this be true? They have not even seen where the rotation curve stops increasing and begins to decline? The situation is more confusing than I thought.Er, yes, it's true. But it's not a problem, because the mass in clusters is constrained by other estimates, and what are clusters but many galaxies (including 'dark' ones) and 'some' IGM?Originally posted by marcus Then there is the business of the Xray-emitting gas. [a.k.a. IGM] Varun Sahni says that even the visible matter in clusters of galaxies is not what I thought it was. It is not stars, he says, most of the mass of visible matter in a cluster is in the form of Xray-emitting gas! Omigosh! hock! Shorror!! Lends a whole new meaning to the phrase 'scum of the universe', doesn't it? [*(] [;)]
Originally posted by marcus
One research group has detected us plowing through dark matter ever since 1996 but no other group is able to sense this happening. So maybe those researchers found something else in 1996?
Anyway, won't it be nice when we detect some real, honest-to-Hoyle neutralinos, binos, winos, higgsinos, axions, Wimpzillas, axinos, or gravitinos?
Originally posted by Nereid
So maybe those researchers found something else in 1996?
Anyway, won't it be nice when we detect some real, honest-to-Hoyle neutralinos, binos, winos, higgsinos, axions, Wimpzillas, axinos, or gravitinos?
they sound like the breakfast cereals from when I was a kid
by MARCUS
I've been distracted from this thread by other (local off-web) activity and also swamped by the complexity of the issues. I need to find a suitable tutorial and an up-to-date review article that surveys how things stand at present and how the various investigations into dark energy are going.
--------------------------------------------------------------------
looking for answers about DM, DE, is akin to a search for the
holy grail, to date i have found only "observational evidence".
http://eu.spaceref.com/news/viewpr.html?pid=7050
Big bang nucleosynthesis theory provides an estimate on the amount of "ordinary" matter in the Universe, and this rules out the possibility that dark matter is from dim stars, dark chunks of solid material or black holes. Dark matter must be exotic, that is, not made of protons and electrons. Various observations with radio, optical and X-ray telescopes aim to determine the distribution and nature of dark matter.
Originally posted by wolram
*SNIP
looking for answers about DM, DE, is akin to a search for the holy grail, to date i have found only "observational evidence". My goodness, what on Earth (M33, Abell 1835 IR1916, ...) is there apart from observational evidence? OK, and maybe coming in to the lab in the morning and finding a nice healthy green Wimpzilla in your SQUID. [:D]
i agree NEREID bad choise of words.
----------------------------------------
IN one of those interesting intersections of particle physics, astrophysics, and cosmology, scientists from Lawrence Livermore National Laboratory, the University of California at Berkeley (UCB), the University of Florida (UF), and the National Radio Astronomy Observatory (NRAO) have joined together to try to pin down an elusive particle. This particle, called the axion, if it is found to exist and is not just a hypothesis, would be a long-sought relic from the first fractional second of the birth of the universe and one of the most weakly interacting particles known. Experimental verification of the existence of the axion would not only help “balance the budget” for the missing mass of the universe but also clear up one of the thorniest issues in particle physics.
---------------------------------------------------------------------
i have lost the url for this paper anyone have a link?
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it seems to me that cold DM is the most favoured candidate
for the missing mass problem, is this correct?
wolfram: it seems to me that cold DM is the most favoured candidate for the missing mass problem, is this correct? Yes.
A not very well known model of Dark energy is the model known as quartessence. In fact, quartessence is multifaceted, plays both the role of dark energy and dark matter.
Eh, I said role marcus, remember? [:))]
http://de.arxiv.org/abs/astro-ph/0401032
I describe recent challenges in hierarchical galaxy formation theory, including the formation of disk galaxies and of ellipticals. Problems with cold dark matter are summarized, and possible solutions are presented. I conclude with a description of the prospects for observing one of the most important ingredients in galaxy formation theory, namely cold dark matter.
--------------------------------------------------------------------
the bottom up theory of evolution has problems ,but they seem
to be mechanical, and not insurmountable.
"Cosmology with tachyon field as dark energy"
http://arxiv.org/abs/astro-ph/0212198
It can be a good idea if they are able to find some day the elusive tachyon [:)]
I insist that is not necessary that dark energy is due to a constant field (e.g. cosmological constant). A model with a variable scalar field would also fit the data. Quintessence is an example of such a model. And, I will go further and I conjecture that quintessence=Higgs field
Ok, call me crackpot now
Any specific predictions from this theory, that sets it clearly apart from other dark energy theories? I mean, other than catching a tachyon in your lab SQUID while you're eating your breakfast cereal. [;)]
Originally posted by wolram
http://de.arxiv.org/abs/astro-ph/0401032
I describe recent challenges in hierarchical galaxy formation theory, including the formation of disk galaxies and of ellipticals. Problems with cold dark matter are summarized, and possible solutions are presented. I conclude with a description of the prospects for observing one of the most important ingredients in galaxy formation theory, namely cold dark matter.
--------------------------------------------------------------------
the bottom up theory of evolution has problems ,but they seem
to be mechanical, and not insurmountable. This was written before the recent INTEGRAL results on the source of the 'diffuse' gammas from the galactic centre, so this 'problem' has now gone away:
"The \gamma-ray flux towards the galactic centre is observed to have a hard spectrum (as expected for annihilations), but the clumps would not survive the tidal disruptions that are inevitable in the inner galaxy [29]. To account for the observed diffuse gamma ray flux from the direction of the galactic centre, one would need to have a very steep density profile (\rho \varpropto~ r−1.5). This would conflict with microlensing observations and the inner rotation curve of the Galaxy."
Silk's introduction is worth repeating:
"Galaxy formation theory must account for the properties and evolution of galaxies, the star formation rate, the spectral energy distribution and galaxy morphologies. Another important confrontation with observation is with the scaling relations. These relations (e.g. Tully-Fisher, fundamental plane) are controlled by the current relaxation time-scales (dynamical and chemical) which are long compared to the age of the universe. This is not an easy task because the theory is almost entirely phenomenological and is driven by the observations. The ultimate aim is to make predictions at high redshift for the current and future generations of powerful detectors and very large telescopes. Progress is inevitably iterative and slow, and observations are usually well ahead of theory. A major hurdle is that there is nof undamental theory of star formation. Major uncertainties include the initial stellar mass function, the star formation efficiency and the star formation rate. Of course, the empirical evidence for star formation is overwhelming, and this leaves cosmologists with little choice but to extract every possible output from their theories."
[Edit: fixed formats]
METEOR.
i think the tachyon is out of favor, it seems to exotic,
the higgs could be tested soon in accelerators, but i am
almost convinced that DE, DM exist, but with 70% of universe
missing anything is possible.
Nereid: I looked through the paper and couldn't find any prediction that differs with the model of cosmological constant. Obviously, the great difference is the presence of the tachyon! But I must admit that I'm quite a layperson and didn't understand half of the paper, and is possible that I have overlooked some distinction with other models
Wolram: is possible that the Higgs have been found. Look this thread
http://www.physicsforums.com/showthread.php?s=&threadid=16078
I will add an alternative model for dark energy that I've discovered this morning
It proposes that dark energy is due to a chaotic scalar field
"Chaotics scalar fields as models for dark energy"
http://arxiv.org/abs/astro-ph/0310479
There's another proposal that postulates that the universe is not really accelerating, but the dimming of the type Ia supernova that triggered all the dark energy business is due to a mechanism known as axion-photon mixing
"Cosmic acceleration vs axion-photon mixing"
http://arxiv.org/abs/astro-ph/0311495
Ahrg, so many papers and so little time to read them all...
by METEOR.
Wolram: is possible that the Higgs have been found. Look this thread
-------------------------------------------------------------------
it would be fantastic if this could be verified, but it seems
the CL is quite low.
by METEOR.
There's another proposal that postulates that the universe is not really accelerating, but the dimming of the type Ia supernova that triggered all the dark energy business is due to a mechanism known as axion-photon mixing
"Cosmic acceleration vs axion-photon mixing"
--------------------------------------------------------------------
this is a hill that we can never get to the top of, the axion
is another thing that is proposed but not found, its good to
speculate but one cant climb an imaginary hill.
Another curious theory [:D]
http://arxiv.org/abs/astro-ph/0308183
Dark energy and dark matter from an inhomogeneous dilaton
Authors: Mikel Susperregi
Comments: 9 pages, 8 figures, uses revtex, submitted PRD
Journal-ref: Phys.Rev. D68 (2003) 123509
A cosmological scenario is proposed where the dark matter (DM) and dark energy (DE) of the universe are two simultaneous manifestations of an inhomogenous dilaton. The equation of state of the field is scale-dependent and pressureless at galactic and larger scales and it has negative pressure as a DE at very large scales. The dilaton drives an inflationary phase followed by a kinetic energy-dominated one, as in the "quintessential inflation" model introduced by Peebles & Vilenkin, and soon after the end of inflation particle production seeds the first inhomogeneities that lead to galaxy formation. The dilaton is trapped near the minimum of the potential where it oscillates like a massive field, and the excess of kinetic energy is dissipated via the mechanism of "gravitational cooling" first introduced by Seidel & Suen. The inhomogeneities therefore behave like solitonic oscillations around the minimum of the potential, known as "oscillatons", that we propose account for most DM in galaxies. Those regions where the dilaton does not transform enough kinetic energy into reheating or carry an excess of it from regions that have cooled, evolve to the tail of the potential as DE, driving the acceleration of the universe.
Here dark energy is caused by the famous dilaton, the scalar field that appears in superstring theory.
Wolram: the axion was postulated to solve the so-called strong CP problem, and is a serious candidate to Cold dark matter
METEOR.
i cant comment on this one the math is to much for me,
as for all these proposed particles i just dont know,
DM,DE is so unintuitive that i would prefer an alternative,
but unless i go out and find one and get it approved by
mainstream science i am stuck with it, what is your view?
from WIKIPEDIA.
--------------------------------------------------------------------------------
The axion is a hypothetical particle postulated by Peccei-Quinn theory to resolve the strong-CP problem in quantum chromodynamics (QCD). The naive first principles formulation of QCD without axions predicts that some strong interactions will violate CP-symmetry. This is never observed in practice, and the axion was postulated to be a particle (specifically a pseudo-Goldstone boson) associated with a new broken symmetry of nature, whose conservation is constructed to exclude all CP-violating terms from QCD.
Axions are predicted to have no charge, very low mass (10-6 - 10-2 eV/c2) and very low interaction cross-sections for strong and weak forces. Hence they are nearly invisible to ordinary matter, and cannot be excluded on the basis of current measurements, though they have never been observed.
The predictions of axion theories would lead to them being created abundantly during the big bang. Because of a unique coupling to the instaton field of the primordial universe (i.e. "misalignment mechanism"), an effective dynamical friction is created during the acquisition of mass following cosmic inflation, this robs all such primordial axions of their kinetic energy. Hence axion theories predict that the universe will be filled with a very cold Bose-Einstein condensate of primordial axions. Depending on their mass, axions could plausibly explain the dark matter problem of cosmology. Observational studies to detect dark matter axions are underway, but they are not yet sufficiently sensitive to probe the mass regimes where axions would be expected to be found if they are the solution to the dark matter problem. Such studies have excluded the possibility of high mass axions.
It should be noted that the existence of axions are also a necessary component of string theory.
So many theories, so little in the way of observational constraints!
Well, that's not entirely accurate - the observations are very extensive (for example, only last week SDSS (http://www.sdss.org/news/releases/20040315.datarelease2.html) announced the release of 6 terabytes of data to the public!), it's more that the observations don't seem (yet) to constrain the theories much. And the theories aren't investigated well enough (in general).
Personally, I rejoice in the observational advances, and am happy to wait for consensus to emerge over a period of five years or so.
Of course, if someone manages to catch a neutralino or three ...
[Edit: fixed spelling [:(] ]
Add this to the zoo of theories trying to explain dark energy:
"Phantom k-essence cosmologies"
http://arxiv.org/abs/astro-ph/0403157
Ah, and beware of the "phantom tachyon" [!:)]
http://arxiv.org/abs/astro-ph/0403480
"Approaching Lambda without fine-tuning"
This paper discuss a model of dark energy known as "Extended quintessence"
"Bose-Einstein condensation as dark energy and dark matter"
http://arxiv.org/abs/astro-ph/0403571
"We study a cosmological model in which the boson dark matter gradually condensates into dark energy. Negative pressure associated with the condensate yields the accelerated expansion of the Universe and the rapid collapse of the smallest scale fluctuations into many black holes, which become the seeds of the first galaxies. The cycle of gradual sedimentation and rapid collapse of condensate repeats many times and self-regularizes the ratio of dark energy and dark matter to be order one."
uhm, we live inside a Bose-Einstein condensate?
[8)] [:D]
its amazing the BEC has only been around for a few years,
now its linked to DE, i have lost count of how many theories
have been proposed ,but it makes no difference, they will
all have to stay on the shelf until evidence is found, i
think NEREID said 5YRS or so before we get some results in,
will it be the dawn of new science?
So, in the model known as "Phantom energy", the strength of the dark energy is not constant, (like in the case of a cosmological constant), but instead increases in time, leading to the scary Big Rip. Do the cosmological constant also leads to the Big Rip?
PS: By strength of dark energy I mean density. In the model of cosmological constant, the density of dark energy stays always constant. In the model of phantom energy, the density of dark energy grows over time
http://arxiv.org/abs/astro-ph/0403614
"Dark matter relic abundance and scalar-tensor dark energy"
This is a model of dark energy for scalar-tensor gravity theory
hellfire
Mar30-04, 02:00 AM
Do the cosmological constant also leads to the Big Rip?
Big-rip means a scale factor going to infinity in a finite cosmological time (AFAIK). This is not the case of an expansion dominated by the cosmological constant. Regards.
kurious
Mar30-04, 02:51 AM
Why dark energy and not dark particles?
Why dark energy and not dark particles?If I understand this question correctly, the answer is that 'dark energy' is a catchy shorthand for something which exerts negative pressure; 'dark matter' refers to something which has mass, but doesn't otherwise interact. Since we have no idea what the 'dark matter' is, we usually assume it to be made up of particles; as there are many theories which predict many different kinds of particles as yet unobserved, this is sometimes convenient because some properties of dark matter (if it were indeed made up of these predicted particles) can be worked out and possible tests described.
More fundamentally, you could say the difference in terminology reflects some core concepts in modern physics - fields and particles.
A couple of weeks ago (16 March)
... I need to find a suitable tutorial and an up-to-date review article that surveys how things stand at present and how the various investigations into dark energy are going.
... Sahni's review article
"Dark Matter and Dark Energy"
http://arxiv.org./astro-ph/0403324
You can tell its meant as a review article because it has 190 references in its bibliography...
Wolram established this thread as a collecting point for stuff about Dark Energy
and he has been consistently warning that the assumption of a cosmological constant may be wrong and either there is a better explanation of observed accelerating expansion or else the observational data may be questionable. Skepticism and constant probing of assumptions is part of what makes the enterprise a success, so it seems like a good thing to keep
bringing papers that challenge the prevailing view. (even tho I lean towards
accepting it)
Sahni's review article gives a good presentation of the prevailing view and what the supporting reasons are IIRC
Now Sahni comes out with an article challenging it!
http://arxiv.org./abs/astro-ph/0403687
"The case for dynamical dark energy revisited"
Ujjaini Alam, Varun Sahni and A. A. Starobinsky
the main practical distinction I see between peoples idea of dark matter and dark energy is that
dark matter clumps
it is gathered together by its own gravity just like ordinary visible material and the fact that there are clumps of dark matter is what holds clusters of galaxies together and the dark matter in and around Milky is what keeps our galaxy from flying apart, and so on
the thing about dark energy is that it doesnt. It either does not clump at all or else it only does so a very very little, much less than dark and ordinary matter
the easiest way to imagine an energy density in space that does not clump is to think of it as a constant energy associated with volume itself
like 0.6 joule per cubic km
a nice bonus you get is that a constant energy density automatically has a negative pressure
if a volume is expanding then the total amount of energy in that volume is growing with the volume----so expansion must involve work
if it takes work to pull the piston out some then there must be neg pressure inside the cylinder
so a constant energy density has to have a pressure-to-density ratio (written w) of -1
the pressure is -1 times the energy density
let us find out what the DE pressure around us is
well the density is 0.6 joule per E9 cubic meters
and that is 0.6 E-9 joule per cubic meter
and minus one times that is -0.6 E-9 newtons per square meter
Not that metric units are so great but anyway the pressure around
us is -0.6 nanopascal.
Science shows its death wish by calling this w, the ratio of pressure to energy-density, an "equation of state"
it is not an equation it is a number, usually assumed to be -1
But like someone who habitually rides too fast on their motorcycle
Science habitually calls simple things by confusing jargon as if it wished
to destroy its connection with the rest of humanity. We try to overlook this.
We call the ratio w of pressure to energy density by the name "equation of state"
SAHNI SAYS THE EQUATION OF STATE MAY BE GRADUALLY CHANGING he says that as long ago as z = 1 it may have been zero and now it is around -1 and that cutting some slack to the equation of state so it can change
gradually helps get a nice fit to the supernova data.
Hardnosed insistance that the equation of state be now and forever equal to minus one produces not so nice a fit. He says.
But to be sure about this kind of thing requires more and more observations
(the old story, they always say more data is needed, right? well?
it probably is)
pelastration
Mar31-04, 02:06 PM
if it takes work to pull the piston out some then there must be neg pressure inside the cylinder
... so a boundary?
by MARCUS.
SAHNI SAYS THE EQUATION OF STATE MAY BE GRADUALLY CHANGING he says that as long ago as z = 1 it may have been zero and now it is around -1 and that cutting some slack to the equation of state so it can change
gradually helps get a nice fit to the supernova data.
Hardnosed insistance that the equation of state be now and forever equal to minus one produces not so nice a fit. He says.
But to be sure about this kind of thing requires more and more observations
(the old story, they always say more data is needed, right? well?
it probably is)
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nice unbiased posts MARCUS, one can only wonder at mans creativity
and imagination, "one problem umpteen solutions", but "best fit", has
to be the deciding factor, the data that hopefully will be collected in the
next 5 YRs should reveal how well we are dressed.
http://arxiv.org/abs/astro-ph/0403292
Using the spectacular new high redshift supernova observations from the HST/GOODS program and previous supernova, CMB and galaxy clustering data, we make the most accurate measurements to date of the dark energy density rho_X as a function of cosmic time, constraining it in a rather model-independent way. We find that Einstein's vanilla scenario where rho_X(z) is constant remains consistent with these new tight constraints, and that a big crunch or big rip is more than 50 gigayears away for a broader class of models allowing such cataclysmic events.
Eric Linder
"Light Thoughts on Dark Energy"
6 page, a review of the different theories
by a prominent cosmologist
http://arxiv.org./astro-ph/0404032
could be a useful summary of the present state of puzzlement
concerning this
(personally I'm not a fan of his but I discount my own lukewarm
reaction. He is a leader in the
field and an author of a textbook on cosmology and
accelerated expansion is a specialty of his, he was involved
in its discovery. this lite not-too-technical survey by such an expert
as Linder ought to be quite useful and probably will be to some people)
maybe someone can clear up a possible misconception of mine,
if a body can radiate gravity where does this expended radiation
go? i have an idea that it is damped out, by spacetime, but then
spacetime would be gaining energy.
http://arxiv.org/abs/hep-th/0404030
"Extended holographic dark energy"
This paper is a variant of the model of holographic dark energy also presented in this thread
http://arxiv.org/abs/astro-ph/0404043
We consider Double Quintessence models for which the Dark Energy sector consists of two coupled scalar fields. We study in particular the possibility to have a transient acceleration in these models. In both Double Quintessence models studied here, it is shown that if acceleration occurs, it is necessarily transient. We consider also the possibility to have transient acceleration in two one-field models, the Albrecht-Skordis model and the pure exponential. We find for all four models that a transient acceleration that has already ended at the present time, is viable. We show that it is even possible for some of the models to have no acceleration at all, neither at the present time nor in the past, still in agreement with observations. The two last scenarios, namely a transient acceleration ended by today or no acceleration at all, can be implemented for the range of cosmological parameters $\Omega_{m,0} \gtrsim 0.35$ and $h \lesssim 0.68$.
Um, two scalars field driving acceleration? Why only not one? Economy...
it seems this model needs two Fields, its the interaction," kinematic
energy exchange ",that drives an axillary Field that powers expansion.
if i understand correctly.
there is another model which does not need any dark energy
(you may already have discussed it in this thread and I missed the post)
it is called the Cardassian model
in this model one can have just matter and radiation (no dark energy) and
the universe can still be flat and accelerating, as observed.
here is a recent paper using the Cardassian model to derive an estimate of the age of the universe (if it operates by Cardassian equation instead of Friedmann equation)
this paper has many references so one can look back at earlier papers about the model
I am not recommending this model but it is one way to explain things without dark energy, so it belongs in this thread
--------------
http://arxiv.org/astro-ph/0403196
Age of the Universe in the Cardassian Model
Christopher Savage, Noriyuki Sugiyama, Katherine Freese
13 pages
"The age of the universe is obtained in a subset of Cardassian models by using WMAP data. Cardassian expansion is a modification to the Friedmann equation that allows the universe to be flat, matter dominated, and accelerating, without a vacuum component. Since this model changes the evolution of the universe, we should not a priori expect the Cardassian age to be the same as the WMAP Friedmann derived result of 13.7 +/- 0.2 Gyrs. However, in the subset of Cardassian models we consider, we discover that the age of the universe varies from 13.4 - 13.8 Gyr over the range of parameter space we explore, a result close to that of the standard Lambda model. The Hubble constant h, which may also vary in these models, likewise varies little from the Friedmann result. "
thankyou MARCUS,
it is nice to have a non DE model, it shows, in a way, that not
everyone is happy with this strange energy, and are attempting
to do away with it, i think this paper does a good job fitting
parameters to observations.
http://arxiv.org/abs/astro-ph/0403012
Observational Evidence from Supernovae for a Contracting Universe
Authors: William Q. Sumner
Comments: LaTex, 11 pages, 4 figures
"New precision in measuring extragalactic distances using supernovae has confirmed with high probability an accelerating increase in redshift with distance. This has been interpreted as implying the existence of dark energy in an expanding and accelerating, flat universe. A more logical explanation of these observations follows directly from an observation made by Erwin Schrodinger in 1939 that in a closed Friedmann universe every quantum wave function changes with spacetime geometry. Double the size of the universe and both the wavelengths of photons and the sizes of atoms double. When the evolution of atoms and photons are combined, the meaning of Hubble redshift is reversed. Redshift is characteristic of contracting universes. The magnitude-redshift curve for a contracting universe has exactly the accelerating form recently observed and is in excellent quantitative agreement with the data of Riess et al. 1998, Knop et al. 2003, and others. An observed maximum redshift of 1.3 gives a minimum age estimate for the universe of 114 billion years. The time until collapse is estimated to be 15 billion years or less. "
Here, the author is suggesting that cosmological redshift has been misinterpreted, and that indeed means contraction of the universe. (crank?)
http://arxiv.org/abs/astro-ph/0404202
A new alternative model to dark energy
Authors: Yungui Gong (CUPT), Xi-Ming Chen, Chang-Kui Duan
Comments: 4 pages, 4 figures,latex
"The recent observations of type Ia supernovae strongly support that the universe is accelerating now and decelerated in the recent past. This may be the evidence of the breakdown of the standard Friedmann equation. Instead of a linear function of the matter density, we consider a general function of the matter density to modify the Freidmann equation. We propose a new model which explains the recent acceleration and the past deceleration. Furthermore, the new model also gives a decelerated universe in the future. The new model gives $\Omega_{m0}=0.46$ and $z_T=0.44$."
In this model of dark energy the universe will decelerate in the future
thanks METEOR.
this one is two pages plus graphs, and requires modification of
uptill now accepted constants.
I was reading an article recomended by marcus in the thread "Accelerated expansion and its rate". The article is called "Making sense of the new cosmology", by M.Turner.
It's rather interesting, and I've discovered 2 news candidates for dark energy. The first is called spintessence, is a spinnig scalar field, and my investigations have lead me to the paper where it was presented
http://arxiv.org/abs/astro-ph/0105318
The second candidate is a network of topological defects(you know, cosmic strings, domain walls, etc); it has lead me to this paper also of M.Turner
http://arxiv.org/abs/astro-ph/9811454
In "Making sense of the new cosmology", Turner highlightes the importance of the so-called Dark Energy equation of state. The DEEOS can cast light over the true nature of dark energy. The equation is:
w=P/d
P is the isotropic pressure of Dark energy, d the energy density
He then give the cipher of the equation of state for various candidates to dark energy.
If dark energy is vacuum energy, then w=1. If it's a network of topological defects, is -N/3. If it's a rolling scalar field w varies between 1 and -1
It's rather interesting, and I've discovered 2 news candidates for dark energy. The first is called spintessence, is a spinnig scalar field, and my investigations have lead me to the paper where it was presented
http://arxiv.org/abs/astro-ph/0105318
METEOR, the math in this paper is beyond me, could you explain the theory?
nickdanger
Apr25-04, 05:15 AM
Can you answer a newbee question? I work in quantum mechanics but with the new ideas in cosmology impacting particle physics, it's hard not to get lured into paying attention to some of the astrophysics.
I understand how a supernova and star production can take us back to a view of what was happening a few hundred thousand or hundred million years after the big bang. (Blanchard's comment is that light will never give us a better view than 400,000 years after the big bang.) But how does CMB give us a picture earlier than light data?
And finally, what I am truely interested in: How is it we believe that there are relatively unevolved (chemically) regions of space that give us a view of a few billionths of a second after the big bang? I understand if this were true it would be our particle physics laboratory in space, but how can anything still be giving us data about a high energy event from so long ago? Why do we think these regions haven't evolved?
What you're saying about dark energy was explained centuries ago and known by kabalists forbidden by Catholic Church. If you read Isaiah 45:7 says in Hebrew the word "hoshek" (darkness) being the REAL CREATION OF GOD ("create" in Hebrew is "barah") and different from "form". Hence, the text is saying darkness was not abscense of light but real creation while light was formed because of that pre-existing black fire. In the Jewish idea there was God's Big Crunch called "tsim tsum" and the "ruach elokim" and vacuum or vague darkness of Genesis 1:1,2 was the super black hole as inflationary event. Hindus believed in Big Bang-Big Crunch scenario related to Brahma's own body swelling or collapsing with universe which was part of himself. Eventually now astronomers and physicist do believe we have to replace the Big Crunch or Big Bang ending of the universe into the Big Rip ending like a whimper in an endless sea of space.
www.cox-internet.com/hermital/holopara1-4.htm
www.icr.org/pubs/btg-a/btg-174a.htm
Can you answer a newbee question? I work in quantum mechanics but with the new ideas in cosmology impacting particle physics, it's hard not to get lured into paying attention to some of the astrophysics.
I understand how a supernova and star production can take us back to a view of what was happening a few hundred thousand or hundred million years after the big bang. (Blanchard's comment is that light will never give us a better view than 400,000 years after the big bang.) But how does CMB give us a picture earlier than light data?
And finally, what I am truely interested in: How is it we believe that there are relatively unevolved (chemically) regions of space that give us a view of a few billionths of a second after the big bang? I understand if this were true it would be our particle physics laboratory in space, but how can anything still be giving us data about a high energy event from so long ago? Why do we think these regions haven't evolved?
Hello Nick, welcome
I just saw a post where you mentioned a talk at U. Penn. Dont remember seeing your posts earlier. It is really nice to have your perspective here since you are already working in a related field but just getting interested in cosmology (the newcomer's perspective is often the best and fits in well here at PF)
I will give you my (not necessarily authoritative) take on your questions and others may chime in too.
the CMB is light (stretched out 1100-fold)
and dates from 300,000 to 400,000 years (estimates vary)
when plasma cooled down and formed neutral (mostly H) atoms and
stopped scattering the light
so people talk about the "last scattering" era and the "surface of last scattering" which is where the CMB came from and which goes back to
year circa 300,000
there isnt any older light so in a sense "light cant tell us" about anything before that, like you say.
but there was a time in the first milli or micro second called "nucleosynthesis" when baryons stuck together----so there was H-nuclei and isotopes of some other light elements: He, Li.
Doubtless there are observable regions of space where the original mix still hasnt condensed very much into stars and therefore hasnt been fused into heavier nuclei. So what you say about "unevolved (chemically) regions of space" telling us about the first small fraction of a second. Doesnt Weinbergs book "First 3 Minutes" talk about this?----there must be a lot of good writng about how conditions during the instant of nucleosynthesis can be inferred by measuring abundances of elements and isotopes
You ask how we can look back.
Just to measure relative abundances we dont have to look back to before 300,000.
We just have to look back to, say 500,000 or one billion, a time when the first stars of the first galaxies were forming of of that original mix.
And then measure the abundances. And from that we can INFER back to
one microsecond and estimate the temperature etc, because that was when the relative abundances were established.
Someday there will be telescopes able to see primordial big bang neutrinos which subsequently cooled down so much that presentday instruments cant detect them. People say that this will allow us to see into the earlier conditions. If you are a graduate student or postdoc today then sometime in your career there maybe
primordial neutrino data, showing you a picture of the first microsecond. Maybe, I'm not too sure about this, or the actual time quantity.
But at present we dont get any light older than 300,000
and are still able to infer back earlier than that, from isotope abundances and suchlike.
I'm probably not the only one here who would be delighted if you would keep us posted on U.Penn particle-cosmology colloquia and stuff
nickdanger
Apr27-04, 03:49 PM
Marcus:
Thanks for the welcome. Yes, I am new the last few days. Still getting the hang of it...some of my posts get lost occassionally. In any case, I will pretty much stick to quantum physics, but I try to pay attention to the cosmology.
kurious
Apr28-04, 12:35 PM
an observation made by Erwin Schrodinger in 1939 that in a closed Friedmann universe every quantum wave function changes with spacetime geometry. Double the size of the universe and both the wavelengths of photons and the sizes of atoms double. When the evolution of atoms and photons are combined, the meaning of Hubble redshift is reversed. Redshift is characteristic of contracting universes.
What is the logic behind expanding photons and atoms giving a hubble redshift.?
Elastic dark energy:
http://arxiv.org/abs/astro-ph/0405096
Well, being this model correct or no, advice to you, dark energy: we will finally discover your nature, so your slippery behaviour will not be of utility for you
http://arxiv.org/abs/astro-ph/0312579
K-Essential Phantom Energy: Doomsday around the corner?
Authors: Pedro F. Gonzalez-Diaz (IMAFF, CSIC, Madrid)
Comments: 10 pages, Latex, 1 Figure, to appear in Physics Letters B
Report-no: IMAFF-RCA-03-10
Journal-ref: Phys.Lett. B586 (2004) 1-4
In spite of its rather weird properties which include violation of the dominant-energy condition, the requirement of superluminal sound speed and increasing vacuum-energy density, phantom energy has recently attracted a lot of scientific and popular interests. In this letter it is shown that in the framework of a general k-essence model, vacuum-phantom energy leads to a cosmological scenario having negative sound speed and a big-rip singularity, where the field potential also blows up, which might occur at an almost arbitrarily near time in the future that can still be comfortably accommodated within current observational constraints.
http://www.physics.ucla.edu/hep/dm04/talks/yunwang.pdf
a pencil survey of 100 type 1A supernova will help in solving dark
energy problem.
an april 2004 review of dark matter energy.
http://arxiv.org/abs/astro-ph/0403324
I briefly review our current understanding of dark matter and dark energy. The first part of this paper focusses on issues pertaining to dark matter including observational evidence for its existence, current constraints and the `abundance of substructure' and `cuspy core' issues which arise in CDM. I also briefly describe MOND. The second part of this review focusses on dark energy. In this part I discuss the significance of the cosmological constant problem which leads to a predicted value of the cosmological constant which is almost $10^{123}$ times larger than the observed value $\la/8\pi G \simeq 10^{-47}$GeV$^4$. Setting $\la$ to this small value ensures that the acceleration of the universe is a fairly recent phenomenon giving rise to the `cosmic coincidence' conundrum according to which we live during a special epoch when the density in matter and $\la$ are almost equal. Anthropic arguments are briefly discussed but more emphasis is placed upon dynamical dark energy models in which the equation of state is time dependent. These include Quintessence, Braneworld models, Chaplygin gas and Phantom energy. Model independent methods to determine the cosmic equation of state and the Statefinder diagnostic are also discussed. The Statefinder has the attractive property $\atridot/a H^3 = 1 $ for LCDM, which is helpful for differentiating between LCDM and rival dark energy models. The review ends with a brief discussion of the fate of the universe in dark energy models.
http://physicsweb.org/article/world/17/5/7
Dark energy
Feature: May 2004
New evidence has confirmed that the expansion of the universe is accelerating under the influence of a gravitationally repulsive form of energy that makes up two-thirds of the cosmos
It is an irony of nature that the most abundant form of energy in the universe is also the most mysterious. Since the breakthrough discovery that the cosmic expansion is accelerating, a consistent picture has emerged indicating that two-thirds of the cosmos is made of "dark energy" - some sort of gravitationally repulsive material. But is the evidence strong enough to justify exotic new laws of nature? Or could there be a simpler, astrophysical explanation for the results?
http://arxiv.org/abs/hep-th/0405054
"Dilatonic ghost condensate as dark energy"
http://arxiv.org/abs/gr-qc/0405038
SO(1,1) dark energy model and the universe transition
Authors: Yi-Huan Wei, Yu Tian
Comments: 11 pages
kurious
May11-04, 04:46 PM
WOLRAM
"Or could there be a simpler, astrophysical explanation for the results?"
What if some parts of the universe are contracting faster than others - wouldn't that give the illusion of accelerating expansion?
Hi kurious.
The data from supernova red shift is compelling evidence for expansion,
Adam Riess used the Hubble telescope to discover 42 new ones including
6 of the most distant known, if some parts of the U are contracting
i think that would show up in the data collected from these S Nova.
Red shift is not the only method used to demonstrate that the U is
expanding, a team lead by Prof G Efstathiou used clustering patterns
of 25000 galaxies and compared it to the CMBR, and found a good
match with S Nova data.
so everything points to an expanding U accelerated by Dark Energy,
a most unintuitive scenario and the only one on offer to date.
http://www.space.com/scienceastronomy/astronomy/universe_expansion_020320.html
Now, a team of 27 astronomers led by Professor George Efstathiou of the University of Cambridge has published strong evidence for the existence of dark energy using an entirely different technique. They used the clustering pattern of 250,000 galaxies in a large volume of the universe surveyed with the Anglo-Australian Telescope at Siding Spring in New South Wales, Australia. By comparing the structure in the universe now, some 15 billion years after the Big Bang, with structure observed in the cosmic microwave background radiation, which preserved information about what the universe was like when it was only 300,000 years old, the Anglo-Australian team could apply a simple geometrical test to elucidate the composition of the universe.
If the nature of the universe at the very first is for accelerated expansion (inflation), even before there were any kind of particles, then doesn't that imply that there is something inherent is space itself that accelerated expansion is a property of space? I can imagine that the creation of matter might slow this process down for a while. But I don't see why the expansion should continue to accelerate.
astro-ph/0305179] Inflation and the Cosmic Microwave Background.
Is continued acceleration "natural", i dont know, this paper
by Lineweaver goes some way in explaining inflation,
unfortunately these thing can not be tested in a laboratory
and can only be deduced from observations.
http://arxiv.org/abs/astro-ph/0405232
"Supernova / Acceleration Probe: A Satellite Experiment to Study the Nature of the Dark Energy"
SNAP: A space-based experiment to study the nature of the "Cosmic yeast"
kurious
May13-04, 05:34 AM
Can't dark energy just be electric charges of the same sign repelling one another -
even if the charge has to be on a massless wave of some sort.Repulsion of charges would stop a singularity from forming too if there were enough of them.
http://arxiv.org/abs/astro-ph/0405267
There are various models that assume that dark energy is a scalar field, for example the models of quintessence and k-essence, but this paper explores the possibility that dark energy could be a vector field. An interesting reading without a doubt
kurious
May15-04, 09:40 AM
There are various models that assume that dark energy is a scalar field, for example the models of quintessence and k-essence, but this paper explores the possibility that dark energy could be a vector field. An interesting reading without a doubt
The three vectors in this paper are all at right angles to one another to guarantee isotropy.Usually vectors in phenomena associated with energy, such as photons,
are at right angles when they represent different kinds of field.Is dark energy a collection of fields.
Hi kurious, I tend to prefer the idea that dark energy is due to a single field, but is because I always tend to simplicity, and I may be biased. In concrete, my favourite model of dark energy is that called quintessence.
Perhaps the most important quantity defining dark energy is the value of its equation of state. If dark energy is the cosmological constant, then the dark energy equation of state takes a fixed value always, this value is -1. However, if it's quintessence, the value of the equation-of-state varies over time. This is very important. But in general, the models of quintessence have a great difficulty, and is that they need that the initial conditions of the quintessence scalar field must have a very precisses values in order to obtain the energy density and the value of the equation of state at present time. In other words, they require fine tuning. This has been alleviate with the appearance of some models called "tracker quintessence", in these models the initial conditions are not important, a wide range of initial conditions converge to a common evolutionary track, hence the name tracker.
kurious
May16-04, 05:00 AM
The difficulties dark energy is causing theorists may mean that we do not currently understand what a scalar field is - the dark energy problem may only be solved if we modify our beliefs about more familiar scalar fields.
Hello!
In 2001, a person called Wetterich wrote this paper
http://arxiv.org/abs/hep-ph/0108266
"Cosmon dark matter?"
where he proposes that quintessence, the scalar field driving the acceleration of the universe, it's not uniformly distributed throughout the U, but its more concentrated at certain points, so these "quintessence lumps" can account for the dark matter of the universe. He dubs this kind of dark matter with the original name of Cosmon dark matter.
I want to present also another version of quintessence, gauge quintessence
http://xxx.arxiv.cornell.edu/abs/hep-ph/0302087
"Gauge quintessence"
kurious
May19-04, 07:18 AM
The gauge quintessence paper mentions that the flatness of a quintessence
field is difficult for physicists to model in a natural way.It makes you wonder if dark energy is a scalar phenomenon.I'm more inclined to think of it as a vector phenomenon that seems to be scalar.A vector has magnitude and direction.But if a vector was spinning quickly it would behave like a scalar. There would be a difference
though - the rotating vector should give rise to a small force in all directions.
http://arxiv.org/abs/astro-ph/0405353
"The Phantom Bounce: A New Oscillating Cosmology"
-----------------------------------------------------------
Perhaps is time to give a bit of attention to another model of dark energy, the Generalized Chaplygin gas. This model was proposed in this paper
http://arxiv.org/abs/gr-qc/0202064
"Generalized Chaplygin Gas, Accelerated Expansion and Dark Energy-Matter Unification"
Authors: M. C. Bento, O. Bertolami, A. A. Sen
and is a generalization of the Chaplygin gas model of dark energy, proposed some time before by other people
kurious
May20-04, 11:09 AM
"The phantom bounce" was very interesting reading. I didn't know that oscillating universe theories were constrained by Hawking's idea that black holes would grow
as large as the horizon and make the cosmological equations invalid.The paper
says that dark energy needs to tear the black holes apart before a bounce occurs for the equations to be useful.In a particular case the author says that the energy density of dark energy can be inversely proportional to the energy density of radiation.
This would be the case if the dark energy was absorbing energy from the radiation.
Is the cosmic microwave background redshifted because dark energy absorbs energy from photons? If Dark energy absorbs photon energy and undergoes a phase change it would be analagous to what water vapour is to ice and water -Give dark energy some input energy and it "evaporates" and yields a higher "vapour pressure."
If the photons and dark energy are exchanging quanta of energy this would mean
electromagnetic theory needs revising and also we would be able to introduce quantum mechanics into the friedman equations.
One of the arguments against tired light theories of redshift is that
there would be blurring because of the change in momentum of photons
interacting with other particles.But dark energy is unusual, perhaps there would be no blurring involved.And what's more explaining redshift with dark energy means the universe is still expanding and so the microwave background is the afterglow of a Big Bang.
kurious
May21-04, 12:46 PM
Does dark energy absorb energy from photons and cause them to redshift?
Someone told me that the redshift would probably be higher than is experimentally observed if this idea was right, but they didn't give details of why. Can someone tell me why the redshift would be higher? And if dark energy isn't uniformly distributed at every point in space - if it was distributed like a uniform volume of atoms- could that reduce the redshift back to the experimentally observed redshift?
Can dark energy consist of individual quantized particle pairs of
some kind?
Apparently quintessence models have difficulty producing the supposed
flatness of the dark energy distribution in a natural way.There is a
paper on the arxiv that models dark energy as phenomenon with three mutually orthogonal Vectors to guarantee isotropy.Is dark energy a
vector phenomenon?
If dark energy absorbs energy from photons and expands it would keep its density constant - in other words, can dark energy be space-time?
If dark energy absorbs energy from photons and expands it would keep its density constant - in other words, can dark energy be space-time?
-----------------------------------------------------------------------
photon energy reduces as space expands also photon density,
but i think photon numbers stay constant, i think your suggestion
is close to the tired light theory which has not found favor.
I'm not even sure if total photonic energy is sufficient to power
accelerated expansion, maybe METEOR has some ideas?
Wolram have you seen
www.cosmologystatement.org
it seems that a number of academic folk (some
emeritus) and technically trained brethren
have decided to rain on the dark energy parade
but I have not checked, just saw this
doesnt mean BB is wrong and they are right, just healthy skepticism
on their part, I suppose.
you tend to maintain a skeptical attitude, even while
getting into the details, so this should not seem all that different
I dont like their statement myself, but there it is
published 22 May in New Scientist
kurious
May21-04, 03:01 PM
photon energy reduces as space expands also photon density,
but i think photon numbers stay constant, i think your suggestion
is close to the tired light theory which has not found favor.
I'm not even sure if total photonic energy is sufficient to power
accelerated expansion, maybe METEOR has some ideas?
Tired light arguments don't apply.The dark energy might not have momentum to cause blurring of photons and time dilation would be fine too because the universe is still expanding but the redshift is caused by space-time (dark energy?) absorbing energy from photons.
If rest masses are also supplying energy to dark energy then they might make up the shortage.We could all be evaporating slowly!!
As for the cosmology statement: alternative theories to the big bang hypothesis should get funding.When any scientific investigation is done
it usually yields posititve or negative results that help scientists decide what the right answer is.
Thanks MARCUS, several learned people giving ms science an itch, i can
only say---------
Good old Halton Arp," im unsure about others," he keeps banging his drum, maybe some will start to like his music, i think he should not be dismissed
as he has some valid reasons for disputing m/stream science, if i were a scientist i would sign his paper, for no other reason than opening the debate.
KURIOUS seems i misunderstood you how could your theory
be tested?
Chronos
May22-04, 03:16 AM
the fluctuations are required by quantum physics. nothing, including consciousness, can remain fixed. universal expansion requires exansion on all levels, including the spectators
kurious
May22-04, 04:39 AM
how could your theory
be tested?
--------------------------------------------------------------------------------
Need to know accurately the acceleration rate of the expansion of the universe and the change in that rate.
Then the amount of mass/energy turning into dark energy could be deduced.
If a planet is losing mass/energy to space-time/dark energy then the radius of its orbit
would change slightly.Perhaps a gyroscope would spin more slowly or the magnetic moment of an electron which is known to 11 decimal places might change.But any measurement of the acceleration rate of the expansion of the universe has a high
degree of uncertainty in it.Maybe the way a photon redshifts in the Earth's atmosphere
could help out.
KURIOUS, i do like your theory, if anything it is more tangible
than some and seems to have a nice intuitive feel to it, maybe
someone will come along and poke holes in it, but i think it is
a theory worth pursuing.
This would be the case if the dark energy was absorbing energy from the radiation.
Is the cosmic microwave background redshifted because dark energy absorbs energy from photons?
This idea was first proposed by marcus in this thread
http://www.physicsforums.com/showthread.php?t=2220
It would be interesting to clear up some things.
According to the persons that defend that dark energy is a cosmological constant, it means that is due to fluctuations of the electromagnetic field. That is to the emergence of virtual photons that rapidly annihilate and disappear. I have never heard that dark energy can be due to fluctuations of other fields, like the gravitational field, the Dirac field, the gluon field or others. Why?
I can imagine like those virtual photons emerge in spacetime and each of them carry negative pressure, so they make spacetime to expand.
It's like you have a cake, and them, POP!, some virtual raisins appear in it, and the cake expands.
It makes me wonder about two different existent theories about the nature of spacetime: substantivalism and relationalism. In short, the first defends the physical existance of spacetime, but the second denies its physical existance.
Rovelli's book (you can find it for free in his web) talks about it in chapter 2. Rovelli is clearly a relationalist, he DENIES the physical existence of spacetime. It has me confused, I tend to be more of a substantivalist. He explains about the "hole argument", taht got confused Einstein about if the field equations should or not be generally covariant. The hole argument gives two alternatives:
a)the field equations cannot be generally covariant
b)there's no meaning to talk about some given physical spacetime point
At the end, the equations of Einstein were generally covariant. SO it must follow that spacetime don't exist. But, I , wonder, how can spacetime stretch the wavelength of the photons of the CMB as it expands then? how can then exist then gravitational waves, ripples of spacetime? how can then exist phenomena like frame dragging, the dragging of spacetime around a massive body as it rotates? I hope that someone can give some ideas
kurious
May22-04, 05:44 AM
METEOR
I have never heard that dark energy can be due to fluctuations of other fields, like the gravitational field, the Dirac field, the gluon field or others. Why?
Probably because the electromagnetic field is the one that's best understood.
People stick with what they know in detail.
how can spacetime stretch the wavelength of the photons of the CMB as it expands then? how can then exist then gravitational waves, ripples of spacetime? how can then exist phenomena like frame dragging, the dragging of spacetime around a massive body as it rotates? I hope that someone can give some ideas
The rubber sheet analogy people often use for spacetime may be more
realistic than is currently thought.What if spacetime is a substance - like the ghost condensate Nima Arkani Hamed is working on at Harvard
http://arxiv.org/PS_cache/hep-th/pdf/0312/0312099.pdf
Then the Earth rotating could be dragging something which physically sticks to it and distorting that something.Gravitational waves could be like sound waves moving through a medium.
If spacetime doesn't exist....
Relativity makes all the right predictions but spacetime is a counterintuitive
idea. People argue that quantum mechanics which makes counterintuitive real predictions has been misinterpreted by the Copenhagen school, perhaps relativity has been misinterpreted too - perhaps spacetime doesn't exist.
The struggle to find a quantum theory of gravity suggests something is wrong.
I should add that the expansion of the universe and increase in dark energy does not have to relate directly to the photon redshift. What if photons have more energy than we think they do and this energy gets released in a similar way that rest mass releases energy in an atom bomb.So if a photon redshifts by a certain number of kg (E= mc^2), perhaps that
number of kg is analagous to rest mass and can split into many units of light which have a more effective energy - a kind of phase change.Photons will be evaporating and turning into dark energy - so no need to invoke a dark energy that absorbs photon energy.It was mentioned on the previous thread Marcus started in 2003 that the amount of cmbr energy would only be 1/10 of what is required to account for dark energy.But what if E = 10 m c^2 and it takes a long time for the other 9 mc^2 to appear.It will be hard to poke holes in the theory because dark energy is unusual.The challenge is to try and reduce it to more familiar physics - if it exists in the real world then it must be reconciled with the real world.
I noticed on the thread started in 2003 that there was mention of moving the thread to theory development.But dark energy discussions have to be highly speculative because little is known about it and whatever it is, it
is likely to be unusual- unless someone comes up with a dramatically improved idea of what vacuum energy is.
This idea was first proposed by marcus in this thread
http://www.physicsforums.com/showthread.php?t=2220
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Well without knowing the source i find myself agreeing with
MARCUS i always knew he knew what he was talking about.
It's worth to mention that the paper about the phantom bounce says that in the model of phantom energy, the density of energy of dark energy becomes infinite in finite time. This seems totally wacky
This idea was first proposed by marcus in this thread
http://www.physicsforums.com/showthread.php?t=2220
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Well without knowing the source i find myself agreeing with
MARCUS...
heh heh
the M :surprise:RC :biggrin: S conjecture
well maybe
Marcus
Well in archeology two stones together is a wall, two stones together
with a shard of roman pottery is a roman wall.
by KURIOUS
It will be hard to poke holes in the theory because dark energy is unusual.The challenge is to try and reduce it to more familiar physics - if it exists in the real world then it must be reconciled with the real world.
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I second that, if dark energy is "the power" in the universe then
nothing can be fully understood without knowing what it is, it
maybe that dark energy is the normal universe stuff, and baryonic
things are the anomaly.
http://www.cnn.com/2004/TECH/space/05/18/dark.energy/
CNN) -- The Chandra Space Telescope has gathered further evidence the universe is expanding at an accelerating rate, scientists at NASA and Britain's Institute of Astronomy announced Tuesday. The finding sheds new light on a force known as "dark energy."
http://xxx.arxiv.cornell.edu/abs/astro-ph/0404601
We discuss the possibility that the existence of dark energy may be due to the presence of a spin zero field $\phi(x)$, either elementary or composite. In the presence of other matter field, the transformation $\phi(x)\to \phi(x) +$ constant can generate a negative pressure, like the cosmological constant. In this picture, our universe can be thought as a very large bag, similar to the much smaller MIT bag model for a single nucleon.
http://arxiv.org/abs/astro-ph/0405430
This paper discuss the possibility that dark energy is composed of decaying vacuum energy. Explicitly, vacuum energy that decays in cold dark matter. That's no great? You solve two problems in the same paper. You explain what's dark energy and explain where dark matter come from. But how can the density of vacuum energy stay constant if it's decaying? Something says me that this is not the correct model
kurious
May24-04, 02:28 PM
Chandra is saying that dark energy looks to be constant in density still.
The paper on a spin 0 field seems to suggest that mass can be attributed to this field but then the Higgs field is spin zero too so that would be logical.Stephen Hawking doesn't think that a spin zero Higgs field will be found
and apparently upset Peter Higgs a few years ago when he heard he'd said this!
If dark energy is made from decaying vacuum energy then is that why the vacuum energy is only about 10^-9 J/m^3 instead of 10^120?
Hawkings, what has he contributed?
http://xxx.arxiv.cornell.edu/abs/astro-ph/0404601
We discuss the possibility that the existence of dark energy may be due to the presence of a spin zero field $\phi(x)$, either elementary or composite. In the presence of other matter field, the transformation $\phi(x)\to \phi(x) +$ constant can generate a negative pressure, like the cosmological constant. In this picture, our universe can be thought as a very large bag, similar to the much smaller MIT bag model for a single nucleon.
This from three weeks ago:http://www.physicsforums.com/showthread.php?t=24176
Contains the Authors work?
http://arxiv.org/abs/astro-ph/0405596
"Acceleressence: Dark Energy from a Phase Transition at the Seesaw Scale"
In this model called acceleressence (seems some kind of gluing of the words acceleration and quintessence), the acceleration of the universe is related to a phase transition, and dark energy is identified with a scalar field called acceleron. I can't say much more given that, for example, I don't have a clue about what's the seesaw scale
http://arxiv.org/abs/astro-ph/0405596
"Acceleressence: Dark Energy from a Phase Transition at the Seesaw Scale"
In this model called acceleressence (seems some kind of gluing of the words acceleration and quintessence), the acceleration of the universe is related to a phase transition, and dark energy is identified with a scalar field called acceleron. I can't say much more given that, for example, I don't have a clue about what's the seesaw scale
Im going to give this a wide berth, but only because of the terminology used!
Just kidding, I believe the paper is refering to the 'post-phase' being Seesaw, a transitional moment where the Universe transfers from one Phase to another, for example from an Expansive mode to a Contracting Mode, the Contracting mode can be derived from the Expansion Mode just as a Deceleration evolves from an Acceleration by the factor of relationary Speed.
The interesting thing to be asked is if this deceleration is prior or post to a Bounce in some Expryotic modeling.
DARK ENERGY and the AHARANOV BOHM effect.
In the Aharanov -Bohm effect, a magnetic field isolated inside a solenoid,
changes the phase of an electron interference pattern.The magnetic field does not exert a force on the electrons - it has been shown that the electromagnetic
vector potential can account for the phase shift.But is it possible to explain the Aharanov-Bohm effect in another way?
I think the answer to this question is yes and can be given by the force the magnetic field of the solenoid exerts on dark energy particles passing through the solenoid and into the "isolated" magnetic field. If the dark energy particles carry an electric charge they will be deflected by the magnetic field into the path of electrons passing through the slits in the interference experiment and will exert a force on the electrons which will change their trajectories slightly and change the phase of the interference pattern.
Let’s assume the dark energy particles are moving at close to the speed of light – 10^8 m / s . The magnetic field strength of the solenoids used in experiments of this kind are typically abouts 1 Tesla,the superconducting solenoid is about 10^ - 2 metres wide, and so a dark energy particle moving at about 10^8 m / s would take about
10^ - 10 seconds to pass through the solenoid.
The force exerted by the magnetic field in the solenoid on the dark energy particles is:
Force = q v B
Force = q x 10 ^ 8 x 1
Force = 10 ^ 8 q
A maximum force of 10 ^ 8 q is transferred to an electron by the dark energy particle.
The acceleration of the electron is given by Force / mass of electron
This is 10 ^ 8 q / 10 ^ -31 m / s ^ 2 = 10 ^ 39 q metres / s ^ 2
But the force only acts for 10 ^ - 9 seconds ( the time the electron takes to cross the path of the solenoid and the region in which the dark energy particles are interacting with the magnetic field of the solenoid – assuming the electron has a speed close to 10 ^ 7 m / s ) so after this time the electron could have a maximum speed displacing it from its main direction of motion of
10^ 39 q x 10 ^ -9 m / s = 10 ^ 30 q m / s.
In the time the electron takes to cross the path of the solenoid and the region in which the dark energy particles are interacting with the magnetic field of the solenoid,
the electron will be displaced a maximum of 10 ^ 30 q x 10 ^ -9 = 10 ^ 21 q metres.
It is known from experiment that the interference pattern shows a phase shift of
10 ^ -6 metres.
Therefore 10 ^ 21 q = 10 ^ -6
But there is more than one dark energy particle so if N is the total number of dark energy particles acting on the electron over a distance of 10 ^ -2 metres then
10 ^ 21 q x N = 10 ^ -6
The minimum uncertainty in energy of a dark energy particle is given by the maximum time over which it could change its energy state ( assuming dark energy particles can change their energy state).This maximum time is given by the age of the universe which is 10 ^ 18 seconds.
using E x t = h bar
E x 10 ^ 18 = 10 ^ - 34
E = 10 ^ -52 joules
Since a dark energy particle must have at least this energy then this means
the minimum rest mass associated with a dark energy particle is ( by E = m c^ 2 )
10 ^ -69 kg.
If we assume that dark energy particles have the same charge / mass ratio as a proton
( they have approximately the same mass density per cubic metre – about 10 ^ -27 kg / m ^ 3) then they have an electric charge of 10 ^ -61 Coulombs.
Using 10 ^ 21 q x N = 10 ^ -6 and q = 10 ^ -61
we get N the number of dark energy particles in 10 ^ -6 m ^ 3
( the width of the solenoid cubed) is 10 ^ 34
So in one cubic metre there would be 10 ^ 34 x 10 ^ 6 = 10 ^ 40 dark energy particles.
Earlier on I said the mass of a dark energy particles was 10 ^ - 69 kg.
In one cubic metre this amounts to a mass of 10 ^ 40 x 10 ^ -69 =10 ^ - 29 kg.
About one hundreth of the mass of a proton.
Here's a curious paper proposing that black holes lose mass as they accrete phantom energy
http://lanl.arxiv.org/abs/gr-qc/0402089
New model of dark energy called Quintom, with dark energy equation of state greater than -1 in the past, and less than -1 today
http://lanl.arxiv.org/abs/astro-ph/0404224
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