What Does XMM-Newton Reveal About Dark Energy in the X-ray Universe?

[wolram]

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.

 

[wolram]

. 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.
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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.

 

[wolram]

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?
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according to first paper on this thread, 5 billion YRS ago.

 

[wolram]

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]

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]

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.

 

[wolram]

HELLFIRE.

may i refer you to NED Wrights website 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.

 

[wolram]

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

 

[marcus]

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

 

[wolram]

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]

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.

 

[Nereid]

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 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 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').

 

[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 doesn't 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

 

[marcus]

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 = 300M_sol/L_sol. 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/M_lum = 20, where M_lum 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 isn't matter, it is DE
but there is some matter
however most of the matter isn't 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

 

[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

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

 

[wolram]

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.

 

[wolram]

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.

 

[marcus]

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.

 

[Nereid]

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 doesn't 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?

 

[Nereid]

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?

 

[marcus]

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

 

[wolram]

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".

 

[wolram]

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.

 

[Nereid]

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 into the lab in the morning and finding a nice healthy green Wimpzilla in your SQUID.

 

[wolram]

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?
---------------------------------------------------------------------
it seems to me that cold DM is the most favoured candidate
for the missing mass problem, is this correct?

 

[Nereid]

wolfram: it seems to me that cold DM is the most favoured candidate for the missing mass problem, is this correct?

Yes.

 

[meteor]

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?

 

[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.
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the bottom up theory of evolution has problems ,but they seem
to be mechanical, and not insurmountable.

 

[meteor]

"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

 

[Nereid]

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.

 

[Nereid]

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.[/color]"

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.[/color]"

[Edit: fixed formats]

 

[wolram]

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.

 

[meteor]

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
https://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...

 

[wolram]

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.

 

[wolram]

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 can't climb an imaginary hill.

 

[meteor]

Another curious theory
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

 

[wolram]

METEOR.
i can't comment on this one the math is to much for me,
as for all these proposed particles i just don't 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?

 

[wolram]

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.

 

[Nereid]

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 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 ]

 

[meteor]

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"

 

[meteor]

Extended quintessence

http://arxiv.org/abs/astro-ph/0403480
"Approaching Lambda without fine-tuning"

This paper discuss a model of dark energy known as "Extended quintessence"

 

[meteor]

"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?

 

[wolram]

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?

 

[meteor]

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

 

[meteor]

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]

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]

Why dark energy and not dark particles?

 

[Nereid]

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.

 

[marcus]

a changing dark energy, revisited (Varun Sahni)

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

 

[marcus]

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)