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

[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)

 

[pelastration]

if it takes work to pull the piston out some then there must be neg pressure inside the cylinder

... so a boundary?

 

[wolram]

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)
--------------------------------------------------------------------------
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.

 

[wolram]

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 giga years away for a broader class of models allowing such cataclysmic events.

 

[marcus]

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)

 

[wolram]

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.

 

[meteor]

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

 

[meteor]

double quintessence

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...

 

[wolram]

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.

 

[marcus]

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. "

 

[wolram]

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.

 

[meteor]

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?)

 

[meteor]

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

 

[wolram]

thanks METEOR.
this one is two pages plus graphs, and requires modification of
uptill now accepted constants.

 

[meteor]

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

 

[wolram]

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]

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 truly 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?

 

[oscar]

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[/URL]
[url]www.icr.org/pubs/btg-a/btg-174a.htm[/url]

 

[marcus]

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 truly 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 isn't any older light so in a sense "light can't 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 don't 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 can't 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 don't 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]

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]

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.?