Exploring the X-ray Universe: XMM-Newton Studies Dark Energy

[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.
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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.
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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.
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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?

 

[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.
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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]

 

[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
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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"
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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.

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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)

 

[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)
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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.

 

[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.