Exploring the X-ray Universe: XMM-Newton Studies Dark 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.?

 

[meteor]

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

 

[meteor]

k-essential phantom energy

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.

 

[wolram]

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.

 

[wolram]

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.

 

[wolram]

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?

 

[meteor]

http://arxiv.org/abs/hep-th/0405054
"Dilatonic ghost condensate as dark energy"

 

[meteor]

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]

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?

 

[wolram]

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.

 

[wolram]

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.

 

[Mike2]

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.

 

[wolram]

astro-ph/0305179] Inflation and the Cosmic Microwave Background.

Is continued acceleration "natural", i don't 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.

 

[meteor]

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]

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.

 

[meteor]

vector dark energy

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]

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.

 

[meteor]

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]

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.

 

[meteor]

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]

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.