# Understanding the accelerating universe.

1. Dec 17, 2013

### Damo ET

For the record, I completely accept the concept of an accelerating universe and the role dark energy plays (as the guys who figured these ideas out are light years beyond what I can hope to wrap my brain around). So, I am hoping someone can explain to me in layman terms why this the case.
Is it a case of an accelerating universe at this moment in time at all points in the universe?
or
Is it a case of an expanding universe in the past which we assume is still accelerating today based on what we see?

As we look at more and more distant galaxies back toward CMB, we are seeing greater and greater red shift due to the expansion of the 'space' between the photons being emitted, and our sensors. That's all well and good, but that was billions (upto 12 billion+) years ago when the universe was far younger and smaller. Even if the expansion of the universe was slowing, the further we look back, the faster it would appear to receding!

One way which I could figure we could assume that expansion is accelerating would be to know the redshift to a galaxy, and actually see an increase in that redshift over time. But this is obviously not the way it is done.

Or is there a halo in all directions at a certain distance/time where there is a change in the characteristics of what we see, which indicates an accelerating universe?

I can't help but think I am missing something very fundamental. Any help would be appreciated.

Damo

2. Dec 17, 2013

### Naty1

Hi Damo: The 'fundamentals' of cosmology take a while to put together....experts can disagree on a number of things.

Very distant points in the universe ARE moving away from each other, distances are increasing. The universe expanded really fast for a very short period of time shortly after the big bang [this is called inflation] then slowed down before picking up acceleration several billions years ago. The role of dark energy in all this remains a matter of discussion and debate....We have been in an 'energy dominated universe' for several billion years and observations confirm expansion is speeding up.

Here are two well regarded papers which are insightful:

Davis & Lineweaver:
http://arxiv.org/abs/astro-ph/0310808

[There is a simplified and abbreviated version of the above paper in Scientific American, "Misconceptions about the Big Bang"....I have a copy but not a valid link. ]
edit: Took me a while but I found it..Linewaver has all his works listed on a Berkely U website...START HERE and maybe pHinds 'Balloon Anlogy' in a post below
http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf

Tamara Davis thesis:

and some experimental data:

What have we learned from observational cosmology ?
http://arxiv.org/pdf/1304.4446v1.pdf

No one is sure about 'the role dark energy plays'....

Why all these prejudices against a constant?
Eugenio Bianchi, Carlo Rovelli
(Dated: April 13, 2010)
but nevertheless it still seems a possibility.

Marcus of these forums explains expansion this way:

Last edited: Dec 17, 2013
3. Dec 17, 2013

### marcus

Naty I think that's a really helpful short list of references! The wide-audience essay "what have we learned from observational cosmology" by Jean-Christophe Hamilton was new to me. I think it's exceptionally clear and complete for non-technical writing. Just now saw the link http://arxiv.org/pdf/1304.4446v1.pdf in your post, clicked on it, and read the section on what we learn from the Cosmic Microwave Background (e.g. wavelengths of acoustic oscillation in the hot gas).
He really knows how to talk to an intelligent lay audience!
I instantly became a J-C H fan and looked him up, here is his webpage at the University of Paris7:
http://www.apc.univ-paris7.fr/~hamilton/JCHweb/Bienvenue.html

Although no way an expert (just a fascinated watcher of the progress in cosmology) I was proud to be quoted along with your links to the pros.

4. Dec 17, 2013

### julcab12

The simplest way to understand expansion(theories) for us laymen is to set misconceptions in line with academic materials, put restrictions on 'words' and focus on the general interpretation/idea itself. It is safe to say that expansion doesn't denote to negative pressure like the usual pressure in a balloon or standard tension of stretched string or rubber band, pulling in rather than pushing out. Which denote the universe accelerates by larger negative pressure than the positive. In some sense, It is fair and true but not quite right. It simply replaces one question /why does dark energy cause acceleration?/ dark energy has negative pressure, and gravity is sourced by a sum of energy and pressure and so on. It will led to dead end rather than any actual understanding.

Imagine DE expansion as a feature of space. The amount of dark energy is constant throughout both space and time using the simplest way Einstien formulation of gravity of which Einstien tensor is proportional to the amount of energy-momentum tensor(curvature of spacetime is proportional to the amount of stuff within it)of which manifestation of spacetime curvature is simply the fact that space is expanding that makes the density of dark energy constant, which means the curvature of spacetime is constant.

The universe expands at a fixed rate or accelerating (not speed) in timescales since galaxies is not a constant number, it’s proportional to their distance making the expansion rate of the universe constant. Meaning it takes a fixed amount of time for the universe to double in size.

Last edited: Dec 17, 2013
5. Dec 17, 2013

### Naty1

hey Damo,

if you have time also check out this; I think you will find it very helpful, especially if you have already encountered THE BALOON ANALOGY

www.phinds.com/balloonanalogy

which helps explain cosmological expansion, ‘stretching’, of space....with input from a bunch of Physicsforums members.

6. Dec 17, 2013

### Naty1

Marcus:
Good, that may keep the 'censors' here off my back!!!...last time I posted helpful references like that, I got demerits for 'too advanced a reply'....or somesuch frivilous admonition.

Regarding expansion and acceleration, I found the 'Misconceptions about the Big Bang'
Scientific American article..... and edited my earlier post to include it. That set off 'ah ha moments' for me when I first read it; Much more approachable than the ARXIV article for beginners. It's so good I'll repeat the link here:

http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf

Being a pro has its advantages, I suppose, but regardless of how you view yourself, you have a talent for explaining things to us dummies.....which we appreciate.

7. Dec 18, 2013

### Damo ET

Thanks heaps guys, it may take me a little while to read through the info you have all referred. Cheers.

Damo

8. Dec 18, 2013

### Naty1

Damo....you posted:

I would say neither of those is quite accurate. [If I were really smart, I would give you short accurate replies, but I am not!] What you can do is test what you think you meant by those statements against the sources of information linked above. In other words, see if you can refine those statements....for example, get a better understanding of 'acceleration' in cosmology. [That took me a while.]

There ARE several 'halos' of sorts, actually appearing as surrounding spheres with us at the center.....one is the Hubble sphere, another which is referred to as the 'surface of last scattering' ....I saved this description on the latter, seems like maybe it was from Marcus also: [Hubble sphere is easy to understand, from say, Wikipedia, to get started. ]

This was emitted about 380,000 years after the Big Bang....early in the universe but did not take place instantaneously.
[for perspective via an illustration, see here: http://en.wikipedia.org/wiki/Expansion_of_the_universe#What_space_is_the_universe_expanding_into.3F]

9. Dec 21, 2013

### bahamagreen

I have been reading Weedman's "Quasar Astronomy" (1986), wherein, writing about the "time scale problem" (age of universe based on globular clusters vs Hubble constant) with respect to Friedman (relativistic) cosmologies...

"An astute reader will perceive that the age discrepancy would disappear, even if Ho>90, if there were a way of accelerating the universe, so that t could exceed T. Within a newtonian cosmology, this is impossible because there is no long range repulsive force to accomplish this acceleration. Nor does such a force exist in relativistic cosmologies, but these can accommodate space whose expansion accelerates. A term in the relativistic equations - the "cosmological constant" - is available for this purpose, although utilization of it violates existing laws of physics. So this term is conventionally taken to be zero, and that convention is likely to stand until there is no longer room for escape by reducing Ho, globular cluster ages, or both in adequate combination to avoid the need for an accelerating universe."

10. Dec 21, 2013

### Chronos

11. Dec 21, 2013

### bahamagreen

Maybe it is just my books that are old... I did have a brief correspondence with Saul Perlmutter a number of years ago about vacuum fluctuations and missing mass. He went on to accelerate the universal expansion, I'm still enjoying reading old books.

12. Dec 23, 2013

### RUTA

This is the way DE is introduced by Unzicker and Jones in "Bankrupting Physics" (Chap 3, A Speedy Revolution: Why Cosmology is Going the Wrong Way). The way I came to understand DE was simply through distance modulus versus redshift. I have my GR students make plots of the Union2 supernova data and create fits using Einstein-deSitter and LCDM. That's an easy way to see the effect of Lambda.

13. Dec 23, 2013

### Naty1

Hi RUTA,

If this is the "Einstein-deSitter' description to which you refer,

1. The Einstein-DeSitter Model
http://ned.ipac.caltech.edu/level5/Watson/Watson2_4_1.html

what's the motivation for plotting data against it...

I assume UNUION2 supernova data uses type1A as standard candles, etc, showing acceleration of more distant points??

In other words, if we have been in an energy dominated era for several billion years,
why use a matter dominated model.....I'm trying to understand the perspective your students might have before plots..........

I never studied this in a logical sequence, so I don't know the progression typical GR students might follow,,,,just curious....

14. Dec 24, 2013

### RUTA

The website you have for EdS cosmology is correct in saying it's the matter-dominated, spatially flat FRW model. They do say incorrectly that it's a Minkowski (zero curvature) spacetime. The spatial surfaces of homogeneity are flat, but the spacetime has non-zero curvature.

Anyway, the reason we use it to do a best fit through mu vs z of the Union2 Compilation data is that the concordance model (LCDM) is just EdS plus Lambda. So, it's easy to see the effect of Lambda when you compare the best fit LCDM with the best fit EdS.

15. Dec 24, 2013

### Chronos

The current 'best' alternative to the LCDM model is probably the Dirac Milne universe, which dispenses with the need for dark matter and dark energy. It requires antimatter with antigravity properties which places it in tension with observational evidence. The case for anti gravity is still under review. Some recent relevant discussions include http://arxiv.org/abs/1110.3054, Introducing the Dirac-Milne universe; http://newscenter.lbl.gov/news-releases/2013/04/30/antimatter-up-down/, Does Antimatter Fall Up or Down?, http://www.nature.com/ncomms/journal/v4/n4/full/ncomms2787.html#additional-information, Description and first application of a new technique to measure the gravitational mass of antihydrogen; http://arxiv.org/abs/1308.0878, Measuring Antimatter Gravity with Muonium;

16. Jan 18, 2014

### ConformalGrpOp

Hey Naty1, maybe we can audit RUTA's course!!! Ok, well, maybe not!!

Thanks Naty1 for the references cited early in this thread. After reading the Hamilton and Lineweaver references, I came away with a much clearer understanding of the foundational role H0 and the phenomena of flat galactic rotation curves play in the structure of the $\Lambda$CDM model. And, the quoted post by Marcus has to go down as an absolute classic! I can say I have never seen the problem of geometry in GR articulated with such clarity and explanatory power.

After making my way through Hamilton, I couldnt help thinking that some day way in the future, someone is going to read this paper and shake their heads and say, "Really?? This is what they thought about the way the Universe is (was, would become)?

I am curious about how the FLRW model would look if we took H0 = 0, and eliminated dark matter altogether, and then just played with $\Lambda$ a bit to see how much explanatory power the model would have. (I have been considering the problem in the context of de Sitter's universe because of the unique role that H0 has in that model.)

In order for this approach to be coherent, it is necessary to assume that the established value of H0 relates the magnitude of velocity independent frequency shifts as a function of distance as light propagates from remote sources. It also requires the the observed flat rotation curves of distant galactic systems are an optical effect such that the more proximate an observer is to the system, the more it appears that the system complies with predicted keplerian motion.

At the very least, it might serve as a thought provoking exam question in a graduate level course on GR!

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