Where, in the Universe, are we?

[marcus]

A perfectly flat universe is spatially infinite...

Could we be getting somewhere?

Be careful. Chronos is stating a personal opinion. Not part of professional cosmology and not suppoted mathematically. So you would be starting your train of reasoning on shaky ground.

You said the one thing you didn't understand was how a flat, spatially finite, boundaryless U could exist. Do you want to try to understand that? Or is it more attractive to you to discuss your train of thought leading to the "eternal universe" conclusion?

It is up to you what we discuss.

 

[Endervhar]

Thanks marcus, warning noted.

Yes, I do want to try to understand how a flat, spatially finite, boundaryless Universe could exist. I am quite willing to accept that my train of thought might contain errors, and I would certainly rather not arrive at a conclusion based on these. Been there, done that!

 

[marcus]

EDIT: Whoah! I didn't see your post #37 while I was typing this. It looks like my post #38 here is completely unnecessary, and perhaps a bit grouchy. I will not delete it, but will try to quickly respond to what you just said in a followup to this.

Perhaps I could ask for comments on my line of thought from here.

If the Universe is flat, it is spatially infinite...
..., it must have existed before the BB.

My main comment would be that although the starting premise is unfounded and the logic is not perfect, the CONCLUSION is accepted by lots of professional cosmologists as a real possibility.

The idea has become increasingly popular with the pros in the past few years, and it dominates the research literature in a subfield of cosmology called quantum cosmology.

So your CONCLUSION ("existed before BB") is not novel or surprising. You are in good company---hundreds of experts.

They come to that conclusion by starting with the classic mainstream equation model of the U based on Einstein's GR equation (the geometric way gravity works) and quantizing the equation model of the U. Then they find that there is no "singularity" at the start of expansion. Typically, or in many cases studied, solving the equation model shows a simple bounce----a contracting collapsing U reaching a high enough density so that quantum effects dominate, gravity repels, and there is a violent rebound. Some work suggests that this bounce was favorable to inflation and may have triggered it, or ensured that the inflation episode was sufficient to make a U matching observations. Ways to test observationally, testing bounce and inflation ideas together, have been proposed. It is an active research field these days!

At this point most cosmologists would be surprised if it turned out that the U did NOT exist before BB. (This is discussed in the Einstein-online link in my sig, public outreach from a branch of the Max Planck Institute. The discussion might interest you.)

At this point you are kind of at a fork in the road. You can try to understand why so many cosmologists are working on equation models which don't develop a singularity and which yield results where the U goes back before bang, i.e. before start of expansion.

AFAICS their reasons are unrelated to your chain of reasoning and do not depend on the concept "eternity" at this point in the game.

Or, alternatively, you can ignore what the mainstream research community is doing and ask us to comment on your personal chain of reasoning.

The question is, which are you more interested in doing? I have no preference---I'm good with either but not with both at the same time, there should be some clear focus.

 

[marcus]

Responding to your post #37, which I didn't see earlier:

OK, it looks like you want to go the route of getting acquainted with this current research initiative into "before bang" models.

There is that public outreach website from the German national research establishment MPI. It has an essay called "A Tale of Two Big Bangs" which gives some of the reasoning.

The MPI (Max Planck Institute) is very big, old, reputable and mainstream. Nothing in science is sure and eventually all theories are seen to be flawed and must be improved on. But MPI is at least not some crackpot outfit. You might try their essay A Tale of Two Big Bangs.

There was a sea-change around 2005. I would ignore outreach writings from before 2006. The MPI stuff "Einstein-online" is at least recent. So you could try it.

I'm going to propose that even if you are a complete tenderfoot (slang for newcomer) you take a look at the actual current research in quantum cosmology (QC) and just be sure not to get bogged down or scared by technical detail you don't understand.

Here is the professional research literature in quantum cosmology after 2006 ordered by citation count---that means the most cited papers (usually the more important, more highly valued by other researchers) are listed first.

If you want a brief summary of any paper, just click "Abstract". If you want a complete PDF of the article you can usually get that by clicking "PDF" on the abstract summary page. Often the first and last paragraphs of a research paper are written in ordinary English. Well, sometimes they are. But the main thing is to scan the TITLES and authors to get an idea what TOPICS and who is doing research that gets cited by other research.

If you have eyeballed the listing of the, say, top 50 papers in QC, post 2005 or post 2006, then it helps me talk to you because then if I say "Abhay Ashtekar" to you it will not be some strange out of the blue incomprehensible signal. The name will ring a bell.

Somewhere on that list (not yet up near the top) there should be papers by a model-testing expert named Aurelien Barrau. He looks for ways to shoot down models by comparing with observational data, like cosmic microwave background maps which are getting constantly more precise. They bear traces of what happened in early U. He is not a theorist with a stake in anyone theory or model. He is what is called a phenomenologist (their specialty is putting models on trial).

So let's look. Spires is the Stanford database for research papers. We will use the keyword "quantum cosmology" and do a search of the professional literature post 2006.

http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=dk+quantum+cosmology+and+date%3E2006&FORMAT=WWW&SEQUENCE=citecount%28d%29

Can you get this on your computer? Sometimes it takes a couple of minutes. If the Stanford search engine is busy. May also time out, but just try again.

I also want to look at the same search but post-2007. To see where the newer papers are going. There may be some trend. Or we could look post-2008

 

[marcus]

Yes! This post-2008 search gives a much clearer idea of where the field is really going.

http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+DK+QUANTUM+COSMOLOGY+AND+DATE%3E2008&FORMAT=www&SEQUENCE=citecount%28d%29

Do have a look! You can see the testing people---the phenomenologists---recent entry into the field. They are rising in the cite-count lists as the importance of testing, and its technical feasibility, is increasingly recognized.

On this list you see a phenom paper by Julien Grain down around #27 and right after it #28 a paper by Barrau (co-authored with Grain and Mielczarek).

Now I see also that #6 and #13 are phenom papers as well. They are deriving from the bounce cosmo model feature that they can look for in the CMB (microwave background temp and polarization maps)

Don't worry about the technical details. The primary interest is how observational testing is coming to the forefront.

Also in post-2008 QC you see a lot of "spinfoam" cosmology papers in the top-cited 25 or 50. That is new. If you look at post-2006 you don't see them because older papers (which have had more time to accumulate cites) dominate the list.

The very recent stuff has not had time to build up a substantial number of cites, so we don't see it. But we see the trend.

There is already quite a contrast between the post-2006 and the post-2008 list.

When I went back, the Stanford Spires was slow, so I am going to try the German mirror site "DESY search"
http://www-library.desy.de/spires/hep/
That worked better:
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=find+dk+quantum+cosmology+and+date+%3E+2008&FORMAT=WWW&SEQUENCE=citecount%28d%29

 

[Endervhar]

Marcus, thanks for the mass of info. I certainly wouldn't have considered your post #38 as grouchy. I post on a couple of other fora (forums?) and compared with some of the responses on those, anything I have seen on PF would be pleasant and immersed in smilies.

In spite of my being retired, time tends to be in short supply, as is my technological expertise, so wading through the links will probably take a while. My possible absence should not be taken as a sign of loss of interest.

 

[marcus]

...

In spite of my being retired, time tends to be in short supply, as is my technological expertise, so wading through the links will probably take a while. My possible absence should not be taken as a sign of loss of interest.

Since time is short, I will offer my take and spare you the trouble of sifting thru the evidence. I find that when you search the QC (quantum cosmo.) field a large portion of the models have a bounce.

I see that people quantize classic Einstein-based cosmo equations and they either solve the resulting system or they run computer simulations, i.e. solve numerically. Time and time again they get a bounce. It is what is called a robust prediction---doesn't depend on fine tuning or special cases. Bounce means that time doesn't end as you go back, dynamical evolution continues on back in time before start of expansion, no singularity.

If you look at the top 50. The fifty QC papers most cited, it is mostly LQG-based quantum cosmology. Loop quantum gravity. Its predominance is a fairly new feature of the situation.
You might expect Hawking papers, or stringtheory cosmology papers, instead of LQG.
But no, I think in the second search I linked, the most cited Hawking paper was only #50. Fortynine other papers were more cited.

If you want to check, you can look down the list of the top 50. Not all LQG papers are labeled as such, but you will often see the word "Loop" in the title, and you will see a lot of papers by Ashtekar, and by people who you see by looking at other papers have coauthored with Ashtekar. His former students, former postdocs, colleagues at PennState, etc.

Loop is the leading type of quantum cosmo that produces a bounce.

Then if you look at the most recent the post-2008 you see a lot of occurrences of the term "spin foam" in the title. Spinfoam models applied to cosmology are a new LQG development. You will see Ashtekar and his team getting into spinfoam cosmology, but also people from a different group, Rovelli's team at Marseille.

And, as I mentioned before, there is the sudden appearance of the phenom'ist people, the professional theory testers.

Basically there has been a big change in quantum cosmology. Much of it quite recent.
A lot of this involves a robust theoretical prediction which runs the model smoothly back into the past before the bang. And which so far agrees with observation as well as the classical model agrees, which is pretty well all things considered.

===============
Now you don't have to work on it. You can take my interpretation and summary on faith, and if you want you can check up. Look at the research publication data for yourself. But only if you want to. I think I've given an objective and fair report, but you should be able to check anything you feel like checking.

 

[Endervhar]

Thanks for the summary. So far I've found some nice simple stuff on MPI, and some nasty complicated stuff on Spires. Currently looking for something in between.

 

[Chronos]

Be careful. Chronos is stating a personal opinion. Not part of professional cosmology and not suppoted mathematically. So you would be starting your train of reasoning on shaky ground.

You said the one thing you didn't understand was how a flat, spatially finite, boundaryless U could exist. Do you want to try to understand that? Or is it more attractive to you to discuss your train of thought leading to the "eternal universe" conclusion?

It is up to you what we discuss.

I'm flattered, but, unworthy. I assumed this was common knowledge and failed to cite a credible source - like NASA:
http://wmap.gsfc.nasa.gov/universe/bb_concepts.html
"... Given the assumption that the matter in the universe is homogeneous and isotropic (The Cosmological Principle) it can be shown that the corresponding distortion of space-time (due to the gravitational effects of this matter) can only have one of three forms ... It can be "positively" curved like the surface of a ball and finite in extent; it can be "negatively" curved like a saddle and infinite in extent; or it can be "flat" and infinite in extent ..."

 

[marcus]

Outreach site, contains a technical error. "Universe 101" does not have footnotes to authoritative sources and is not one itself. Popularization like that often glosses over finepoints and may be re-written largely by staff or dumbed-down in the editing process.
Again, general warning, be careful with info off the internet.

 

[Chronos]

Acknowledged, marcus. In cosmology, the truth is rarely as simple as it appears in popular accounts. Another source:

arXiv:0802.2236v1
The Shape and Topology of the Universe
Authors: Jean-Pierre Luminet

page 11
“In most studies, the spatial topology is assumed to be that of the corresponding simply connected space: the hypersphere, Euclidean space or the 3D-hyperboloid, the first being finite and the other two infinite.”

Footnote: a "hypersphere" represents a closed [omega > 1] universe, "Euclideans space" is flat [omega = 1], and "3D hyperboloid" [omega < 1] represents an open universe. See excerpt from p13.

p 12
“From an astronomical point of view, it is necessary to distinguish between the “observable universe", which is the interior of a sphere centered on the observer and whose radius is that of the cosmological horizon (roughly the
radius of the last scattering surface), and the physical space. There are only three logical possiblities. First, the physical space is infinite - like for instance the simply connected Euclidean space. In this case, the observable universe is an infinitesimal patch of the full universe and, although it has long been the preferred model of many cosmologists, this is not a testable hypothesis.”

p 13
“The “concordance model" of cosmology describes the Universe as a flat infinite space in eternal expansion, accelerated under the effect of a repulsive dark energy. The data collected by the NASA satellite WMAP [11] have produced a high resolution map of the CMB which showed the seeds of galaxies and galaxy clusters and allowed to check the validity of the dynamic part of the expansion model. However, combined with other astronomical data [12], they suggest a value of the density parameter 0mega = 1:02 +/-0:02 at the 1 sigma level level. The result is marginally compatible with strictly flat space sections. Improved measurements could indeed lower the value of omega closer to the critical value 1, or even below to the hyperbolic case. Presently however, taken at their
face value, WMAP data favor a positively curved space, necessarily of finite volume since all spherical spaceforms possesses this property.”

As the author notes in this paper, a flat universe is not necessarily spatially infinite. There are any number of possible bounded [finite] 3D manifolds with flat topologies - cylinder, torus, poincare dodecahedron, etc. Signs of these other possible manifolds have been searched for, but, have not been found. The simply connected, spatially infinite model is the simplest explanation, and the one preferred by most cosmololgists.

 

[Xander756]

I've always wondered about this question myself. I don't mean to sound like an *** but there has been a lot of posts in this thread since the question was asked and without having to go back and read every single post, could someone give me a brief rundown of the discussion/answer thus far?

I'd also like to ask my own question on top of this (hoping it hasn't already been asked) but couldn't the universe be more than 13.7 billion years old and we just can't see out past that because the light hasn't reached us yet? So the observable universe goes out to 13.7 billion years but it could actually be much larger and older?

 

[Chronos]

The universe could be infinitely old, just not the part we can observe. That part is 'only' 13.7 billions years old. Light from more distant regions can never reach us. We are forever trapped by the CMB boundary. In fact, the light 'currently' emitted by many objects we presently observe will never reach us. They will not just go 'poof' and vanish, merely redshift into obscurity. Our observable universe is temporally, not spatially bounded.