Is the universe still expanding?

Simetra7

Yes, please do....I'm all ears (eyes).

Chronos

Evidence favoring Big Bang theory: The cosmological microwave background radiation, primordial elemental abundance, WMAP power spectrum. large scale structure of the universe... Big Bang theory does not require a finite, or infinite, universe.

johninf

If scientists are so blissfully happy, why are they so paranoid
when the big bang THEORY is critisized?
In reality they are afraid of infinity because it cannot be described mathematically.
The present math is inaccurate and innappropriate to infinity.
They are also afraid of the idea of no beggining!

johninf

"The true size of the universe is probably much larger than the visible universe. The geometry of the universe suggests that it may have an infinite size and that it will expand forever. Even if the universe is not infinite, our visible universe must be a minute speck in a much larger totality."
NASA
If they concede that the univerese may be infinite NOW, then its a simple step in deduction to say that it always was.and always will be.

johninf

"The cosmological microwave background radiation,"
is not supporting evidence of the big bang.
Background radiation is conatantly caused by an infinite number of supernova.
Its the ambient temperature of the universe.

marcus

"We" is a fine choice---meaning mainstream working astronomers and cosmologists. Let's include ourselves, as long as we are trying to understand the mainstream consensus of experts. I regret having sounded persnickety at that point.

Did you have a look at the Sci Am article---Lineweaver and Davis? If it was not right for you let us know and I or someone can look for other stuff about the current picture in cosmology.

right now in this thread we are getting some trouble because of a very common misconception----a lot of people don't understand the big bang set of models and think that any big bang picture has to be spatially finite. that just isnt so. From the very start----going back I think to the NINETEEN TWENTIES----people were proposing and studying expanding universe models which were spatially infinite and which started off with a singular hypersurface of infinite spatial extent

the singularity in the model has always been considered possible to be either infinite in extent or finite.

Nick (Tiger) who is a gradstudent going on to be a professional can correct me if I am wrong but this is pretty standard.

the popular misconception is that expansion has to start from a point, which it doesnt, mathematically an infinite thing can start expanding----so this is a "FAQ" type of thing we encounter over and over again at messageboards like PF. I think it comes from several reasons

1. the famous physicist John Wheeler CALLED it onset of expansion by the NAME of "big bang". He loves colorful terminology and is very imaginative. I think he also invented the term "black hole".
After a good name crops up it will often take off on its own and develop its own signif. in the public imagination.

2. people find it hard to imagine a "bang" that happens in an infinite region, they picture some comicbook explosion all emanating from one point. It is a verbal thing where the original mathematical idea gets lost and a verbal preconception dominates.

3. another verbal trouble comes from the word "singularity". In mathematics a singularity can occur at an infinite set of points. It means a PLACE WHERE THE MODEL BREAKS DOWN and fails to compute, often because it starts giving infinite or otherwise unreasonable answers, or no answers.

the classical 1915 Einstein model fails to compute right at the beginning of expansion, so that is called a singularity. It also fails to compute right around the center of a black hole.

but singularity does not mean "single point". Nobody who knows what they are talking about ever said that universe expansion has to begin at a single point, or even at a finite set of points. IT COULD but that hasnt been settled yet. It could also very well begin at an infinite set.

even a black hole singularity does not have to be a single point, in some models it is a ring (I find this hard to picture but mention it only because it is just a common truism that singularties do not in general consist of single points. It is only the WORD that gives people that idea.)

instead of meaning "single point" a singularity is an "oddity" or "peculiarity" that happens when you push a model to the limit of where it applies and it breaks down.

in some proposed new cosmologies (e.g. Loop Quantum Cosmology) the big bang singularity is smoothed out and spacetime evolution extends back to an earlier contracting phase. Eventually something like this will probably become mainstream----some improved theory which does NOT fail at the beginning of expansion will check out and be accepted by cosmologists and will replace the classical 1915 Einstein based cosmology.
then the singularity (or failure) in the classical picture will be FIXED. this has happened in other situations---improved theories (like of the atom) eliminate singularities in older classical theories.

================
so don't be misled by the words "bang" and "singularity" into thinking that the mathematical model necessarily begins at a point or has to be spatially finite.

this is actually one of the most interesting questions in big bang cosmology!

there is a number Omega, which can be MEASURED, and for which the measured value is around 1.01 or 1.005 plus/minus something.

and the uncertainty is such that it could easily be exactly 1.000, in which case the universe might actually be of infinite spatial extent! and on a very large scale UNCURVED.

but if this number is slightly more than 1, say it is 1.005, then the universe can be very huge (much bigger than what has been observed so far) but still finite.

and we really dont know, this is one of the big questions in big bang cosmology, and why they sent up satellite observatories like COBE and WMAP----to measure Omega more precisely, to shrink down the errorbounds.

a lot of mainstream cosmologists seem to think Omega is exactly one (infinite bigbang, infinite universe) I guess just because the measurement gives an answer that is so close--maybe they think "it is so close to one, why shouldnt it be exactly one?" But in all honesty the issue is still undecided and there is still an errorbar and more accurate measurements need to be made.

Hope this is helpful, and not too long winded.

Simetra7

Yes I read the article and looked at the other links too. It shed some light on what is definitely a hard concept to get your head around. Any other stuff you could dig up would be much appreciated. Thanks.

marcus

this is normally SpaceTiger's beat. or Chronos. keep asking specific questions and see if you can get some more suggested reading. I am going to retire to my usual haunts and not take up Astronomy room for now.

turbo

For tutorials on cosmological concepts in the most widely-accepted views on cosmology and astrophysics, you can try Ned Wright at UCLA:

http://www.astro.ucla.edu/~wright/cosmolog.htm

He props up alternate theories and concepts like straw-men and knocks them down with concepts from the "concordance view" of physics, sometimes more convincingly than others. It is a good place to visit to see how concepts in physics are developed and tested and how experiments and observations can place constraints on theory.

Mike2

The universe cannot be infinite in extent. That would violate causality. For if the universe were infinite in extent, then you could never trace the cause for its existence, and that would deny any reasonable explanation for its existence.

turbo

Thats a pretty silly assertion, Mike2. If you want to say "god made the universe', I would expect you to show not only how god made the universe, but also to explain how god came to be... This is not a trivial exercise...it is key. You may reject the concept of infinity, but if you do, you must define the qualities that constrain its finiteness.

Sherlock

Wrt what you were responding to, I'm assuming that
what johninf was talking about has been exhaustively
checked out and found to not as closely approximate
the observational record as the big bang - expansion
model. I'm assuming it because I'm also assuming
that I don't have the time to learn how to do the
comparison myself. :-)

Anyway, I've got a question. Somewhere in my
brief readings and musings about things cosmological
I've gotten the idea that the rate of expansion should be
connected somehow to the speed of light in a
vacuum.

Is there anything to this in the standard
cosmological model(s)?

Chronos

Expansion / inflation does not require a finite, or infinite universe. It also has nothing to do with the finite speed of light. The finite speed of light only constrains how much of the universe is visible to us at any given time after the Big Bang. And, as an added bonus, you can become greatly confused just thinking about that! [SpaceTiger has patiently tried, and I much appreciate his efforts to elevate my understanding on that point.]

Of course, that assumes there was a Big Bang. But you don't need a BB event to arrive at the same conclusion. Most cosmologists favor the BB model because it's the one that agrees with the preponderance of evidence.

Mike2

How did "God" enter the discussion?

The quantity that constrain its finiteness is cause and effect, premise and conclusion. If the universe is or has always been infinite in extent, then the only way this can happen is some sort of instant creation of an infinite emount of structure with no development whatsoever. When it all just instantaneously IS, then no form of cause and effect can possibly be traced. And so it defied any logical explanation or reasonable development. It is YOU who is supposing some sort of magic at work.

SpaceTiger

Mike, I think it's pretty presumptuous to assume that our traditional notions of cause and effect apply to the beginning of time. Even if they did, I don't see any reason why the "cause" should have to exist in our spacetime. If either of those assumptions were wrong, so would be your conclusions.

It's really a waste of time to debate about it, though. We have about as much proof and understanding of an initial singularity as we do of God. Let's focus on those things that can conceivably be observed in the near future.

SpaceTiger

I don't like this view of things. There is no "territory" on PF -- anyone who feels qualified to answer a question in any forum should do so. Besides, I'm a busy guy, so I won't be able to answer every question in as much detail as I would like. I've already cut back on my responses in other forums and, as the semester gets busier, I will likely have to cut back on my responses here as well.

Anyzok, I have no objections to what you said, nor do I have much to add. Johninf's criticisms of cosmology seem agenda-driven (religious perhaps?), so I suspect he has low standards for what constitutes an "understanding" of something.

marcus

likely maybe----I know the pressure of grad school
but I nevertheless hope that you will not need to cut back.
It would diminish the interest of this forum considerably.