Clarification of Arch's post

[Arch2008]

To avoid confusion agentchange, the value for omega determines the fate of the universe. An infinite universe in the contex of this value means that it expands forever. That doesn't mean that it is now infinite in size. A universe that starts as a finite space smaller than an atom and expands at a finite rate for 13.7 billion years is not going to suddenly be inifinite in size.

 

"Is the Universe really infinite or just really big?

We have observations that say that the radius of curvature of the Universe is bigger than 70 billion light years. But the observations allow for either a positive or negative curvature, and this range includes the flat Universe with infinite radius of curvature. The negatively curved space is also infinite in volume even though it is curved. So we know empirically that the volume of the Universe is more than 20 times bigger than volume of the observable Universe. Since we can only look at small piece of an object that has a large radius of curvature, it looks flat. The simplest mathematical model for computing the observed properties of the Universe is then flat Euclidean space. This model is infinite, but what we know about the Universe is that it is really big."

http://www.astro.ucla.edu/~wright/cosmology_faq.html#RB

(P.S., this is obviously a quote, not plagiarism.)

[Martin]

To avoid confusion agentchange, the value for omega determines the fate of the universe. An infinite universe in the contex of this value means that it expands forever. That doesn't mean that it is now infinite in size. A universe that starts as a finite space smaller than an atom and expands at a finite rate for 13.7 billion years is not going to suddenly be inifinite in size.

...

 

Hi Arch, there seemed to be some things here that needed clarification, so I split it off from the other thread.

 

I've been distinguishing between two cases, spatial infinite and spatial finite.

I cant remember anyone saying "infinite" universe meaning simply one that expands forever. Infinite means spatial infinite.

 

So it could be misleading for you to suggest that an infinite universe in this context means one that expands forever. It blurs an essential distinction.

 

In conventional cosmology one of the possible cases is a finite universe that expands forever. Indeed we just may be in that case!

 

that was one thing. If you disagree, I hope you explain.

====================

 

Another thing is you said " the value for omega determines the fate of the universe."

 

Perhaps you should be more explicit. Let's say Omega = 1.03,

now is the fate of the universe determined? If so, what is it?

 

I remember back 10 years or more ago people used to say that a lot. Back say around 1990 they would say " the value for omega determines the fate of the universe."

 

But I haven't heard that much recently, have you? It could be misleading. Could confuse people. Because as I understand it the actual situation is slightly more complicated than that suggests.

 

Perhaps I don't understand you. Would you like to explain what you meant?

In any case, better discuss this outside where it won't distract the other thread.

=======================

 

EDIT TO REPLY TO ARCH's next post:

look at the passage you yourself quoted from Ned Wright. He is not saying that we know U is spatially finite. He is not saying that we know U is spatially infinite.

I've read a number of his papers including quite recent, and even cited them here. He would never say the U is spatial finite. And he certainly doesnt say that in what you quoted! This is something to get really clear about, Arch. Read it again carefully.

Professor Wright is saying that as of April 7, 2008, the universe is not spatially infinite, just really big.

 

No, he says we don't know whether it is finite or infinite, but we do know that in either case it is really big.

 

As I pointed out, a finite universe expanding at a finite rate for a finite amount of time cannot suddenly become infinite.

 

Of course, that is never been the issue. BTW that is why the volume of the big bang state must be assumed infinite, in treating that case.

Edited October 21, 2008 by Martin

[Arch2008]

(Martin, we have to stop meeting this way )

 

Well the cosmologist who did that FAQ I linked to says otherwise.

© 1996-2008 Edward L. Wright. Last modified 07 April 2008

http://www.astro.ucla.edu/~wright/intro.html

(I believe he is "the Ned Wright" of Wright's calculator and the WMAP5 paper you linked to in the other post, too)

 

Professor Wright is saying that as of April 7, 2008, i.e., after the WMAP5 paper, the universe is not spatially infinite, just really big. Our models of the universe may be infinite, but they are only models. As I pointed out, a finite universe expanding at a finite rate for a finite amount of time cannot suddenly become infinite. People have forgotten that over the last 10 years and that is why so many people get confused when they hear “infinite universe”. An open universe expands forever, i.e. is spatially infinite at an infinite point in the future. That’s what Professor Wright seems to have heard recently.

 

Martin, what part of-

"This model is infinite, but what we know about the Universe is that it is really big."

is unclear to you?

Your models are infinite, the universe is not. A finite universe that expands at a finite rate for a finite time does not make an infinite universe. The product of three finites doesn't equal an infinite, that's basic middle school math.

I'll e-mail Professor Wright and ask him what he meant.

Edited October 21, 2008 by Arch2008

[npts2020]

Arch2008: On what basis are you saying time is finite?

[Arch2008]

The WMAP5 reports that the universe is 13.72 billion years old (give or take), which is considerably less than infinite.

[npts2020]

So you are claiming either no time exists before the beginning of our universe or that time was created with our universe? On what are you basing this?

[Arch2008]

We know that time existed at the point of the Big Bang. We do not have evidence of anything before the BB, yet. So that is what I base that on. Besides, if there was time before the BB, it still would not have an effect on the expansion time of the universe since the BB.

[Martin]

... Besides, if there was time before the BB, it still would not have an effect on the expansion time of the universe since the BB.

 

This is something I think you get absolutely right! I agree with you on this.

What astronomers call, by convention, the "age of the universe" is really the expansion time, or as, it's sometimes called, the expansion age of the universe.

 

Essentially what they do is run the classical (vintage-1922) Friedmann model back in time until the density blows up---and get an estimate for how long expansion has been happening: namely 13.7 billion years.

 

This doesn't say anything about how long some prior process may have been happening. It is simply the age of expansion.

 

There are newer models which match the classical on this side of the bang, but differ right around the bang and then continue onwards back in time from there.

 

Notice I am not claiming the new (nonsingular) bounce models are correct nor am I saying the old model (that blows up) is correct. What I am saying is there is no scientific basis for believing that time stops at that point.

 

You have equally good models and one blows up and one doesn't. They will differ slightly as to their predictions about future CMB observations and we eventually be able to prefer one or the other, but so far there is not enough evidence to decide which is right.

 

Now I want to remember what Carl Sagan said:

Extraordinary claims require extraordinary evidence.

http://en.wikiquote.org/wiki/Carl_Sagan

 

It would be quite extraordinary for time to stop. Total failure of the kind of causality we are used to. So anybody who tries to claim that time stops at the bang, and doesnt exist before the bang (as you seem to be doing) should be required to provide plenty of scientific evidence.

 

I am not claiming anything. I don't say this model is right or that model is right. I take a skeptical position on this. I don't have to provide evidence.

 

But you, if you are claiming time doesnt extend back before bang, should be required to back that up. And, with respect, I don't think you have, or that anybody (scentist or nonscientist) has.

[Arch2008]

Exactly. I'm open to evidence that time existed before the BB. It just doesn't change the fact that it hasn't been an infinite amount of time since the BB.

[Martin]

Exactly. I'm open to evidence that time existed before the BB. It just doesn't change the fact that it hasn't been an infinite amount of time since the BB.

 

Good. I agree. (Probably 13.7 billion years would be an acceptable estimate to both of us and, I'm guessing, most people here. I'm not sure that was ever in question, was it?)

[npts2020]

Thanks for the clarifications. I really have no concrete opinion about time before the big bang even though I don't particularly like the idea of infinity applying to the real world. However, IMO if you obviate time before then, some sort of explanation of how time comes into existence is desirable. Is time finite in one direction only, both directions or not at all?

[Martin]

...I really have no concrete opinion about time before the big bang even though I don't particularly like the idea of infinity applying to the real world. However, IMO if you obviate time before then, some sort of explanation of how time comes into existence is desirable...

 

Heh heh, "desirable" is putting it mildly, n-Points! The idea of a total breakdown of causality and time-evolution abruptly stopping (as you work back) or starting (as you go forwards) is to say the least extraordinary. And it is not forced on us---it's simply the classic vintage 1915 model model breaking down, where some others don't.

 

I said somewhat the same thing in my post. And I don't think you need to worry about infinities in the case of time. It's not like when a theoretical model blows up and predicts infinite energy or density or curvature at some location and point in time. That is just a failure-to-compute, a breakdown of the model---the polite term for it is singularity. The model fails to compute something sensible at some location and point in time. And that makes people suspicious of the model. That has happened in physics several times in the past and they've cured the singularities by improving or replacing the model.

 

I don't think we need to worry about time being infinite as if it were a pathological localized infinite density or energy----caused by a blow-up or glitch. In the case of time the non-stopping is simple continuity. Normal behavior.

 

In any case the existence of models in which time does not stop, whether looking forward or backwards, should not be of general concern. It's what we ordinarily expect!

 

Here's how I put it---I think this agrees in large part with what you said:

 

 

...I am not claiming the new (nonsingular) bounce models are correct nor am I saying the old model (that blows up) is correct. What I am saying is there is no scientific basis for believing that time stops at that point.

 

You have equally good models and one blows up and one doesn't. They will differ slightly as to their predictions about future CMB observations and we eventually be able to prefer one or the other, but so far there is not enough evidence to decide which is right.

 

Now I want to remember what Carl Sagan said:

Extraordinary claims require extraordinary evidence.

http://en.wikiquote.org/wiki/Carl_Sagan

 

It would be quite extraordinary for time to stop. ...anybody who tries to claim that time stops at the bang, and doesnt exist before the bang (as you seem to be doing) should be required to provide plenty of scientific evidence.

...

[npts2020]

Martin: The clarifications are nice and help toward having a more exact understanding of what is being talked about. Cosmology is, imo, a very interesting subject that really goes to the heart of what science is all about, describing the world around yourself from observation and experiment. Many things are universally well defined and understood from this method but more than a few are not. Who has ever seen a universe created to know if there was time before it? We are getting close to being able to see the creation of our own but I am skeptical as to how definitive the findings from things like LHC and W-MAP are going to be. The exciting part of these projects to me is that they will almost certainly rule out some if not all but one of the scenarios. But isn't that the fun of cosmology, wild speculation, within the limits of applying what has been observed and is known to be true?