Why do scientists believe the universe is infinite?

[Pavel]

I was reading the answers by some scientists (including Krauss) to the question "What do you believe but cannot prove" on edge.org, who say there are good reasons to believe the Universe is infinite. They don't mention what those reasons are, but I was wondering if somebody could comment on that. Besides the anthropic principle, which I'm not even sure is popular among scientists, I'm not aware of any other compelling reasons.

I also presume they're not talking about unbounded universe.

Thanks,

Pavel.

 

[JesseM]

One reason is that if the curvature of the space is zero or negative (and the evidence points to it being very close to zero), the only way to keep space from being infinite would be to choose a weird topology where the space would be like a cube or some other geometric shape where if you go through one face you reappear inside the shape at a corresponding point on another face, a bit like the video game "Asteroids" where if you disappear off one side of the screen you reappear on another. Here are some articles on what it would mean for the universe to have a weird topology like that:

http://plus.maths.org/issue10/features/topology/

http://web.archive.org/web/20070118...edu/home/web/olinto/courses/A18200/nbower.htm

And here are some articles on how physicists might look for evidence that our universe had such a topology by looking for repeating patterns in the cosmic microwave background radiation:

http://space.mit.edu/home/angelica/topology.html

http://www.etsu.edu/physics/etsuobs/starprty/120598bg/section7.htm

 

[TR345]

I find it hard to comprehend that the universe could be infinite, but I also find it hard to believe that the universe is finite. Both seem unattainable, yet one must be true. I think that an infinite universe is easier in a way because then you don't need to worry about what is on the other side.

The most compelling reason I have for an infanite universe is:
The inevitable singularities at time equals zero.

 

[Pavel]

One reason is that if the curvature of the space is zero or negative (and the evidence points to it being very close to zero), the only way to keep space from being infinite would be to choose a weird topology where the space would be like a cube or some other geometric shape where if you go through one face you reappear inside the shape at a corresponding point on another face, a bit like the video game "Asteroids" where if you disappear off one side of the screen you reappear on another. Here are some articles on what it would mean for the universe to have a weird topology like that:

http://plus.maths.org/issue10/features/topology/

http://web.archive.org/web/20070118...edu/home/web/olinto/courses/A18200/nbower.htm

And here are some articles on how physicists might look for evidence that our universe had such a topology by looking for repeating patterns in the cosmic microwave background radiation:

http://space.mit.edu/home/angelica/topology.html

http://www.etsu.edu/physics/etsuobs/starprty/120598bg/section7.htm

Jesse, I'm afraid you're talking about unbounded, but finite universe (like the surface of a sphere). They're talking about infinite universe. Alexander Vilenski, for example, believes:

There are good reasons to believe that the universe is infinite.

If so, it contains an infinite number of regions of the same size as our observable region (which is 80 billion light years across). It follows from quantum mechanics that the number of distinct histories that could occur in any of these finite regions in a finite time (since the big bang) is finite. By history I mean not just the history of the civilization, but everything that happens, down to the atomic level. The number of possible histories is fantastically large (it has been estimated as 10 to the power 10 to the power 150), but the important point is that it is finite.

Thus, we have an infinite number of regions like ours and only a finite number of histories that can play out in them. It follows that every possible history will occur in an infinite number of regions. In particular, there should be an infinite number of regions with histories identical to ours. So, if you are not satisfied with the result of the presidential elections, don't despair: you candidate has won on an infinite number of Earth's.


Krauss has some reasons too:

...There are likely to be a large, and possibly infinite number of other universes out there, some of which may be experiencing Big Bangs at the current moment, and some of which may have already collapsed inward into Big Crunches. From a philosophical perspective this may be satisfying to some, who find a universe with a definite beginning but no definite end dissatisfying. In this case, in the 'metaverse', or 'multiverse' things may seem much more uniform in time...

Hence my question in the OP.

Thanks,

Pavel

 

[JesseM]

Jesse, I'm afraid you're talking about unbounded, but finite universe (like the surface of a sphere). They're talking about infinite universe. Alexander Vilenski, for example, believes:

There are good reasons to believe that the universe is infinite.

You misunderstood my point. I was saying that one reason someone might believe the universe is infinite is that the only way that a universe with zero or negative curvature could be finite is for it to have a weird topology (or I suppose it'd be logically possible for it to be 'bounded', but that's an even weirder notion)--the simplest topology would make it infinite, and the idea of the universe having the shape of a cube or dodecahedron or other geometric object is a bit hard to swallow. That's why I said "the only way to keep space from being infinite would be to choose a weird topology".

 

[marcus]

... good reasons to believe the Universe is infinite. They don't mention what those reasons are, but I was wondering if somebody could comment on that. Besides the anthropic principle, which I'm not even sure is popular among scientists, I'm not aware of any other compelling reasons.
...

I am not aware of any scientific reason to believe the Universe is spatially infinite.

It is an ASSUMPTION which is convenient mathematically and which is bound up in some people's minds with certain inflation scenarios that depend on exotic matter (the "inflaton field").

The actual data is consistent with the universe being NEARLY spatially flat but with a slight positive curvature (mathematically, an Omega of around 1.01) leading to a boundaryless spatially finite picture.

The data is also consistent with infinite space and curvature that is either zero or miniscule (corresponding to an Omega of say 1.00001, such as might result due to random variation in certain inflation stories.)

My advice would be to disregard what is said for public consumption (whether by Krauss or by Vilenkin) and look at recent technical papers by working observational cosmologists. Ned Wright would be a good one to start with: he came out with a paper in January 2007 that has a "best fit" value of Omega.

Whether someone says spatial finite or spatial infinite depends on things like prejudice, convenience, and what model they are using. Strictly on scientific grounds one cannot say it is one or the other.

Ned Wright is just an observational cosmologist, he doesn't favor one or the other. On page 17 of the paper he says
"Using all the data together gives the plot shown in Figure 6. The best fit model is slightly closed with Omega_tot = 1.011..." If you want to check out the other options he discusses, here is the paper:
http://arxiv.org/abs/astro-ph/0701584
Constraints on Dark Energy from Supernovae, Gamma Ray Bursts, Acoustic Oscillations, Nucleosynthesis and Large Scale Structure and the Hubble constant
Edward L. Wright (UCLA)

 

[Pavel]

You misunderstood my point. I was saying that one reason someone might believe the universe is infinite is that the only way that a universe with zero or negative curvature could be finite is for it to have a weird topology (or I suppose it'd be logically possible for it to be 'bounded', but that's an even weirder notion)--the simplest topology would make it infinite, and the idea of the universe having the shape of a cube or dodecahedron or other geometric object is a bit hard to swallow. That's why I said "the only way to keep space from being infinite would be to choose a weird topology".

Ah, my apology. I did misunderstand your point. This would be more of a reductio ad absurdum "proof" for the infinite universe, rather than direct evidence. Thanks for the insight.

 

[Pavel]

My advice would be to disregard what is said for public consumption (whether by Krauss or by Vilenkin) and look at recent technical papers by working observational cosmologists. Ned Wright would be a good one to start with: he came out with a paper in January 2007 that has a "best fit" value of Omega.

I see. Well, you really answered my question to the level of detail I needed. Thank you for doing that.

Pavel.

 

[Sundance]

Hello All

The universe is infinite in time and space. The question is:

Is it ifinite with respect to matter?

We know that the space matter that we see are finite in size and are part of the known universe.

Observations in deep field of the size a rice seed shows thousands of galaxies. Now imagine how many rice seeds are up there.

If you apply it to the N degree than matter is to be found to infinity and beyond as Buzz light Year would say

 

[JesseM]

The universe is infinite in time and space.

There are plenty of mainstream models of the universe in which one or both of these is false--for example, if you model the Big Bang scenario using general relativity, the universe has a finite past (although quantum gravity might change this).

 

[Sundance]

Hello Jesse

People hide behind models

We have enough observations and info to allows to deduce some form of reality.

The problem we have is that the main stream controls the "current", the cash flow, the religious workings, schooling and political ways. Now if you think this is my opinion than think again.

Do your own research.

WE have issues:

The universe is infinite.

We have enough info to deduce star formation and evolution in the phases changes.

We have info on compacted star cores and degenerate matter.

We have info on the evolution and changes to the form of galaxies and their relationship to the centre Neucleon.

We know the properties of the planets within our solar system and the probable origin.

We have moved away from the standard models that have kept us locked up in a closed room for decades. By this I mean people looked up to the standard models without question because it was backed up by schools, church and politics. People who questioned the standard model were removed from their jobs or have their cash flow reduced.

and so on

Observations rule over ad hoc ideas.

 

[Ratzinger]

A question: if the curvature is positive, is projective space RP^3 possible?

 

[JesseM]

Hello Jesse

People hide behind models

We have enough observations and info to allows to deduce some form of reality.

OK, but there are abundant observations in support of the basic Big Bang model--the idea that all the matter around us started from an extremely hot dense state which expanded out 14 billion years ago (approximately). The question of whether the Big Bang represents the "beginning of time" or whether there was something before that is one that cannot currently be settled by observation. And there is also plenty of evidence that the universe is very close to spatially flat, but as marcus said, observations can't decide between the possibility that it has a slight positive curvature (which under the simplest topology would make it spatially finite) or whether the curvature is zero or negative (which under the simplest topology would make it spatially infinite).

 

[Sundance]

Hello Jesse

Hold your horses for 2 sec's

Do you understand the Big Bang?

Have you read the evidence for it?

Have you read the critics?

Have you observed some of the super massive cluster of clusters of clusters of galaxies: This is one object.

If you need info on the Big Bang just let me know.

Keep cool

 

[JesseM]

Do you understand the Big Bang?

I think I have a good layman's understanding, although I'm not a professional physicist.

Have you read the evidence for it?

A fair amount of it, yes.

Have you read the critics?

I'm not aware of any mainstream physicists who are "critics" of the Big Bang. And you're aware of this forum's policy on non-mainstream theories, right? If you want to discuss them you need to post in the "Independent Research" forum. If you want to understand why mainstream physicists consider the Big Bang pretty thoroughly established, this is the place. For example, are you familiar with the evidence from the pattern of galaxy redshifts, the evidence from the cosmic microwave background radiation, and the evidence from the Big Bang theory's predictions about nucleosynthesis and the ratios of different elements and isotopes? If not, this page would be a good place to start. There's also the fact that supercomputer assumptions based on the Big Bang model give excellent predictions about the types of structures that astrophysicists observe in the universe--see the Millennium simulation.

Have you observed some of the super massive cluster of clusters of clusters of galaxies: This is one object.

What are you referring to here? Can you link to a source (or provide a published reference) to whatever it is you're talking about?

 

[Sundance]

Hello All

Policy or no policy science is science and is not controlled by main stream or non main stream. When you have policies in place its like scence going mad.

Reading many posts I find people not understanding the theory behind the Big Bang.

I do not agree with the BBT, but! that does not make me right. I'm not emotionally attached to any theory.

Here are some links in support of the Big Bang, later I will post against. For now maybe discuss the supporting issues.

Tango at your speed.

Big Bang Nucleosynthesis
http://www.astro.ucla.edu/~wright/BBNS.html

A Glimpse of the Young Milky Way
http://www.eso.org/public/outreach/p.../pr-19-02.html

Evidence for the Big Bang
http://www.talkorigins.org/faqs/astr...html#firstlaw

Frequently Asked Questions in Cosmology
http://www.astro.ucla.edu/~wright/co...y_faq.html#XIN

History of the Big Bang Theory
http://astrophysics.suite101.com/art...ig_bang_theory

Chapter 10 Origin of the Elements
http://www.lbl.gov/abc/wallchart/tea...pdf/Chap10.pdf

Mysterious iron factory in the Early Universe
http://www.mpe-garching.mpg.de/Highl...r20020708.html

Phase Transitions in the Early Universe
http://www.damtp.cam.ac.uk/user/gr/public/cs_phase.html

THE BIG BANG:
http://www.umich.edu/~gs265/bigbang.htm

Foundations of Big Bang Cosmology
http://map.gsfc.nasa.gov/m_uni/uni_101bb2.html

If anybody has links that can support the Big Bang, please post them.

 

[JesseM]

Hello All

Policy or no policy science is science and is not controlled by main stream or non main stream. When you have policies in place its like scence going mad.

But the purpose of this forum is not to "do science" in the professional sense, the purpose is primarily educational, for people to learn about aspects of physics they don't understand. So, I think that's the reason for the policy, if half the posts in the relativity forum were saying relativity was wrong and aether theories correct, then this would be likely to confuse or distract anyone trying to learn about relativity. As I said, there is the Independent Research sub-forum for discussions of new non-mainstream ideas.