# Observable Universe and Beyond

1. Aug 7, 2008

### Diffy

I was just reading a bit about our universe and the observable universe which stirred up a few questions. I am mostly a student of mathematics, so I apologies if I am assuming things that are incorrect or if I am not using proper terminology. Maybe some of the quotes from my readings are inaccurate, please feel free to correct.

1) "If the size of the observable earth were a quarter, the entire universe would be the size of earth". How do we know that there is a finite size to the universe? I can see how one would have to subscribe to the finite notion if they accepted the big bang theory. But I see no reason other than that why the universe can't be infinitely large.

2) The cosmic light horizon - is our only limitation on how far we can see into the universe the age of the universe itself? In other words, since light from galaxies beyond our universe hasn't had enough time to get to earth then we can't possibly see it. If yes though, at the time of the big bang, there was a moment when all galaxies and planets and everything was condensed in a singularity. At this time the distance between what is now earth and every other object in space was 0. After the big explosion, unless objects traveled faster than the speed of light to get away from us (which I assume is impossible by the theory of relativity) then we should be able to view light from any object in space. (I don't think I did a very good job at explaining that one)...

3) "Since space is roughly flat..." Can someone justify this claim? How do we know this?

4) If you accept that our Universe only extends a finite length, then is it possible that there are other universes "next to" ours? I am not talking about parallel universes, I am more using the term universe to define an enormous collection of galaxies. Are there any viable theories like this out there?

Thanks for your help! I am quite interested in this stuff, but could never find (or fit into my schedule) a class in university about it.

2. Aug 7, 2008

### Chronos

The universe is observationally finite - about 13.7 billion light years in every direction from earth. For all practical purposes, that makes it spatially finite.

The space between 'objects' is hypothesized to have expanded much faster than light immediately after the big bang. Astrophysicists refer to this as inflation.

Universes 'external' to our own remain speculation. No means to affirm their existence has been devised and validated.

3. Aug 7, 2008

### offtheleft

so... it just ends?

4. Aug 8, 2008

### marcus

many if not most professional cosmologists would agree with you on that. nearly everybody uses the LCDM model which can either be spatially infinite or spatially finite, depending on the value of a certain parameter. the tendency is to assume the spatially infinite case (in some sense mathematically simpler)

Last edited: Aug 8, 2008
5. Aug 8, 2008

### marcus

Please read the March 2005 SciAm article by Lineweaver called Misconceptions about the Big Bang. It was not an explosion, at least not in the standard cosmology picture. Distances did increase at a rate faster than c. The model conforms to General Relativity. The special relativity "speed limit" does not apply here. Lineweaver's article is clear and nontechnical. Many people at PF have recommended it or had ti recommended to them. I have a link to it in my signature.
Flat is jargon, or science slang. Basically what it means in this context is that if you double the radius of a sphere the area will go up by a factor of 4 and the enclosed volume by a factor of 8.

In a spatially nonflat universe, if you took a very large sphere and doubled the radius the volume might increase by LESS than you expect, by less than a factor of 8.

we have lots of kinds of evidence for what you say. One kind is simply count the galaxies within a given distance----then count how many within twice that distance and see if it is 8 times as many. since galaxies are roughly uniform distributed, counting them gives an estimate of volume.

This should be intuitive to you. On the earth's surface if you take a large circle, the area does not increase as the square of the radius. It increases more slowly. We explain this by saying that the earth's surface is not flat. If you double the radius of a large circle on the earth's surface, the area enclosed will increase by LESS than a factor of 4. All we are doing is considering the 3D analogy.

Galaxy counts show that our universe is approximately spatially flat, but it may not be exactly so. Some measurements have shown the possbility of slight non flatness at large scale.

There is as yet no reason to assume that our Universe extends only a finite distance.
If, for the sake of argument, one assumes that, there is still no scientific basis for postulating other separate universes. You CAN imagine anything, and people do make up pretty fantastic multiverse fantasies. there is an old principle called Occam's Razor which says don't make things any more complicated than you need...

Last edited: Aug 8, 2008
6. Aug 8, 2008

### marcus

much of the stuff we are observing right now is much farther away than 13.7.
The farthest stuff that is being imaged and studied at present is estimated to be about 46 billion LY from us at this time. That's the CMB temperature map.

there is no evidence that anything just ends
(the evidence is more consistent with the idea that things continue on without any abrupt change)

7. Aug 8, 2008

8. Aug 8, 2008

+1 :)

9. Aug 8, 2008

### chroot

Staff Emeritus
Though the universe is about 13.7 billion years old, we can see further than 13.7 billion light-years away. The reason? The universe used to be much smaller, and objects that are distant now were much closer in the past. The furthest objects that we can see are at the so-called "particle horizon," which is about 46 billion light-years away.

- Warren