PDA

View Full Version : Is the universe infinate?

Zachary
2004-Dec-08, 08:36 PM
I'm just curious about this question - I know that the universe has no pysical 'edge' so to speak, but is it infinate in volume? If that is the case then I assume there is a finite amount of planets (unless you delve into alternate realities)

if that is the case then, as a well renowned author once said -

"Any finite number divided by infinity is as near nothing as makes no odds, so the average population of all the planets in the Universe can be said to be zero. From this it follows that the population of the whole Universe is also zero, and that any people you may meet from time to time are merely products of a deranged imagination." * *

Could somebody refute this calumny? :wink:

Russ
2004-Dec-08, 09:02 PM
I think if you question your own existence as well as the input of your senses, you are in trouble before you begin your thought problem. eg. The last time I took a punch in Tae Kwon Do class, the other guy seem pretty real to me! :lol:

Nowhere Man
2004-Dec-08, 09:12 PM
He's quoting from the Hitchhiker's Guide to the Galaxy. See the wink?

But anyway, the universe can be unbounded but still finite in volume. Remove a dimension and the best analogy is the surface of a sphere. Whether the universe is a 4-dimensional sphere is up for discussion. I don't know.

Fred

Russ
2004-Dec-08, 09:54 PM
He's quoting from the Hitchhiker's Guide to the Galaxy. See the wink?
Fred

Hence my :lol: .

Some day I will learn to write so the reader can hear my joke. :( :-? :lol:

2004-Dec-08, 10:03 PM
It may very well be infinite, but at the moment there is not enough information known to answer that question definitively.

2004-Dec-08, 10:08 PM
He's quoting from the Hitchhiker's Guide to the Galaxy. See the wink?

But anyway, the universe can be unbounded but still finite in volume. Remove a dimension and the best analogy is the surface of a sphere. Whether the universe is a 4-dimensional sphere is up for discussion. I don't know.

Fred

Basically the spatial part of the FRW metric when the curavture is postive is a 3-sphere (i.e. the surface of a four diemnsional sphere) and has a finite volume, howveer the curvature can be negative or 'flat' in which case we get a universe with an infinite volume.

TrAI
2004-Dec-08, 10:28 PM
I'm just curious about this question - I know that the universe has no pysical 'edge' so to speak, but is it infinate in volume? If that is the case then I assume there is a finite amount of planets (unless you delve into alternate realities)

if that is the case then, as a well renowned author once said -

"Any finite number divided by infinity is as near nothing as makes no odds, so the average population of all the planets in the Universe can be said to be zero. From this it follows that the population of the whole Universe is also zero, and that any people you may meet from time to time are merely products of a deranged imagination." * *

Could somebody refute this calumny? :wink:

Well, I do not think the scientists are completely sure about the size or volume of the universe right now, and it is probably a long time, if ever, until we know for sure.

As for the other question, well, it all depends on your view of it...

It is in a way similar to the problem posed in the "0,999...=1" (http://www.badastronomy.com/phpBB/viewtopic.php?t=17953) poll/thread, I guess.

So if you lean to the yes in that, then it follows that there is no life, planets, mass, energy or anything else of a finite number/amount in an infinite universe, since any infinitely small number is zero.

If you feel that even an infinitely small number is something, then I would say that that saying is false, but you have quite a lot of space with nothing in it, and a few places with mass.

worzel
2004-Dec-12, 12:44 PM
He's quoting from the Hitchhiker's Guide to the Galaxy. See the wink?

But anyway, the universe can be unbounded but still finite in volume. Remove a dimension and the best analogy is the surface of a sphere. Whether the universe is a 4-dimensional sphere is up for discussion. I don't know.

Fred

Basically the spatial part of the FRW metric when the curavture is postive is a 3-sphere (i.e. the surface of a four diemnsional sphere) and has a finite volume, howveer the curvature can be negative or 'flat' in which case we get a universe with an infinite volume.
I've asked this question before and never got an answer. So, your're saying that the claim that you would come back to where you started if you kept going in the same direction long enouhg is only true if the universe is closd - i.e. if the universe will eventually collapse in a big crunch. Is that right?

Ut
2004-Dec-12, 03:00 PM
I've asked this question before and never got an answer. So, your're saying that the claim that you would come back to where you started if you kept going in the same direction long enouhg is only true if the universe is closd - i.e. if the universe will eventually collapse in a big crunch. Is that right?[/quote]

Pretty much. The flat and open models do not have a finite volume, and never stop expanding. There sphere analogy only holds for a closed universe, because a closed universe has the geometry of a sphere. The flat universe is, uh, flat, and so you'd have to use the analogy of an infinite sheet of paper, where as the open universe has the geometry of a hyperboloid.

umop ap!sdn
2004-Dec-12, 06:34 PM
FWIW, Zachary, I understood your joke. Didn't get the reference though. :lol:

The simplest way to refute it is to point out that it is confusing zero with infinitessimal. :D

2004-Dec-12, 06:46 PM
[quote=Bad jcsd]I've asked this question before and never got an answer. So, your're saying that the claim that you would come back to where you started if you kept going in the same direction long enouhg is only true if the universe is closd - i.e. if the universe will eventually collapse in a big crunch. Is that right?

Yes, but actually the length of time it would take to do such a journey is such that you wouldn't make it back before the big crunch!

Grey
2004-Dec-12, 07:06 PM
I've asked this question before and never got an answer. So, your're saying that the claim that you would come back to where you started if you kept going in the same direction long enouhg is only true if the universe is closd - i.e. if the universe will eventually collapse in a big crunch. Is that right?

Pretty much. The flat and open models do not have a finite volume, and never stop expanding. There sphere analogy only holds for a closed universe, because a closed universe has the geometry of a sphere. The flat universe is, uh, flat, and so you'd have to use the analogy of an infinite sheet of paper, where as the open universe has the geometry of a hyperboloid.
However, in a passing conversation with one of my professors, he pointed out that the curvature and ultimate fate only absolutely correlate in a universe without a cosmological constant, which isn't looking like it's the universe in which we live. With a cosmological constant, it's possible to have a universe with positive curvature (finite, unbounded) which expands forever. I think it's also possible to have a universe with flat or negative curvature (and therefore infinite) which eventually collapses again. Sadly, I didn't have time to discuss the matter further, and have had a chance to explore it myself, but my experience has been that he generally knows what he's talking about.

Astronomy
2004-Dec-13, 12:32 AM
However, in a passing conversation with one of my professors, he pointed out that the curvature and ultimate fate only absolutely correlate in a universe without a cosmological constant, which isn't looking like it's the universe in which we live. With a cosmological constant, it's possible to have a universe with positive curvature (finite, unbounded) which expands forever. I think it's also possible to have a universe with flat or negative curvature (and therefore infinite) which eventually collapses again. Sadly, I didn't have time to discuss the matter further, and have had a chance to explore it myself, but my experience has been that he generally knows what he's talking about.

A cosmological constant will cause an exponential expansion of the universe as time goes on, no matter what the rest of the universe balances out to be.

Ut
2004-Dec-13, 12:42 AM
A cosmological constant will cause an exponential expansion of the universe as time goes on, no matter what the rest of the universe balances out to be.

Eh? And what about a cosmological constant equal to Lambda_E? I'd hardly call the Einstein static universe exponentially expanding... Not that there's any measure of reality assigned to the ESU.

2004-Dec-13, 12:57 AM
This site has a nice graph of different cosmological models and the cosmologicla constant

Astronomy
2004-Dec-13, 02:18 AM
A cosmological constant will cause an exponential expansion of the universe as time goes on, no matter what the rest of the universe balances out to be.

Eh? And what about a cosmological constant equal to Lambda_E? I'd hardly call the Einstein static universe exponentially expanding... Not that there's any measure of reality assigned to the ESU.

My apologies. I was talking about an FRW universe.

Grey
2004-Dec-13, 03:26 AM
And of course, if we really want to get complicated, there's not any a priori reason that the "cosmological constant" actually has to be a constant. If it were to change with time (or some other parameter), we could get theoretically get all kinds of wild universes.

Ut
2004-Dec-13, 03:35 AM
You know, I've always wondered what the universe would be like if it had the geometry of a player piano...

Normandy6644
2004-Dec-13, 03:42 AM
You know, I've always wondered what the universe would be like if it had the geometry of a player piano...

Or a donut. Mmmm....donut.

Grey
2004-Dec-13, 04:15 AM
You know, I've always wondered what the universe would be like if it had the geometry of a player piano...
That would explain why I sometimes have songs running through my head for no apparent reason.
:D

Evan
2004-Dec-13, 07:54 AM
Since it appears the universe is expanding at an increasing rate and the limit of our exploration of this universe is the speed of light then it must be infinite, by definition. We cannot get there from here.