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Is the universe infinite?

by QuantumJG
Tags: infinite, universe
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Chalnoth
#19
Dec28-10, 06:00 PM
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Quote Quote by afennah View Post
I thought that 'Oblers Paradox' proved that the universe could not be infinite. If it was...the night sky should be brilliant white (caused by the starlight from an infinite number of stars).
Olbers' Paradox takes two assumptions, and shows they cannot both be true:
1. The universe is infinite (in time and space).
2. The universe is static (no expansion).

The discovery of the expansion of the universe demonstrates that the second assumption fails, which means that Olbers' Paradox cannot provide any additional information about the truth or falsity of the first.
Chronos
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Dec28-10, 10:20 PM
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A clarification appears to be in order. Olber hypothesized the universe cannot be both spatially and temporally infinite. We are fairly certain it is not temporally infinite. The jury is still out on the spatially infinite part. A temporally finite universe could be spatially infinite. My guess is it is not. My reasoning is an infinitely spatial universe would have observationally irregular 'edges'. I think this would be fairly obvious from WMAP data - and it is not. The alleged 'axis of evil' thing has been discredited due to selection effects - unsurprisingly.
Chalnoth
#21
Dec28-10, 10:27 PM
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Quote Quote by Chronos View Post
A clarification appears to be in order. Olber hypothesized the universe cannot be both spatially and temporally infinite. We are fairly certain it is not temporally infinite. The jury is still out on the spatially infinite part. A temporally finite universe could be spatially infinite. My guess is it is not. My reasoning is an infinitely spatial universe would have observationally irregular 'edges'. I think this would be fairly obvious from WMAP data - and it is not. The alleged 'axis of evil' thing has been discredited due to selection effects - unsurprisingly.
With expansion, the universe can still be both spatially and temporally infinite without impinging upon Olbers' paradox.

What edges would a spatially infinite universe have?
Deuterium2H
#22
Dec28-10, 10:36 PM
P: 59
Quote Quote by afennah View Post
I thought that 'Oblers Paradox' proved that the universe could not be infinite. If it was...the night sky should be brilliant white (caused by the starlight from an infinite number of stars).
Dr. Edward Harrison gave the "definitive" answer/solution to Olber's Paradox, in his 1987 book "Darkness at Night: A Riddle of the Universe".

While it is an open question wether the Universe is infinite in extent (space), it is finite in time...i.e., it had a beginning (The Big Bang). We can only look back a finite distance (our Cosmological Horizon), so the light from any stars existing beyond the radius of the Hubble Sphere has not had a chance to get to us yet. Combined with the fact that stars themselves have a finite life-time, there is simply not enough visible stars in our observable universe to make the night sky bright.

As Chalmoth pointed out above, the expansion of the Universe also has the effect of red-shifting any distant luminous objects. Even though stars did not exist at the time of Recombination (Surface of Last Scattering), even the unbelievably intense, incandescent light from this epoch (approx. 370,000 years after Big Bang) has been red-shifted to such low frequency/long wavelengths that it is no longer in the visible spectrum...hence the Cosmic Microwave Background Radiation.

Finally combined with observation that the expansion of the Universe is now accelerating, eventually all galaxies (with the possible exception of the local galaxies that are gravitationally bound with the Milky Way) will pass beyond our particle horizon, and will forever become unobservable. Note that I do not subscribe to the so-called "Big Rip". So, in theory, billions of years from now, our Milky Way will truly become an "Island Universe", just as it was once thought of, up until the early twentieth century.
afennah
#23
Jan6-11, 03:42 AM
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Quote Quote by Brain Dwarf View Post
You got that wrong, the paradox you mention was more to do with a static universe.
Since our universe is all that there is (that is after all the definition of universe), it must then extend forever - yet it is still expanding!
Hi Brain Dwarf, I don't agree with your reasoning (in your statement): 'because the universe is all there is...then it must be infinite'. Professor Brian Cox touched on the subject of 'Oblers Paradox' during his astronomy program this week. (BBC Stargazing Live, Pt1). He seems to believe (as I do) that the universe is not infinite.

I thought Deuterium's comment about 'red shifted light' was very interesting though.

Happy New Year!
Al.
Chronos
#24
Jan7-11, 12:53 AM
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The universe may well be spatially infinite, although I think that is unprovable. I too am inclined to agreee with Professor Cox.
Nordic
#25
Jan10-11, 09:43 AM
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Quote Quote by QuantumJG View Post
I argued that it must be finite in size, since the universe contains a finite amount of matter and since no space is truly empty, how could the universe be finite.
Where did you get all of this information from? I'm just wondering.

I think the answer to this would be that we do not have a precise answer to this.

My own speculation on this (without almost any support at all, it is just a speculation of my own) is that our evolved universe that still continues to grow is finite, but is located in a space that is infinite.

I believe it is not total trash, but it's also just an opinion.
josewrivera
#26
Jan10-11, 11:35 AM
P: 5
first at all, what happened a few seconds before the big bang, I know something happened that tells me that time is infinite, no beggining, no end
josewrivera
#27
Jan10-11, 11:38 AM
P: 5
I agree with our universe located in a space that is infinite
jobigoud
#28
Jan10-11, 12:56 PM
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Quote Quote by Nordic View Post
our evolved universe that still continues to grow is finite, but is located in a space that is infinite.
Quote Quote by josewrivera View Post
I agree with our universe located in a space that is infinite
It would seem that you guys do not use the accepted definition for the word "universe".
Universe: The totality of everything that exists.

It cannot be in something, or located. It is everything.
Nordic
#29
Jan10-11, 01:18 PM
P: 9
Quote Quote by jobigoud View Post
It cannot be in something, or located. It is everything.
Are you sure about that?

I am not going to argue about anything for cosmology is my hobby, and not a specialty or anything.

I thought that a universe is located in space, or aspace, call it however you want. And I believe that it is a possibility that the universe is finite, while space is infinite. I am not kicking the idea of a multiversity around here, don't be misguided by my sentences into thinking that.

So, again, are you sure about what you are stating?
nlsherrill
#30
Jan10-11, 04:57 PM
P: 322
Quote Quote by josewrivera View Post
I agree with our universe located in a space that is infinite
This doesn't make sense. The universe contains everything, even space. Cosmologist have told and shown us that the universe is expanding, and accelerating in its expansion.

Now think about this: If the universe were infinite, then why would be observe that space was expanding? If its infinite in span, then the idea of the universe growing in size doesn't make sense because if its infinite in span, it can't be any larger than infinite.

At least, this is what I just thought of...
Nordic
#31
Jan11-11, 01:04 AM
P: 9
Quote Quote by nlsherrill View Post
Now think about this: If the universe were infinite, then why would be observe that space was expanding? If its infinite in span, then the idea of the universe growing in size doesn't make sense because if its infinite in span, it can't be any larger than infinite.

At least, this is what I just thought of...
I really have never thought of that. It is a really good idea.

But again, my opinion on this is that the universe is located in space. A space that is infinite since it still lets the universe expand. Now, think about this: What if we were out of this space? What will the universe do then? Just crash into the walls of it? I do not think so...

You can search my opinion up, I'm sure some other people have got it as well. I am not trying to prove anything, just giving suggestions. :)
TungstenX
#32
Jan11-11, 02:44 AM
P: 20
The definition according to the mighty Wikipedia:
The Universe comprises everything we perceive to exist physically, the entirety of space and time, all forms of matter and energy. However, the term Universe may be used in slightly different contextual senses, denoting such concepts as the cosmos, the world, or Nature.
en.wikipedia.org/wiki/Universe

As for an infinite "thing" expanding, it makes a lot of sense; the observed distances of the "points" inside this "thing" will increase. (I think I read that it is not only the mass objects / collections moving away from one another but also more space being "created" at the "centre" of the universe that accelerate this observation).

I think our perception / definition of something expanding is limited (having a human frame of reference; e.g. living on a planet). We want to think that something is expanding in / into something else, so how can we view the collection of everything (including space and time) expanding, when we are in it?

(Sorry English no good today )
TungstenX
#33
Jan11-11, 03:06 AM
P: 20
BTW, what will the impact be if the universe is infinite or finite (lets say 10x billion light years, where x is a very large number)

Or even is x is not that big number....

Are we going to fall of the edge of the earth, erm... I meant universe.

We can't even send a man to Mars yet or any object out of our galaxy, thus a finite view of the universe approach an infinite view on our scale. (To plankton, the ocean must seem infinite; except for the ones reaching the edge)

Seeing that we can not go and find (or not find) the edge of the Universe; is there a thought experiment that will help.

Wow, that is a lot of ramblings
josewrivera
#34
Jan12-11, 07:51 PM
P: 5
It's my universe, just one of millions, and is not expanding, just moving in space, we havent been that far nothing is written on stone sorry about spelling
Nordic
#35
Jan13-11, 12:39 AM
P: 9
Quote Quote by josewrivera View Post
It's my universe, just one of millions, and is not expanding, just moving in space, we havent been that far nothing is written on stone sorry about spelling
lol. Millions of universes? I don't think that's right.
Macocio
#36
Jan13-11, 03:12 AM
P: 16
Is the universe infinite?

The question should be:
Why should the universe be infinite?

- to accommodate for the infinity of both the gravitational forces and the electromagnetic forces which are present in the universe we know of.

It is still something we can't be sure of until we find the end of the universe.


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