Register to reply

Is the universe infinite?

by QuantumJG
Tags: infinite, universe
Share this thread:
Radrook
#37
Jan16-11, 03:46 AM
P: 334
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).
Was he taking into consideration that the majority of those stars might be beyond detectable range because space is expanding and their light would never reach us?
mirsislam
#38
Jan16-11, 04:06 AM
P: 2
do anybody find the greatest number, so why tell that universe in finite?
Nordic
#39
Jan16-11, 05:40 AM
P: 9
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).
Wrong.

Olber also had a counter-argument to that. He stated that the light from the distant stars would be dimmed since the matter between those stars and us would absorb the light. But that was wrong, because then, that matter would eventually heat up and shine like the stars.

But what Obler did not consider was that the stars had not been shining forever, but were formed at some point in time(finite time).
Chalnoth
#40
Jan16-11, 07:03 AM
Sci Advisor
P: 4,800
Quote Quote by Nordic View Post
Wrong.

Olber also had a counter-argument to that. He stated that the light from the distant stars would be dimmed since the matter between those stars and us would absorb the light. But that was wrong, because then, that matter would eventually heat up and shine like the stars.

But what Obler did not consider was that the stars had not been shining forever, but were formed at some point in time(finite time).
He also didn't consider the possibility of expansion and its effect on the light coming from those stars.
Nordic
#41
Jan16-11, 07:26 AM
P: 9
Quote Quote by Chalnoth View Post
He also didn't consider the possibility of expansion and its effect on the light coming from those stars.
Yes, absolutely, thank you for adding.

Olber's Paradox has been confusing people ever since. It has just been a huge mix up, and it really bugs me. I believe everyone should be informed that it is absolutely wrong.
chivasregal
#42
Jan16-11, 10:47 PM
P: 1
Quote Quote by Chalnoth View Post
He also didn't consider the possibility of expansion and its effect on the light coming from those stars.
It might just be me that is a complete idiot, but is it not widely accepted that the speed of light is the ultimate speed at which anything can travel? And also that the speed of which light travels is not influenced in relation to movement of the object producing the light?

If this is the case I don't see why the expansion of the universe has anything to do with the fact that stars do not cover the sky entirely at night (presuming the universe is indeed infinite).

Obviously as new stars ignite far far away from us, the light that they produce will not reach us in a long time - but that is of course not due to the expansion of the universe.

Pardon my french, I have just recently gotten an interest in cosmology and I am pretty much clueless on all these subjects.
Chalnoth
#43
Jan17-11, 12:13 AM
Sci Advisor
P: 4,800
Quote Quote by chivasregal View Post
It might just be me that is a complete idiot, but is it not widely accepted that the speed of light is the ultimate speed at which anything can travel? And also that the speed of which light travels is not influenced in relation to movement of the object producing the light?

If this is the case I don't see why the expansion of the universe has anything to do with the fact that stars do not cover the sky entirely at night (presuming the universe is indeed infinite).
Olbers' Paradox applies to a universe that is infinite and unchanging in both time and space. In such a universe, no matter where you looked, in every direction there would eventually be a star. Thus everything would be the same temperature as the surface of a star.

You can solve this paradox in three ways:
1. Allow the universe to be finite in time. In such a universe, light wouldn't have had time to come from every location in the universe yet, as you mention.
2. Allow the universe to be finite in space. In such a universe, obviously not all directions would necessarily point to some star or other, since there would be a finite number of them.
3. Allow the universe to expand with time. In such a universe, the light from further-away stars is redshifted more, such that the temperature of the night sky is only affected by the most nearby stars, which are also finite in number.
mirsislam
#44
Jan17-11, 12:43 AM
P: 2
we know number system, but we don't know the smallest and largest number. We dont know the limit of universe, so why we say that it is finite.................
Nordic
#45
Jan17-11, 09:16 AM
P: 9
Quote Quote by Chalnoth View Post
You can solve this paradox in three ways:
1. Allow the universe to be finite in time. In such a universe, light wouldn't have had time to come from every location in the universe yet, as you mention.
2. Allow the universe to be finite in space. In such a universe, obviously not all directions would necessarily point to some star or other, since there would be a finite number of them.
3. Allow the universe to expand with time. In such a universe, the light from further-away stars is redshifted more, such that the temperature of the night sky is only affected by the most nearby stars, which are also finite in number.
I assume the first one would be the best explanation out of all three. I definitely do not agree with the idea of finite space though. It just doesn't seem to be right, for some reason. Although the first might be the best solution out of the three as separate solutions, #1 and 3 combined would create the solution that is accepted today.
Chalnoth
#46
Jan17-11, 11:35 AM
Sci Advisor
P: 4,800
Quote Quote by Nordic View Post
I assume the first one would be the best explanation out of all three. I definitely do not agree with the idea of finite space though. It just doesn't seem to be right, for some reason. Although the first might be the best solution out of the three as separate solutions, #1 and 3 combined would create the solution that is accepted today.
Actually, #3 is the direct solution, with the finite age of our universe necessarily stemming from the fact that it's expanding. The light travel time isn't really significant, in other words, but the redshift is.
afennah
#47
Jan17-11, 11:52 AM
P: 6
Quote Quote by Nordic View Post
my opinion on this is that the universe is located in space. A space that is infinite since it still lets the universe expand. :)
I think it's hard for us to imagine that: the universe could expand... without the need for something for it to expand into... but, maybe it does.
Tanelorn
#48
Jan17-11, 11:57 AM
P: 725
Perhaps instead we can view the expansion as creating new separate island universes no longer connected to our own? Or to coin an old term, other dimensions.. And an infinite number if them no less.
ehilge
#49
Jan30-11, 12:06 AM
P: 161
Quote Quote by chivasregal View Post
It might just be me that is a complete idiot, but is it not widely accepted that the speed of light is the ultimate speed at which anything can travel? And also that the speed of which light travels is not influenced in relation to movement of the object producing the light?
From what I have read, it is mostly accepted that nothing can move faster than the speed of light WITHIN space, however, the infamous space-time fabric itself can expand faster than the speed of light. I feel like now would be a good time to re-introduce the attached article on misconceptions about the big bang. I found it on these forums some time ago and it really is an excellent piece.
Attached Files
File Type: pdf misconceptionsBigBang.pdf (384.2 KB, 25 views)
GODISMYSHADOW
#50
Jan31-11, 02:48 PM
P: 40
Quote Quote by QuantumJG View Post
Ok so me and a few of my physics (& Maths) friends were arguing this.

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.

My friend who's a mathematician said that in her geometry subject this question was actually brought up. She said that the universe may be a 3-manifold (3D surface?) and it depends on the curvature (negative or positive) as to whether the universe is finite or not.

I just want to know what the consensus is.
What do you mean by the universe? Do you mean everything within a radius of 10 parsecs of Earth? Everything within a radius of 20 parsecs of Earth? Everything within a radius of 40 parsecs of Earth? What do you mean?
afennah
#51
Feb1-11, 04:51 AM
P: 6
Quote Quote by GODISMYSHADOW View Post
What do you mean by the universe? Do you mean everything within a radius of 10 parsecs of Earth? Everything within a radius of 20 parsecs of Earth? Everything within a radius of 40 parsecs of Earth? What do you mean?
In your comment you appear to be putting the Earth at the centre of your universe. I'm not sure that is a good idea but, your questions are relevant to what we are debating! Is the universe infinite or not. Some think yes others think no. I personally, do not believe it is infinite (Surely something infinite can't expand?... because it's infinite to start with! Ugh...Someone give me a headache tablet). I am finding the various stances on this subject fascinating. The PDF on ',Big Band misconceptions' was a good read.

I await with trepidation further posts informing me that I am talking Bol*@ks. lol.
Regards,
Nordic
#52
Feb1-11, 06:00 AM
P: 9
Quote Quote by afennah View Post
(Surely something infinite can't expand?... because it's infinite to start with! Ugh...Someone give me a headache tablet)

I await with trepidation further posts informing me that I am talking Bol*@ks. lol.
Regards,
If that is the only fact that supports your position on this, then I don't think you know the topic well. Don't get me wrong here, I'm just curious. The matter in the universe is expanding, yes, but not the SPACE of the universe. I think you should re-check your that though :)
afennah
#53
Feb2-11, 04:26 AM
P: 6
Quote Quote by Nordic View Post
If that is the only fact that supports your position on this, then I don't think you know the topic well. Don't get me wrong here, I'm just curious. The matter in the universe is expanding, yes, but not the SPACE of the universe. I think you should re-check your that though :)
Nordic, I think you'll find you have that the wrong way round. Matter is not expanding!
It is 'space' which appears to be expanding. If it was matter expanding then our galaxy would be getting bigger (which it's not). Matter is being 'carried' along with the expansion of space which is why everything we see appears to be moving away form everything else. Hey... maybe I know a little more than you think! lol.

So, my point still stands...How can the universe be infinite if it's expanding? Cheers,
darkhorror
#54
Feb2-11, 02:48 PM
P: 140
Quote Quote by afennah View Post
Nordic, I think you'll find you have that the wrong way round. Matter is not expanding!
It is 'space' which appears to be expanding. If it was matter expanding then our galaxy would be getting bigger (which it's not). Matter is being 'carried' along with the expansion of space which is why everything we see appears to be moving away form everything else. Hey... maybe I know a little more than you think! lol.

So, my point still stands...How can the universe be infinite if it's expanding? Cheers,
Look at the set of numbers 1, 10, 100, 1000,... that is infinite
now look at the set 1, 5, 10, 50, 100, 500, 1000,... that is also infinite

Now there are more numbers between 1 and 1000 than there were in the first set.


Register to reply

Related Discussions
The Universe - infinite or not ? Cosmology 139
If the universe is infinite General Discussion 3
Infinite-infinite universe explains all General Discussion 42
Does an infinite universe effect an infinite outlook? Astronomy & Astrophysics 11
Universe Infinite Astronomy & Astrophysics 37