Is the Universe Finite or Infinite?

  • Context: Graduate 
  • Thread starter Thread starter QuantumJG
  • Start date Start date
  • Tags Tags
    Infinite Universe
Click For Summary

Discussion Overview

The discussion centers around the question of whether the universe is finite or infinite in size. Participants explore various theoretical perspectives, assumptions about matter and space, and implications of cosmological models, including Olbers' Paradox and the nature of the universe's expansion.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory
  • Technical explanation

Main Points Raised

  • Some participants argue that the universe must be finite due to the finite amount of matter it contains, while others suggest that space itself may be infinite.
  • A participant mentions that the universe could be a 3-manifold and its finiteness depends on its curvature.
  • Another participant states that our understanding is limited, and while the universe is larger than the observable region, its exact size remains uncertain.
  • Some participants discuss Olbers' Paradox, suggesting it implies that an infinite universe would result in a bright night sky, while others argue that the paradox relates to a static universe and does not definitively prove the universe's finiteness.
  • There are claims that matter does not create space, and that the expansion of space is an intrinsic property of the universe, independent of matter.
  • Questions are raised about the implications of a spatially infinite universe and whether it could have observable edges, with some participants expressing skepticism about the existence of such edges based on observational data.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether the universe is finite or infinite. Multiple competing views are presented, with ongoing debate about the implications of various cosmological models and assumptions.

Contextual Notes

Participants express uncertainty regarding the definitions of the universe and the assumptions underlying their arguments, particularly concerning the relationship between matter and space. The discussion reflects a range of interpretations of cosmological theories and observational evidence.

  • #31
nlsherrill said:
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 let's 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. :)
 
Space news on Phys.org
  • #32
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.
http://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 :frown:)
 
Last edited by a moderator:
  • #33
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 :smile:
 
  • #34
It's my universe, just one of millions, and is not expanding, just moving in space, we haven't been that far nothing is written on stone sorry about spelling
 
  • #35
josewrivera said:
It's my universe, just one of millions, and is not expanding, just moving in space, we haven't been that far nothing is written on stone sorry about spelling
lol. Millions of universes? I don't think that's right.
 
  • #36
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.
 
  • #37
afennah said:
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?
 
  • #38
do anybody find the greatest number, so why tell that universe in finite?
 
  • #39
afennah said:
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).
 
  • #40
Nordic said:
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.
 
  • #41
Chalnoth said:
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.
 
  • #42
Chalnoth said:
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.
 
  • #43
chivasregal said:
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.
 
  • #44
we know number system, but we don't know the smallest and largest number. We don't know the limit of universe, so why we say that it is finite....
 
  • #45
Chalnoth said:
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.
 
  • #46
Nordic said:
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.
 
  • #47
Nordic said:
my opinion on this is that the universe is located in space. A space that is infinite since it still let's 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.
 
Last edited:
  • #48
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.
 
  • #49
chivasregal said:
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.
 

Attachments

  • #50
QuantumJG said:
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?
 
  • #51
GODISMYSHADOW said:
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,
 
  • #52
afennah said:
(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 :)
 
  • #53
Nordic said:
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,
 
  • #54
afennah said:
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.
 
  • #55
Here is a different argument. Feel free to poke holes in it.

I will begin with 6 postulates...

1. The universe is finiate in age
2. The universe is infinite in space
3. The universe began with a big bang at which time all mater began to exist and space began to expand.
4. The above 3 are true for all observers.
5. On a large scale the universe is homogenious.
6. General relativity is accurate.

There was a time when all the matter in the observable universe was compressed into an inch radius. (postulates 1 and 3) that time was within a fraction of a second after the big bang. This was not the only matter in existence but this super dense soup must have extended infinitly in all dirrections (postulates 2, 3, and 5). All this mater came into existence simultaniously or there would have been pressure waves which would have made the universe non-homogenious. (postulate 5). If 2 different observers were observing the big bang a fraction of a second after it happened they could disagree on wether the matter at point A in the universe was the same age as the matter at point B if points A and B were separated by more distance then light could have traveled in the age of the universe. If the 2 observers are in wildly different frames of reverence A and B could be simultanious for 1 but separated by billions of years for the other. (postulate 6)

It would seem to me that not all 6 original postulates can be true. I'm inclined to through out #2 and suppose that the universe was not infinite at inception and therefore the entire universe at a point and could therefore come into existence simultaniously from all points of view.

Sorry about my horendsous spelling. I'm on a computer without spell check.
 
  • #56
Put another way, it looks like the whole universe began simultaneously but if it were infinite at inception then you have to decide what "simultaneous" means for widely separated points in space and GR makes that very difficult.
 
  • #57
Well, we know that 6 is wrong on some level, so I'm not sure that this argument gets you anywhere.
 
  • #58
afennah said:
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,

Einstein got rid of the notion of "absolute now." So by infinite Universe, you can't mean "everywhere right now," can you? I'd rather think about something finite so I won't lose my mind. Our Milky Way galaxy is othen referred to as an "island universe."
 
  • #59
mrspeedybob said:
Put another way, it looks like the whole universe began simultaneously but if it were infinite at inception then you have to decide what "simultaneous" means for widely separated points in space and GR makes that very difficult.

Very very shortly after the big bang would a lot of the GR not really work the way we have it work now? Could you have two frames of reference that were very close to each other but moving away from each other at huge speeds due to the rapid expansion of space that don't really go with what relativity would say in normal space like around the Earth now? Also what about that spacetime only existed for a short time how does that work along with the finite speed of light.

Now I am sure this stuff has already been solved, I am going to have to look some of this stuff up but it seems like it may be interesting.
 
  • #60
I haven't read the whole thread so excuse me if this has already been brought up, but has anyone thought about the universes expansion as possibly being "driven" by an outside force?

By outside I mean literally outside of our universe...kind of like outside the membrane that may contain everything that lies inside it(stars, gas, etc). Its hard to explain but think about a bunch of bubbles clustered together. Now think of those bubbles as being individual universes. We see that when a lot of bubbles are near each other, they tend to burst and combine into larger bubbles right? What if all of these bubbles were universes, and they were "combining" to make bigger and bigger universes, such that the universe as seen from someone INSIDE one of the bubbles kept getting larger and larger because it was always combining with other bubbles?

The above probably sounds insane, and drug related, but has anyone else thought of something like that? In a way, that could also work with black holes. Maybe black holes are "holes" in the bubble that is our universe, and our matter is leaving and being contributed to another bubble universe.

I have no idea what I'm talking about, just speculation.
 

Similar threads

High School Big Bang and Infinite Universe
  • · Replies 20 ·
Replies
20
Views
2K
Graduate Polling for estimates of probability our universe is infinite
  • · Replies 1 ·
Replies
1
Views
2K
High School How is the Big Bang compatible with an infinite universe?
  • · Replies 53 ·
2
Replies
53
Views
9K
Undergrad Finite density at the BB preinflation epoch implies a finite universe?
  • · Replies 26 ·
Replies
26
Views
3K
Undergrad Calculating the probability that the Universe is finite
  • · Replies 70 ·
3
Replies
70
Views
5K
Undergrad The Expanding Universe: A Scientist's Perspective on Infinite vs Finite Space
  • · Replies 79 ·
3
Replies
79
Views
12K
Undergrad How can the universe be infinitely big?
  • · Replies 5 ·
Replies
5
Views
2K
Graduate Infinite universe from a finite start
  • · Replies 14 ·
Replies
14
Views
4K
Graduate Can the universe be both infinite AND expanding
  • · Replies 2 ·
Replies
2
Views
3K
Graduate Can the Universe Transition from Finite to Infinite?
  • · Replies 8 ·
Replies
8
Views
2K