Is the Universe Finite or Infinite?

[QuantumJG]

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.

 

[Chalnoth]

There is no consensus. Basically, our vision is limited in time and space, so we can't exactly say anything about whether or not distances are infinite. All we can say for sure is that our universe is many times larger than our observable region. But how much larger is currently up in the air.

 

[brerabbit]

What is the/a universe?? Scientists suggest that there many universes emanating from their own "big bang". Space is infinite but a universe is bounded.

 

[universe11]

The size of the Universe: the speed of light multiplied with 13.7 billion years...

 

[-1/12]

The size of the Universe: the speed of light multiplied with 13.7 billion years...

lol no

 

[universe11]

if the matter that originally emitted the oldest CMBR photons has a present distance of 46 billion light years, then at the time of decoupling when the photons were originally emitted, the distance would have been only about 42 million light-years away...

 

[TungstenX]

Assumption: mass creates space, right?

If space were finite, then just by standing at the edge of space will create more space?

 

[HallsofIvy]

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

What evidence do you have of this?

and since no space is truly empty, how could the universe be finite.

What evidence do you have of this?

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.

 

[darkside00]

space is infinite, energy/matter has a maximum value

 

[TungstenX]

Checking my assumptions:
1. Matter create space?
2. Matter expanse space?
3. Matter needs space to exists?

 

[Tanelorn]

Space can create particles i.e. matter and energy seemingly from nothing? And the reverse?

 

[Deuterium2H]

Checking my assumptions:
1. Matter create space?
2. Matter expanse space?
3. Matter needs space to exists?

Answers to your assumptions:

1) No, matter does not create space. Matter exists in space.
2) No, matter does not expand space. At the earliest epoch of the Universe, there was no matter, per se, but rather pure radiation. The expansion of space is an intrinsic property, set by the initial conditions (Big Bang). Technically, the expansion of space would still occur even if there was absolutely no matter.
3) As physical matter has spatial extension, then I guess yes, matter "needs" space within which to exist.

 

[TungstenX]

Ah, thank you Deuterium2H

 

[mrspeedybob]

... All we can say for sure is that our universe is many times larger than our observable region. But how much larger is currently up in the air.

How can we say that for sure? What in our observable universe would be different if the universe were 1.1 times larger then the observable universe versus 1,000,000,000,000,000 times larger?

 

[Chalnoth]

How can we say that for sure? What in our observable universe would be different if the universe were 1.1 times larger then the observable universe versus 1,000,000,000,000,000 times larger?

Well, there are a number of possibilities that have to be considered. But let's just consider one where the universe wraps back on itself. If the universe wraps back on itself based upon its spatial curvature, then current measurements place the spatial curvature to be within 1% of zero, which makes the radius of curvature at least 10 times the Hubble radius, which is a factor of a few larger than the observable universe.

If the universe is flat but still wraps back on itself, then this induces anisotropies, which we should be able to see in the CMB. But we don't. Thus if it does wrap back on itself, it would have to do that very, very far away (again, a factor of a few times the observable universe).

 

[afennah]

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).

 

[Brain Dwarf]

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).

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!

 

[TungstenX]

http://en.wikipedia.org/wiki/Olbers'_paradox#The_mainstream_explanation". Note the part of the "fabric" space expanding (not just the distances between objects)

If I understand the Big Bang theory; It all started with a finite amount of energy. If space is finite as well then what is the ratio between space and the other finite "stuff" in it?

 

[Chalnoth]

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]

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]

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]

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]

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]

The universe may well be spatially infinite, although I think that is unprovable. I too am inclined to agreee with Professor Cox.

 

[Nordic]

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]

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]

I agree with our universe located in a space that is infinite

 

[jobigoud]

our evolved universe that still continues to grow is finite, but is located in a space that is infinite.

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]

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]

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]

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. :)

 

[TungstenX]

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 )

 

[TungstenX]

BTW, what will the impact be if the universe is infinite or finite (lets say 10^x 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]

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

 

[Nordic]

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.

 

[Macocio]

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.

 

[Radrook]

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]

do anybody find the greatest number, so why tell that universe in finite?

 

[Nordic]

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]

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]

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]

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]

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]

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....

 

[Nordic]

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]

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]

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.

 

[Tanelorn]

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]

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.

 

[GODISMYSHADOW]

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?