Is the Universe Finite or Infinite?

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SUMMARY

The discussion centers on the debate regarding whether the universe is finite or infinite. Participants argue that the universe may be finite due to the finite amount of matter it contains, while others suggest it could be infinite, depending on its curvature as described by geometry. Key points include the implications of Olbers' Paradox, which challenges the notion of an infinite universe by highlighting the observable night sky's darkness. Ultimately, there is no consensus, as the universe's true size remains uncertain, with current measurements indicating it is significantly larger than the observable region.

PREREQUISITES
  • Understanding of Olbers' Paradox and its implications on cosmology
  • Familiarity with the concept of spatial curvature in geometry
  • Knowledge of the Big Bang theory and its role in cosmic expansion
  • Basic grasp of the Cosmic Microwave Background Radiation (CMBR) and its significance
NEXT STEPS
  • Research the implications of Olbers' Paradox on cosmological models
  • Study the properties of 3-manifolds and their relevance to the universe's shape
  • Explore the concept of cosmic inflation and its effects on the universe's size
  • Investigate the role of dark energy in the accelerating expansion of the universe
USEFUL FOR

Astrophysicists, cosmologists, students of physics and mathematics, and anyone interested in the fundamental questions of the universe's structure and size.

QuantumJG
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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.
 
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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.
 
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.
 
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The size of the Universe: the speed of light multiplied with 13.7 billion years...
 
universe11 said:
The size of the Universe: the speed of light multiplied with 13.7 billion years...

lol no
 
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...
 
Assumption: mass creates space, right?

If space were finite, then just by standing at the edge of space will create more space?
 
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
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.
 
space is infinite, energy/matter has a maximum value
 
  • #10
Checking my assumptions:
1. Matter create space?
2. Matter expanse space?
3. Matter needs space to exists?
 
  • #11
Space can create particles i.e. matter and energy seemingly from nothing? And the reverse?
 
  • #12
TungstenX said:
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.
 
  • #13
Ah, thank you Deuterium2H
 
  • #14
Chalnoth said:
... 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?
 
  • #15
mrspeedybob said:
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).
 
  • #16
I argued that it must be finite in size 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).
 
  • #17
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).

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!
 
  • #18
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  • #19
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).
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.
 
  • #20
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.
 
  • #21
Chronos said:
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?
 
  • #22
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).

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.
 
  • #23
Brain Dwarf said:
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.
 
  • #24
The universe may well be spatially infinite, although I think that is unprovable. I too am inclined to agreee with Professor Cox.
 
  • #25
QuantumJG said:
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.
 
  • #26
first at all, what happened a few seconds before the big bang, I know something happened that tells me that time is infinite, no beginning, no end
 
  • #27
I agree with our universe located in a space that is infinite
 
  • #28
Nordic said:
our evolved universe that still continues to grow is finite, but is located in a space that is infinite.

josewrivera said:
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.
 
  • #29
jobigoud said:
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?
 
  • #30
josewrivera said:
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...
 

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