Is our universe finite or infinite?

[Space Drifter]

I say it is finite.

That is, if we agree that the speed of light is constant irrespective of its surroundings.

Let me explain what I mean. A car traveling 60 mph is only doing so relative to its surroundings on earth, such as trees and homes along the roadside. But what if the car had no surroundings? Then it would be impossible to determine any speed whatsoever.

Just like if a photon (a particle of light) in space had no apparent surroundings, no stars, no comets, no nebulae, just empty space... then how could you know tat it was traveling at the speed of light, if there were nothing to measure against? Is there such a thing as absolute speed irrespective to surroundings? Well, I propose that such absolute speed is only possible in a finite universe. i.e.- the photon's velocity is determined relative to the start and end points of said universe.

For in an open or infinite universe, there would be no official distance markers, no start point, no end point. In other words, there would be no way to determine, between two passing objects, say a photon and a comet, whether both are moving, or one is stationary and the other is moving past it.

I think that is a very simple and solid case for why our universe is finite, and not infinite.

What do you think?

 

[Drakkith]

See the other post we were in Drifter. There is no absolute speed other than c. And we ALWAYS measure that the same no matter how fast or in what direction we are travelling.

 

[Radrook]

I say it is finite.

That is, if we agree that the speed of light is constant irrespective of its surroundings.

Let me explain what I mean. A car traveling 60 mph is only doing so relative to its surroundings on earth, such as trees and homes along the roadside. But what if the car had no surroundings? Then it would be impossible to determine any speed whatsoever.

Just like if a photon (a particle of light) in space had no apparent surroundings, no stars, no comets, no nebulae, just empty space... then how could you know tat it was traveling at the speed of light, if there were nothing to measure against? Is there such a thing as absolute speed irrespective to surroundings? Well, I propose that such absolute speed is only possible in a finite universe. i.e.- the photon's velocity is determined relative to the start and end points of said universe.

For in an open or infinite universe, there would be no official distance markers, no start point, no end point. In other words, there would be no way to determine, between two passing objects, say a photon and a comet, whether both are moving, or one is stationary and the other is moving past it.

I think that is a very simple and solid case for why our universe is finite, and not infinite.

What do you think?

Please clarify:

Assuming that the two situations in both universes are identical

How does the finite universes enhance our ability to determine relative movement?
How does the infinite universe hinder it?

 

[Chronos]

Under special relativity motion is always - relative. Every observer perceives they are stationary and everything else is moving. The is no easy way to dispell this illusion and the finiteness of the universe [or lack thereof] has nothing to do with it. The only refererence frame with any legitimacy is the cosmic microwave background. We can measure its redshift across the sky and determine our relative direction and velocity.

 

[L2E]

if an object has 'nothing' around it then it cannot be observed to be traveling at any speed because there are no observers.

The car in your example surrounded by nothing could be stationary or traveling, the people inside the car would not even know. The only way you can tell that you or other objects are moving is because of the relative velocity between you and other objects without that, you wouldn't know.

 

[Space Drifter]

Please clarify:

Assuming that the two situations in both universes are identical

How does the finite universes enhance our ability to determine relative movement?
How does the infinite universe hinder it?

Hi Radrook,

My original thinking is that in a finite universe, the boundaries at its edges, would serve as markers by which every moving object's speed, in such universe, would be relative to the "boundaries." In other words, each object's position would be special, in relation to the boundaries.

Whereas in an infinite universe, there is absolutely nothing that determines whose position or whose is more "correct." There are no boundaries. Like a football field with no out-of-bounds or endzones.

But someone since pointed out that even in a closed universe, there still are really no boundaries. Sort of like if everything was on a sphere, there would be no end to the sphere, even though it is finite. Every point or object on the sphere has no more special position than any other.

So it's up for debate.

 

[bcrowell]

FAQ: Is the universe finite, or is it infinite?

Standard cosmological models come in two flavors, open and closed. The open type has negative spatial curvature and infinite volume. The closed one has positive curvature and finite volume; spatially, it is the three-dimensional analog of a sphere. Since both types are mathematically self-consistent solutions to the Einstein field equations, the finiteness or infiniteness of the universe is something that cannot be determined by logic but only by observation.
Current observations of the cosmic microwave background's anisotropy show that our universe is very nearly spatially flat (on the cosmological scale). If it is exactly flat, then it is a special case lying between the more general open and closed cases. The flat case has infinite volume. However, the range of uncertainty in the curvature is wide enough to be consistent with either positive or negative curvature, so right now the finiteness or infiniteness of the universe is an open question.
Sometimes people use the word "universe" when they really mean "observable universe." The observable universe is finite in volume because light has only had a finite time to travel since the Big Bang.

 

[Chronos]

I like 'observable universe' as a definition. Anything beyond that is scientifically irrelevant, IMO. This is my primary objection to multiverse theories - they lack credible observational support.

 

[AstrophysicsX]

I assume the universe is finite. The big bang theory tells us that everything was at one point a very small, dense nugget of energy. That nugget of energy was finite. Hubble even confirmed that our surrounding galaxies were moving away from us, proving the universe is expanding. An infinite universe couldn't expand.

 

[Space Drifter]

I assume the universe is finite. The big bang theory tells us that everything was at one point a very small, dense nugget of energy. That nugget of energy was finite. Hubble even confirmed that our surrounding galaxies were moving away from us, proving the universe is expanding. An infinite universe couldn't expand.

Chronos: I agree with you completely. An expanding universe could not be infinite. That makes a lot of sense to me.

Also, the way I see it, the big bang really should be called the "big expansion." The universe was once a finite singularity, and to me, it still is a finite singularity. Albeit, a larger one.

And while I do think this particular universe that we live in, is finite, I am not ruling out the existence of other universes.

 

[bcrowell]

I like 'observable universe' as a definition. Anything beyond that is scientifically irrelevant, IMO. This is my primary objection to multiverse theories - they lack credible observational support.

The distinction between the observable universe and the rest has nothing to do with multiverse theories. The observable universe is simply the part of the universe from which light has had time to reach us. Tomorrow more of the universe will be in the observable part.

I assume the universe is finite. The big bang theory tells us that everything was at one point a very small, dense nugget of energy. That nugget of energy was finite.

This is incorrect. Open FRW models start with infinite energy. Closed FRW models start with finite energy. See #7.

Hubble even confirmed that our surrounding galaxies were moving away from us, proving the universe is expanding. An infinite universe couldn't expand.

This is incorrect. Open FRW models are spatially infinite, but they expand. See #7.

It seems like some of the folks in this thread are just getting started on cosmology and GR. A good starting place IMO is Gardner, Relativity Simply Explained. Astro textbooks designed for an introductory gen ed course all have discussions of this kind of thing as well. Some people like Relativity from A to B by Geroch. For those with a couple of years worth of university-level math and physics under their belts, there are good GR textbooks like Hartle that are designed for upper-division physics majors. Taylor and Wheeler's Exploring Black Holes is also a good book at the undergrad level, although it doesn't deal with cosmology.

 

[Chronos]

The distinction between the observable universe and the rest has nothing to do with multiverse theories. The observable universe is simply the part of the universe from which light has had time to reach us. Tomorrow more of the universe will be in the observable part.

Tomorrow the universe will be larger and we will merely continue to observe photons that originated between us and the CMB. I admit I assume photons more ancient than the CMB are inherently unobservable.

 

[Tanelorn]

Chronos, my understanding of what I have read is that in the future the universe will grow greatly in size until everything in the universe except the local cluster of galaxies becomes gravitationally and electromagnetically completely disconnected from us. But it would all still there somewhere, so we wouldn't have cause to believe that it no longer exists. Going backwards in time there is likely a large amount of the universe that is now no longer visible today. (Penrose says 10^30 more!). Of course this assumes that everyone that worked on this is right!