How can empty space expand? (Reality behind the GR equations.)

[TR345]

So it's just that the momentum of gallaxies via the big bang excedes the ability for gallaxies to attract each other?

 

[TR345]

If so, then eventually gravity will overcome momentum and they will get closer and closer until the universe is one big black hole?

 

[JesseM]

So it's just that the momentum of gallaxies via the big bang excedes the ability for gallaxies to attract each other?

I think it's a little more complicated since this picture doesn't really convey the idea that their momentum and energy cause the expansion of space in general relativity, but I'd say this is on the right track.

If so, then eventually gravity will overcome momentum and they will get closer and closer until the universe is one big black hole?

It depends on the density of matter/energy. If it's high enough, everything will start falling back towards each other and space will begin to contract, leading not to a black hole but to a Big Crunch which is the mirror image of the Big Bang, all of space collapses to a point. But if it's not high enough the expansion continues forever at an ever-slowing rate, a bit like how if you throw an object away from the Earth at a velocity equal to or higher than the escape velocity, its speed will keep decreasing but it will never actually come to rest and fall back towards the Earth. But this is only what happens to the galaxies if there is no cosmological constant...the cosmological constant is a kind of negative pressure filling empty space which is possible in general relativity, and which could actually cause the galaxies to fly apart with ever-increasing speed (current cosmological observations suggest the rate of expansion is increasing, but whether this is due to general relativity's cosmological constant or to some non-constant phenomenon that can only be explained in terms of quantum physics is unknown).

 

[pervect]

I've moved a post that goes off on a tangent by "notknowking" to the "beyond the standard model" forum.

The post is at:
https://www.physicsforums.com/showthread.php?t=189014

 

[TR345]

The cosmological constant can be used to predict an accelerating universe, but that doesn't explain why there is a cosmological constant. Some new weird effect could be at the root couldn't it?

 

[JesseM]

Even if the cosmological constant can be used to predict an accelerating universe, that doesn't explain why there is a cosmological constant. So some new weird effect could be at the root couldn't it.

That's true, the cosmological constant might be based on some phenomenon we haven't thought of yet. My main point was just that you don't need new physics to explain the fact that galaxies are flying apart alone (without the additional fact of acceleration), since this can be explained just using general relativity without a cosmological constant.

 

[pervect]

A continuing theme in this thread is that science does not tell us "why", and it does not necessarily even tell us "how" (at least in the sense of a single unique how). What science tells us is "what" - what happens. The goal of science is to make correct predictions. It seems to me, though, that many posters to this thread haven't realized this, and they keep asking "why", and blaming science of not having an answer.

Thus from a scientific viewpoint, two theories, even if they "look different", will be fundamentally the same if they make the same predictions.

Thus it doesn't fundamentally matter if the universe expands, or if our rulers shrink,as long as the two theories makes the same predictions.

A difference that makes no difference is no difference, at least as far as science is concerned. (It may make a difference to philosophy. The fact that philosophy can't be settled by experiment is one reason philosophical arguments tend to go on for a long time).

On a similar note, most theories of quantum gravity may be formulated in totally different terms than relativity. It doesn't really matter as long as they make the same predictions. The interesting thing is that under extreme enough conditions, quantum gravity does make different predictions than GR. The practical problem is that it is generally difficult to create conditions extreme enough where these differences would matter.

 

[TR345]

Why bother making predictions about an accelerating universe if you don't care why it accelerates. Isn't the point of predictions and observations understanding.

 

[pervect]

What is understanding if it is not the ability to make accurate predictions?

The point is that theories that look different on the surface can make exactly the same predictions, and that as long as the predictions are accurate, there isn't any way to tell which of the ways is "right". In general, any formulation that yields identical results can be regarded as OK, and it is a matter of convenience or personal preference as to which way one choses.

To put it another way, science is about falsifiability, ala Popper.