Expansion

[geordief]

Since the Big Bang I understand that the distances between galaxies and ,generally regions not bound together gravitationally have been  increasing in an accelerating way. 

I have seen this process described as either space expanding or space-time expanding.

Which is the more correct way of describing it?

To my mind it should be the former.

I see space time as a mathematical model (and don't see how a model can expand)

I see space as the distances between objects and can understand how these measurements can be continuously  increasing.

On the other hand  I think I can also see that the space time intervals between the galaxies might also be increasing.....

Edited December 10, 2023 by geordief

[Genady]

1 minute ago, geordief said:

To my mind it should be the former.

You are correct.

5 minutes ago, geordief said:

Since the Big Bang I understand that the distances between galaxies and ,generally regions not bound together have been  increasing in an accelerating way. 

Not since the Big Bang. It started accelerating only several billion years ago: Evolution of the universe - Expansion of the universe - Wikipedia

[KJW]

22 minutes ago, geordief said:

I have seen this process described as either space expanding or space-time expanding.

Which is the more correct way of describing it?

To my mind it should be the former.

 

20 minutes ago, Genady said:

23 minutes ago, geordief said:

To my mind it should be the former.

You are correct.

Actually, on Page 4 of the thread "Cosmological Redshift and metric expansion", I have shown that expanding space and expanding spacetime are interconvertible by a coordinate transformation, and therefore they are actually the same thing and neither is more correct than the other.

 

[Markus Hanke]

6 hours ago, KJW said:

expanding space and expanding spacetime are interconvertible by a coordinate transformation, and therefore they are actually the same thing and neither is more correct than the other.

While this is of course true, I think it’s very important to remember that such a transformation changes the physical meaning of the time coordinate - it will no longer correspond to a clock co-moving with the cosmological fluid.

[geordief]

6 hours ago, Markus Hanke said:

While this is of course true, I think it’s very important to remember that such a transformation changes the physical meaning of the time coordinate - it will no longer correspond to a clock co-moving with the cosmological fluid.

Which is which?

The space time expansion is the one where the clock does co-move with the cosmological fluid?

But if we are talking only if space expanding  this is not so?

First time I have heard mention of "cosmological fluid"...

 

[Genady]

2 minutes ago, geordief said:

Which is which?

The space time expansion is the one where the clock does co-move with the cosmological fluid?

But if we are talking only if space expanding  this is not so?

First time I have heard mention of "cosmological fluid"...

 

The space expansion is the one where the clock co-moves.

[KJW]

26 minutes ago, geordief said:

Which is which?

The space time expansion is the one where the clock does co-move with the cosmological fluid?

But if we are talking only if space expanding  this is not so?

First time I have heard mention of "cosmological fluid"...

Actually, the clock is co-moving with the cosmological fluid in both expanding space and expanding spacetime. But what @Markus Hanke is saying is that only in the expanding space does the time coordinate correspond to the proper time indicated by a clock co-moving with the cosmological fluid.
 

[Markus Hanke]

31 minutes ago, geordief said:

First time I have heard mention of "cosmological fluid"...

It’s called that because the energy-momentum tensor that corresponds to this metric (FLRW) is actually one describing a perfect fluid, with its constituents being the galaxies, clusters etc that make up the cosmos. So FLRW is basically an interior fluid solution on a very large scale.

4 minutes ago, KJW said:

But what @Markus Hanke is saying is that only in the expanding space does the time coordinate correspond to the proper time indicated by a clock co-moving with the cosmological fluid.

Indeed. This coordinate system is called Gaussian normal coordinates. 

[studiot]

18 minutes ago, Markus Hanke said:

It’s called that because the energy-momentum tensor that corresponds to this metric (FLRW) is actually one describing a perfect fluid, with its constituents being the galaxies, clusters etc that make up the cosmos. So FLRW is basically an interior fluid solution on a very large scale.

A perfect fluid is one with no turbulence and no viscosity.

[KJW]

7 hours ago, Markus Hanke said:

While this is of course true, I think it’s very important to remember that such a transformation changes the physical meaning of the time coordinate - it will no longer correspond to a clock co-moving with the cosmological fluid.

In the coordinate system of expanding space and time, if one has a light clock based on a pair of mirrors that are at rest in the co-moving frame, then this clock will tick coordinate time and not proper time.