What would a big crunch look like observationally?

[Jarvis323]

Suppose one day the expansion of the universe halted, and then reversed towards a big crunch.

1) Do the laws of physics change? How?

2) Would it be the case that the change from expanding to contracting occurs everywhere instantaneously, or could some parts of the universe begin contracting before others?

2) Supposing light propagates the same way that it does now, what would be the observational effects for objects that are far enough away, that the distances between us and them decreases faster than the speed of light?

 

[Orodruin]

Suppose one day the expansion of the universe halted, and then reversed towards a big crunch.

1) Do the laws of physics change? How?

So the question is essentially "Suppose the laws of physics change. How would they change?" The tautological answer would be that it depends on how you changed the laws of physics.

A better way of phrasing your question might be to ask how things would appear in a contracting FLRW universe.

2) Would it be the case that the change from expanding to contracting occurs everywhere instantaneously, or could some parts of the universe begin contracting before others?

There is no such thing as "instantaneously" as simultaneity is a convention in relativity and not something that is measurable. In a FLRW universe that has a sufficient mass or radiation density to start contracting, the scale factor is a global property and contraction would start at the same cosmological time. But the FLRW universe has many assumptions that go into it and so the question is not really clear.

2) Supposing light propagates the same way that it does now, what would be the observational effects for objects that are far enough away, that the distances between us and them decreases faster than the speed of light?

You will not be able to observe them "now". You are observing what is on your past light cone. Depending on the exact contraction of the universe, which would be governed by the Friedman equations, there may be regions of the universe from which the light will never reach you before the crunch.

 

[Jarvis323]

So the question is essentially "Suppose the laws of physics change. How would they change?" The tautological answer would be that it depends on how you changed the laws of physics.

A better way of phrasing your question might be to ask how things would appear in a contracting FLRW universe.

I suppose what I meant, is whether the laws of physics necessarily have to change in order for a contracting universe to workout. Or, based on what we know, is there a reason to expect that the laws of physics would change in such a case. I suppose that the answer is, essentially, we have not much of a clue? We are already out pretty far on a limb with current theory. I would prefer not to phrase the question absolutely in terms of an FLRW universe if I had a choice. I suppose very few people exist that could give insightful answers outside of FLRW frameworks though.

There is no such thing as "instantaneously" as simultaneity is a convention in relativity and not something that is measurable. In a FLRW universe that has a sufficient mass or radiation density to start contracting, the scale factor is a global property and contraction would start at the same cosmological time. But the FLRW universe has many assumptions that go into it and so the question is not really clear.

I don't like to ascribe to the opinion that measurability or crude theory are arbitrators of existence. I realize FLRW is a model that uses fairly major simplifications that certainly hold only at large scales. So is it likely that, in reality, there are differences on smaller scales that could result in local differences in expansion rates throughout the universe? Then, if so, there could be differences, in reality (as opposed to according to FLRW), when contraction begins depending on location? In that case, I assume you would have interesting things happening locally, and wonder what those would be.

 

[Orodruin]

is whether the laws of physics necessarily have to change in order for a contracting universe to workout.

This is a better defined question and the answer is no. What you would need is a different energy composition of the universe.

Or, based on what we know, is there a reason to expect that the laws of physics would change in such a case.

Now you fell back into the question ”based on the laws of physics, is there a reason to change the laws of physics”. There are some instances where you take a theory to its limit and discover inconsistencies. A contracting universe is not one of those.

in local differences in expansion rates

You would have to define what you mean by this. Without the global view, the split of spacetime into space and time does is not necessarily well defined and without such a split your question lacks some of the necessary foundations.

 

[Jarvis323]

You would have to define what you mean by this. Without the global view, the split of spacetime into space and time does is not necessarily well defined and without such a split your question lacks some of the necessary foundations.

So essentially, FLRW is a global view which lays foundations for slicing up spacetime, but it also assumes away local differences. So this is why we don't have any good physics for describing these aspects of the universe?