What would a big crunch look like observationally?

  • I
  • Thread starter Jarvis323
  • Start date
In summary, the conversation discusses the possibility of a contracting FLRW universe and whether the laws of physics would change in such a scenario. It also explores the concept of simultaneity and how it relates to the contraction of the universe. The experts conclude that a contracting universe does not necessarily require a change in the laws of physics, but the FLRW model may not be sufficient to fully describe the complexities of the universe at smaller scales.
  • #1
Jarvis323
1,243
986
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?
 
Space news on Phys.org
  • #2
Jarvis323 said:
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.

Jarvis323 said:
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.

Jarvis323 said:
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.
 
  • #3
Orodruin said:
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.
 
  • #4
Jarvis323 said:
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.

Jarvis323 said:
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.

Jarvis323 said:
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.
 
  • Like
Likes Jarvis323
  • #5
Orodruin said:
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?
 

1. What is a big crunch?

A big crunch is a hypothetical scenario in which the universe collapses in on itself due to the force of gravity, ultimately leading to a singularity.

2. How would we observe a big crunch?

We would observe a big crunch by studying the movement and behavior of galaxies and other celestial objects. As the universe collapses, the space between objects would shrink, causing them to move closer together and potentially collide.

3. What would happen to the Earth during a big crunch?

During a big crunch, the Earth and all other matter in the universe would be pulled towards the center of the collapsing universe. Eventually, everything would be compressed into a single point, resulting in a singularity.

4. Is a big crunch likely to happen?

Currently, the most widely accepted theory is that the universe is expanding at an accelerating rate, making a big crunch unlikely. However, there is still much debate and research being done on the ultimate fate of the universe.

5. How would a big crunch affect time and space?

A big crunch would likely cause time and space to become distorted and eventually cease to exist as we know it. As the universe collapses, the laws of physics would break down, making it difficult to predict exactly what would happen to time and space.

Similar threads

  • Cosmology
Replies
3
Views
820
Replies
9
Views
986
  • Cosmology
Replies
28
Views
2K
Replies
29
Views
2K
Replies
2
Views
947
Replies
54
Views
3K
Replies
19
Views
2K
  • Cosmology
Replies
24
Views
2K
Replies
15
Views
1K
Back
Top