[cvanuitert (OP)]

I assume most of you reading this are familiar with the most accepted theories on how our universe will come to it's inevitable ending. The crunch, rip, decay and freeze theories all have logical consistencies and scientists today are still debating amongst these four hypothetical endings. Lots of astrophysicists tend to agree on the big crunch, which is considered to be the reversal of the big bang. While it's still unclear to them in what time frame this would happen, we won't have to worry about it just yet. This theory implies that at some point, all the matter in our universe will return to it's previous state, a singularity. This raises some (maybe obvious) questions to me, which I hope one of you could answer as my interest in the origin of the universe seems to be gradually growing each passing day.

As all matter collapses back into a singularity, I assume a new big bang like event will take place. Fairly recently, a scientist (forgot the name) stated that before the big bang a different universe might have existed before ours. The ending of which resulted in the creation of our current universe. The well known footprint of this event is cosmic microwave background radiation. However, considering our universe is just one amongst infinite others. One would think that like ours, other universes might go through the same hypothetical process. My question is, could this mean that other universes go from singularity to an expanded universe and back? And should this happen, do you think there's any hypothetical way to detect or measure other universes going or having gone through this process?

As you probably might be able to tell, I'm anything but a scientist. But that doesn't mean I don't share your interests. These questions spontaneously popped into my mind and I attempted to find some articles and/or papers mentioning my current dilemma, but with no current success. I hope one of you might be able to explain this or refer me to other sources discussing the topic.

[Halc]

The crunch, rip, decay and freeze theories all have logical consistencies and scientists today are still debating amongst these four hypothetical endings. Lots of astrophysicists tend to agree on the big crunch

Some may still be debating, but only a slow heat death model fits the observed data. Yes, I know Penrose is still pushing a cyclic model, despite the observed acceleration of expansion and the violation of thermodynamic law implied by such a model.

The ending of which resulted in the creation of our current universe.

A subsequent cycle is still the same universe, not a new one. By definition universes cannot interact, else they'd each be part of the physics of the other and thus the same universe.

The well known footprint of this event is cosmic microwave background radiation.

The CMB is an artifact of the recombination period, which happened hundreds of thousands of years after the big bang, not something resulting from an interaction from a prior cycle.  OK, it is theorized that non-uniform state of a prior cycle can give rise to nonuniformity in the next, which in turn can effect the CMB indirectly, so while the cause of that nonuniformity is now well known as a prior crunch, I can at least agree that if there was a prior crunch that somehow could violate thermodynamic law, the CMB would plausibly be affected by it.

However, considering our universe is just one amongst infinite others. One would think that like ours, other universes might go through the same hypothetical process. My question is, could this mean that other universes go from singularity to an expanded universe and back?

Assuming these other universes exist by however you define such things, yes, definitely there are some that crunch back, and evolve from there in whatever way it does. Some might do it almost immediately, giving no time for anything interesting to form.

And should this happen, do you think there's any hypothetical way to detect or measure other universes going or having gone through this process?

By definition, no. One universe cannot measure another.

[evan_au]

Lots of astrophysicists tend to agree on the big crunch,

Not since the discovery of the accelerating expansion of the universe, in the 1990s.
See: https://en.wikipedia.org/wiki/Accelerating_expansion_of_the_universe

There is a hypothesis that during the Big Bang, there was a period of "inflation", where the size of the universe grew very rapidly.
- This left an imprint on the Cosmic Microwave Background Radiation
- The rules of physics would have been radically different before that - for example, it is thought that the 4 fundamental forces that we know today may have had similar strengths (today, gravitation is far weaker than the other forces). Effectively, it would have been like a very different universe
- There is a hypothesis that this could happen again, with runaway expansion - the Big Rip
- And in fact, vacuum energy could produce a series of such state changes in the universe, with perhaps different physics in each epoch.
See: https://en.wikipedia.org/wiki/Inflation_(cosmology)
https://en.wikipedia.org/wiki/False_vacuum

Whether we can detect the influence of other universes from inside our own depends on how well isolated different universes are.
- There have been suggestions that maybe there are subtle effects in our universe that can't be explained by things that we can see within our own universe, but you would have to say that is very speculative
- After all, we have Dark Matter & Dark Energy that we don't have good explanations for, as yet, and these do have good observational evidence for their existence...

[cvanuitert (OP)]

Thank you both for your in depth explanations, this is such an interesting topic. I'll attempt to absorb this information and keep posted on new data and theories on the subject.