Evidence for a Cyclic Universe

[117]

http://physics.princeton.edu/~steinh/lambda16.pdf

In this research article the authors suggest a cyclic universe, specifically one involving collisions of higher dimensional branes (an idea taken out of string theory), could indirectly explain why the observed cosmological constant is so small. I personally like this idea because I find it much more logical than the random anthropic interpretation which posits that fine-tuned universes are very rare; mostly as it provides a mechanism for which the cosmological constant can be fine-tuned so life can exist.
However cosmological theories involving higher dimensions and stuff outside of what we can observe seem to be notoriously hard to verify.
I am curious whether a cyclic model would leave a smoking gun; something we could use to prove that the actual universe is much older than the observable universe. Not necessarily just for the string theory cyclic model.
Also, this may sound like a dumb question, but supposing the universe is infinitely old, would it be possible to see 'before' the big bang.

 

[bapowell]

I am curious whether a cyclic model would leave a smoking gun; something we could use to prove that the actual universe is much older than the observable universe. Not necessarily just for the string theory cyclic model.

All we have is the observable universe to observe, so we can say nothing about the "actual universe". There are observational consequences of cyclic models, however. In particular, these models do not produce primordial gravitational waves and so can be falsified with a detection of primordial B-modes in the CMB.

Also, this may sound like a dumb question, but supposing the universe is infinitely old, would it be possible to see 'before' the big bang.

If the universe was infinitely old, would it have a big bang?

 

[skydivephil]

It depends on how you define big bang. If you define big bang as an evolution from a hot dense state then an infinitely old universe could easily have a big bang. In many cosmological models there are is a period of contraction prior to a period of expansion. This sort of evolution still has a big bang in some sense but not in the singularity sense. these universe could be infinitely old.

 

[Chronos]

It is worth asking about observational consquences in an infinitely old universe. Let's take the cyclical model of example. In such models the universe undergoes an unending series of reincarnations. One problem is every last scrap of matter from the previous cycle must be recycled in the next bang otherwise these remnants would accumulate and an infinitely old universe would become dominated by ashes [e.g., black dwarves] after a finite number of cycles. It's not just matter that needs to be accounted for, you also have the well known entropy problem. With each new cycle the universe must find a way to shed the vast entropy it acquired during the previous iteration. Otherwise an infinitely old universe would become dominated by entropy sinks [e.g., black holes] after a finite number of cycles. Thus, given an excess of black dwarves and black hole are not observed, can we safely conclude the cyclical proposition survives its observational challenges? If we ponder carefully the warts begin to show. There is no obvious reason an infnitely old universe should not be spatially and energetically infinite. Such a universe would have a natural right to demand an infinite amount of time to reclaim all of its expended energy [given it is scattered across an infinite volume of space]. Temporality, it would seem is the achilles heel of a cyclical universe. We have plenty of reasons to believe this incarnation of the universe is of finite age. This same temporality can also account for the lack of black dwarves and why it is not teeming with black holes.

 

[skydivephil]

Which cyclic model are you talking about? Ekpyrotic? CCC? VSl? Baum Frampton? Higgs Bang? There are many cyclic models so one should not talk of cyclic model generically.
Moreover one does not even need to assume a cyclic universe to assume an infinitely old universe. Many models of quantum gravity such as string gas comsology, loop quantum cosmology, pre big bang and Horava lIfshitz gravity seem to predict a contracting universe mirroring our expanding universe. The past could be infinite but there is no cyclic behaviour.

 

[117]

thank you everyone, good point phil. I was too general and don't really know :P

 

[Chronos]

A cyclic universe that does not repeat is not cyclic - can we agree on that? Mirror universes are not cyclic, merely magic.

 

[skydivephil]

Yes agreed a cyclic universe by definition repeats. Can we also agree that a cyclic universe is not the only model where the universe is infinitely old ? Can we also agree that there are many different types of cyclic universe so if you are going to critique one you need to specify which one it it and not assume your critique applies to all models as they are often quite different?

Your comment that mirror universes are magic is not one I think that deserves to be on this forum. Giving a serious critique of proposals in the scientific literature is perfectly reasonable, simply dismissing them as magic is not.

Most cosmologists agree we need a quantum theory of gravity to understand the true nature of the big bang. Given so many different independent approaches to quantum gravity predict a mirror universe this makes this idea a plausible one. Of course plausibility is not the same as verified by experiment. But there is no experimental evidence that there is a singularity at the big bang either. Yet many scientists basically mislead the public all the time by saying rubbish like the universe is 13.8 billion years old. That is not correct, its 13.8 billion years since the big bang. But the justification for saying the big bang is the beginning of the universe is extremely dubious. It assume GR is a valid description of gravity all the way to the PLanck scale, something virtually no professional cosmologist would agree to. Yet we keep hearing the same wrong information from professional scientists. So let's change the record , the universe is of unknown age. It is also of unknown size. Both the age and the size of the universe may be infinity, we don't know. There was an event, the big bang, that represents a barrier to our knowledge of the past. Right now we can't probe beyond this barrier. But that does not imply we will never be able to do this. There are currently serious cosmologists who are trying to do this. For example here:

http://arxiv.org/pdf/1504.07559.pdf

or here:
http://arxiv.org/abs/1505.02381

Go ahead and critique this and other such works if you wish. I have nor problem with that , they may well be wrong. But simply dismissing them as magic is not a serious contribution to the discussion.