[charli (OP)]

Listener David has a question:

"But is there any evidence that points towards A Big Bang - that this is not the first in a never-ending explosion/expansion/contraction cycle?"

Can you point him in the right direction?

[alancalverd]

The idea of a cyclic universe is certainly more intellectually satisfying and open to analysis than a single big bang, and is addressed to some extent in Stephen Hawking's "Black Holes and Baby Universes".

[Eternal Student]

Hi.    David is obviously aware of the various theories that involve cyclic cosmologies.
You've tasked us with trying to show our Big Bang was the first. 

   Well, how do you define "time"?   Many Physicist's will call upon Thermodynamics and entropy to define some properties of time.  If entropy isn't increasing then there's little reason to assume time is passing.   Where there is no space and no distribution of energy it's impossible to evaluate entropy.  So there doesn't have to be any time before the big bang.  If there isn't any time before the Big bang then the question about our Big bang being the first is obsolete,  there is no way to order other Big bangs by the thing we understand as time.

   Maybe David could start by looking at how Physicist's (and Philosophers if they wish) try to define time and then re-examine their original question.  If the time they are using is uniquely tied to the properties of our universe, then our Big Bang must have been the first with respect to that time.

[evan_au]

is there any evidence that points towards A Big Bang

Yes.
Cyclic models of the universe were considered feasible up to the 1990s.
- It was known that gravity would slow the expansion of the universe, but it wasn't known whether gravity was strong enough to bring it back together into a "Big Crunch" or "Gib Gnab", as it was called
- If it did come back together, a cyclic universe would be one possible outcome
- Studies in the 1990s showed that in fact there was an additional force causing the universe to expand, and that this new expansion force (called "Dark Energy") is now stronger than the contraction force of gravity
- Conclusion: The universe will not contract, and a cyclic universe is not an option

That means, in our universe, there can only be one Big Bang (provided the current dominance of Dark Energy over Gravity continues)
- That says nothing of multiple phases of the Big Bang in our universe (see Cosmic Inflation), or of what might happen in other hypothetical universes.

Many Physicist's will call upon Thermodynamics and entropy to define some properties of time.

One of my High School teachers commented that "Entropy applies everywhere, except maybe at the level of an atom, or the whole universe".
- By the universe, he may have been thinking of a Big Crunch, which (in some views) may have reset entropy. But we now know that a Big Crunch is not an option. So Entropy applies to the whole universe.
- Some theories suggest that the proton may decay (with some extremely long half-life). If so, entropy may also apply at the level of an atom.

[yor_on]

Well, I wouldn't define it from entropy ES, although I agree to your conclusions. If entropy is the ability to unsort ones socks, then it also should be able to define patches in where they don't get unsorted. Time as such is a one way arrow experimentally and practically, as far as I know, although mathematically defined sometimes something of nuisance it seems.