But the speed of light changes. It can be slowed down. It just doesn't change while moving through outer space (a vacuum). The maximum speed is the constant.

The works of Roger Penrose have shown that it's conceivable or potentially even provable that at the very largest scales of time and space, there is no meaningful difference between the accelerating "cold" end of our universe and the collossal expansion that began the universe as we know it, and in fact those two states are perpetually cycling, birthing new universes from the explosion of old ones. This is based on the idea that when there is no more physical mass in the universe, you can look at the universe from a reference frame that only looks at the geometry of the energy expanding through space and it's identical to the beginning states.

I would recommend PBS Spacetime youtube channel for a lot better explanations of conformal cyclic cosmology than my feeble mind can try to relate.

Maybe there were other big bangs, but we need evidence of that, and that evidence doesn't exist.

Jyst saying "but we don't know" isn't a replacement for evidence.

Then the other more applicable point is that what makes you think "the big bang" was the first big bang?

Well, again you're using terms of time to describe the birth of time, so no that's not what I think because that statement doesn't make sense. But I'm being pedantic, I'm sure you meant "what if our's wasn't the only big bang?" And to that I can confidently say "maybe?". It's an interesting question but it's just not a scientific question. According to big bang theory, our universe, space-time and all the matter and energy in it, began with the big bang and we still exist inside it. Other big bangs, if they exist in some higher medium, are simply outside our scope. We just can't design tests to answer those questions. Best we can tell scientifically is where our universe started.

You think mass and entropy and radioactive decay and all this shit in the nothingness of space all started with "the big bang" but it only happens once and then in a ridiculously long time from now when everything reaches absolute 0 and there's no energy left anywhere, that it's just done? A one trick pony?

Again maybe? You're kinda putting words in my mouth. Idk if our universe is the only one, it's impossible to know. My original point was that time as defined by general relativity could not exist before the big bang because it was itself a product of the big bang.

I'm not a physicist, I don't know one way or another. But it's possible that there's a leading explanation for the formation of the universe based on a mathematical model that predicts exactly one big bang.