system
Given the following post, respond as if you're commenting on Reddit:
human
How certain is the scientific community actually about the big bang theory?

We have been observing the expansion of the universe for only a very limited duration, given the assumed age of the universe and we are extrapolating this behavior until the start of time. I mean anything could happen during the time we are not observing. The function we are measuring could be as well a wave, but the time frame of our existence wouldn‘t be long enough to ever measure a compression of the universe. My point is, are there any factors, that makes it certain that the universe is ever expanding and how certain are scientists about the big bang theory.
gpt
We actually get to see almost the entire history of the universe. Light doesn't travel instantly, so when we look at very distant galaxies, we are seeing them as they were billions of years ago - not as they are now. And as we can see these galaxies, we can observe that the speed of light appears to be unchanged in these distant galaxies - i.e. we can see that the speed of light hasn't changed over time, at least not by much. The speed of light is an important constant that affects the chemistry of elements. But we can get a peek into the chemistry of distant galaxies, because specific atoms and molecules in specific states emit light at very specific frequencies. If the speed of light was radically different, then we'd be able to see radical changes in the emission of these specific elements, and we don't see that.

From our picture of the history of the universe, we see that the universe is *evolving* in an irreversible way. Gas gets turned into stars, and much of that gas stays in stars - so the star:gas ratio increases over time. Hydrogen and helium gets turned into heavier elements, so we see fewer and fewer heavier elements as we look at more and more distant galaxies. Galaxies merge with each other, and don't separate apart from each other again, and so on. We see lots of different pieces of evidence that all support the idea that the universe has been expanding and evolving since a fixed point in the distance past, rather than either being eternal and unchanging, or very young.

And of course, we can directly measure the expansion of the universe with redshifts, and see that the galaxies are indeed mostly moving apart from each other, and the more distant they are, the faster they are moving away, to the point where it utterly dominates the large-scale motion of galaxies in the universe. We also see, in the very distant universe, a nearly featureless opaque "wall" of material. This is the cosmic microwave background, and it's difficult to explain how such background emission could be so featureless, and such a perfect blackbody, unless it is the final emission from the very early universe, as it transitioned from being opaque and hot and dense into being transparent and cooler and less dense.

Now, there's plenty of wriggle room in debating what the origin of the universe was, and what happened *before* the big bang and so on. But the idea that the universe has expanded from a hot dense nearly-uniform state at some finite point in the past is very robust.