system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did we know that the Big Bang existed? Why can't the universe exist before the Big Bang, just at a smaller scale and expand forever since?
gpt
The Big Bang doesn't say what you think it does. It simply states that some point 13.8 billion years ago the universe was in a incredibly dense state. At this point the universe rapidly expanded and took on a new form. The forces that govern our universe like electromagnetism and gravity would become distinct thing, along with a distinction between matter and energy. Nothing before this point matters because it is outside the scope of science. We know that expansion happened because we can see evidence of it today. Both in the continued expansion of the universe and in the cosmic background radiation. We know when it happened by running back the clock back. We can roughly guess at what the universe was like immediately after T=0, but anything before that doesn't have to follow the laws of physics. The Big Bang only cares about the things immediately after that first moment.