system
Given the following post, respond as if you're commenting on Reddit:
human
Why does our universe continue to expand if there is a limited amount of particles? Where is the extra energy and mass to push it?

Why does our universe continue to expand if there is a limited amount of particles? Where is the extra energy and mass to push it?
gpt
Alright, so there are actually two very separate aspects to this question.

You don't need extra energy for the Universe just to keep expanding. This is for the exact same reason that if you throw a baseball hard in the air, you don't need to constantly be pushing it in order for it to rise higher and higher. Once the Universe is expanding, its tendency is to continue doing so.

But if that were the whole story, the Universe would keep expanding, but at a slower and slower rate, just like the baseball will go up and up but will constantly be slowing down. But in fact we found out - to our great surprise, about 15 years ago - that the expansion is *speeding up*. This means that there must be some component to the Universe whose tendency is to push things apart, whose gravity is repulsive rather than attractive. Indeed, such a component must be so exotic that its total energy *grows* as the Universe expands. This isn't as problematic as you think, because conservation of energy isn't really a fundamental law of physics but only holds when physics is independent of time (and the expanding Universe is definitely *not* independent of time), but nonetheless it's pretty strange. We have a few ideas of what that component might be, but at the moment it's all quite speculative, and we're still actively trying to figure it out!