Big Bang Theory
According to the Big Bang theory the universe is still expanding, my question is where does it get heat to expand as expansion needs heat?
Outside United States science education
According to our local physicist:
The expansion of the universe isn’t usefully analogous to, say, the expansion of a gas in a container. In the case of a gas, it needs energy to expand against the environment. In the case of the universe’s “expansion”, there is no environment into which the universe is expanding. The expansion of the universe is the expansion of space-time itself.