Why did the universe cool when it expanded ?

Question

it is said everywhere "as the universe expanded, it cooled and so........" whats the physics behind why the universe cools when it expands?

many say that it does so the same way a hot gas cools when it expands

if this is so, why does the gas cool when it expands?

Answer 1

When Volume increases, the heat per unit volume decreases.

Thus, the body cools.

Answer 2

All I know is that when things are under extreme pressure, the temperature tends to go way up. And when things are that close to each other, they rub against each other and create heat, (like when you rub your hands together). That's all heat is - particles rubbing together. So if the tiny point that contained all the matter in our universe expanded and became less dense, one would expect the temperature to decrease (because the particles aren't right up next to each other and aren't rubbing together). I don't know the specifics behind it, but if the "stuff" has more places to go, it's further away from each other, so yea... I'm sorry if that sounded incoherent :P

Answer 3

PV = nRT yo.. assuming of course the universe is ideal, which we all know it isn't, at least not mine :P

another stray thought I might add, and I should point out that this has no scientific foundation whatsoever and I am basically talking out of my ***, but you have to also I think consider all the energy that is being radiated by light. I would guess that most of the photons that are produced by the mass in the universe never get re-absorbed because they just fly off into nothingness. Now, we all understand conservation of energy, but if the universe is expanding faster than the radiative light can travel, then of course those photons are still part of the universe and don't violate CoE, but they no longer contribute to the tempertature of the universe because that energy is no longer bound up in mass. So over time, the net balance of bound energy should decrease as light is radiated but not re-absorbed, and thus the universe should cool. I'm probably full of ****, but it would sound good to someone who doesn't know any better :D