How can the universe get "colder"? Does it just mean that the average energy-per-square-meter (or whatever unit) is less? If so, wouldn't that mean that saying "less dense" and "colder" mean basically the same thing at that scale?
Maybe I'm asking something stupid and obvious, but I keep thinking that energy can't be destroyed, ergo, there's exactly as much energy (and thus heat?) as before. Or is heat energy density? But doesn't that make "less dense and colder" a redundant statement, since they'd be the same thing?
Less dense implies colder, as the volume is not constant, but expanding. Temperature and density are different things though - one is how much energy something has, the other is how far away from other things it is. Related, but different.
With respect to destroying energy - energy converted into matter is 'colder'. There's not the exact same amount of energy - it's not that it can't be destroyed, it must be conserved with matter (E=MC^2). Great example - kinetic energy is used in a particle accelerator to create new particles, and it takes a ton of energy to make even a tiny bit of mass, so the average temperature would go down as more mass is created (heat being the transfer of energy - once it's trapped in a particle, it's not transferred as freely).
How does converting energy into mass make it "colder"? If you have 1 protons worth of energy in a volume, and you have 1 nonmoving proton in another area with the same volume, isn't there the same amount of energy in both regions? Or is it "colder" because you can't have a region with energy and nothing else, it must be something like kinetic energy, which gives heat?
Temperature is the average energy of the particles in a space. So if the proton isn't moving, it is at absolute 0 temperature even though it has some mass-energy left.
Yes, heat is energy density which is why pressure is basically the same thing as heat. That's why a pressure cooker can generate incredible pressure just by heating it. Similarly, your refrigerator cools things by expanding a liquid into a gas (state change is actually the dominant mechanism in a refrigerator but it would work in principle with out the state change)
How can the universe get "colder"? Does it just mean that the average energy-per-square-meter (or whatever unit) is less? If so, wouldn't that mean that saying "less dense" and "colder" mean basically the same thing at that scale?
Maybe I'm asking something stupid and obvious, but I keep thinking that energy can't be destroyed, ergo, there's exactly as much energy (and thus heat?) as before. Or is heat energy density? But doesn't that make "less dense and colder" a redundant statement, since they'd be the same thing?