2 ms·

Wow, really? Isn't energy conserved? I would expect that expanding the volume of the box would have a negligible effect on the mass of the contents, whether the

Wow, really? Isn't energy conserved? I would expect that expanding the volume of the box would have a negligible effect on the mass of the contents, whether they are light or matter. And I say negligible only because my understanding is that even vacuum has a tiny amount of energy in it due to Heisenberg's uncertainty principle, so a box twice as large would contain a tiny bit more energy even if it was equally empty in both cases. What's the name of the phenomenon you describe? I'd like to learn more.

amluto 7y ago
Energy is indeed conserved [0], but you have to take pressure into account. Suppose you build a cylinder with a piston, just like in a car engine or a bike pump. Start with the piston in a position that puts the interior of the cylinder at half its maximum volume. For simplicity, imagine that the outside of the cylinder is in a vacuum [1]. Now put some gas, light, or dark energy in the cylinder and then pull the piston out to its full extension. You've doubled the volume, but you're also done some work by pulling out the piston. If the cylinder is full of gas or light, the pressure is positive, so you did negative work (received energy) by pulling out the piston, and the energy in the cylinder decreases. For dark energy, the pressure is negative (crazy, I know), and you had to pull hard to extend the piston, so you put energy into the cylinder and, in fact, doubled the total energy in the cylinder. [0] In general relativity, on large scales, conservation of energy is complicated. [1] With a real bike pump or car engine, nothing fundamental changes if you have air outside the cylinder, but when you're talking about cosmology, you start considering that the whole universe is full of more or less the same stuff and it gets a bit more complicated. It all works out in the end, though.

amluto 7y ago

Energy is indeed conserved [0], but you have to take pressure into account. Suppose you build a cylinder with a piston, just like in a car engine or a bike pump. Start with the piston in a position that puts the interior of the cylinder at half its maximum volume. For simplicity, imagine that the outside of the cylinder is in a vacuum [1]. Now put some gas, light, or dark energy in the cylinder and then pull the piston out to its full extension. You've doubled the volume, but you're also done some work by pulling out the piston. If the cylinder is full of gas or light, the pressure is positive, so you did negative work (received energy) by pulling out the piston, and the energy in the cylinder decreases. For dark energy, the pressure is negative (crazy, I know), and you had to pull hard to extend the piston, so you put energy into the cylinder and, in fact, doubled the total energy in the cylinder. [0] In general relativity, on large scales, conservation of energy is complicated. [1] With a real bike pump or car engine, nothing fundamental changes if you have air outside the cylinder, but when you're talking about cosmology, you start considering that the whole universe is full of more or less the same stuff and it gets a bit more complicated. It all works out in the end, though.