Well, energy conservation isn't terribly simple in curved space-time. If you want a good, fairly in-depth description of what's going on here, take a look at this:kasperm said:How is energy conserved in an expanding universe? As space expands between, say, stars in a galaxy, don't they gain potential energy in the gravitational field of the galaxy? Which mechanism lessens the total kinetic energy?
Not quite. Normal matter does this because it is largely pressureless. But any matter that experiences pressure (such as photons) loses energy per comoving volume.Chronos said:I'm with you edpell. IMO, the energy content of the universe is essentially fixed [save for dark energy]. Expansion merely dilutes it.
Well, there are a couple of different issues here. First, if the two are in orbit around one another, they won't be affected at all by the expansion. If the two are far away, and there is no dark energy, then they will simply coast along away from one another, slowing down the entire way. It's only with the existence of dark energy that things start to get a little weird.kasperm said:Im sorry, my question might have been a bit unclear.
Consider this: Two stars a unit distance apart has a well defined potential energy in their gravitational field, or equivalently, a well defined amount of work needs to be done against the field to move each of them from the center of mass out to their respective positions. If we wait a given amount of time and measure their distance once again, it will have increased due to the expansion of the universe so the amount of work need to move them from CoM to their new positions has increased proportionally.
Well, bear in mind that this statement is only true for a closed universe.yogi said:Ed Tryon was probably the first to advance the idea that the total energy of the universe is always zero - specifically the potential gravitational energy at all times is balanced by the expansion energy. Within the Hubble sphere, equating the 1/2 mv^2 energy of each shell of the expansion using Hubble's law to the potential results in a required matter content equal the critical density of an Einstein - de Sitter universe
Chalnoth said:the fundamental flaw with your analysis is that the potential energy is not well-defined in General Relativity.
Well, locally, any space-time is flat. In flat space-time, energy is conserved.kasperm said:This makes sense. So this is why people say energy is conserved on a local scale but not on large scales?
Energy conservation in an expanding Universe refers to the principle that the total energy in the Universe remains constant, even as the Universe expands. This is based on the law of conservation of energy, which states that energy cannot be created or destroyed, only transformed from one form to another.
The expanding Universe does not violate the law of conservation of energy. As the Universe expands, the energy density decreases, but the total energy remains constant. This means that although the Universe is getting larger, the amount of energy within it remains the same.
Energy conservation is important in an expanding Universe because it helps us understand the fundamental laws of the Universe and how it evolves over time. It also allows us to make predictions about the future of the Universe and the fate of energy within it.
No, energy cannot be created in an expanding Universe. The law of conservation of energy states that energy cannot be created or destroyed, only transformed. Therefore, the total energy in the Universe remains constant, even as it expands.
Dark energy is a mysterious force that is thought to be responsible for the acceleration of the expansion of the Universe. While its exact nature is still not fully understood, it does not violate the law of conservation of energy. It is believed that dark energy is a property of space itself, and does not add or subtract from the total energy in the Universe.