Universe losing energy?

[waht]

So when photons are created they carry the energy away from the system at the speed of light,

so basically can you say that since most processes in the universe like the star formations involve the exchange of photons, as a result the total energy of the universe is decreasing because recovering that energy is virtually impossible since the universe is so large,

Although, convervation of energy is still preserved, but it all turns into something you can't really recover.

[chroot]

In a homogenous universe that is not expanding, the energy density of any large region of space is constant. Some photons from distant places enter each such region, while others leave that region for other distant places. The net change in energy density is zero.

Since the universe is expanding, the energy density is constantly going down.

The actual sum of the universe's energy must, however, be constant, because energy cannot, by definition, enter or leave it.

- Warren

[SpaceTiger]

So when photons are created they carry the energy away from the system at the speed of light,

so basically can you say that since most processes in the universe like the star formations involve the exchange of photons, as a result the total energy of the universe is decreasing because recovering that energy is virtually impossible since the universe is so large,

Although, convervation of energy is still preserved, but it all turns into something you can't really recover.

We generally include these emitted photons in our definition of the universe, so this wouldn't really correspond to it losing energy. Rather, the concept you describe bears somewhat of a resemblance to the second law of thermodynamics and the ever-increasing entropy of the universe.

As for the conservation of total energy in general relativity, things can get a bit tricky. Here's a link that explains the problem in more detail:

Is Energy Conserved in GR? (http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html)

[Garth]

As for the conservation of total energy in general relativity, things can get a bit tricky. Here's a link that explains the problem in more detail:

Is Energy Conserved in GR? (http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html)
Indeed ST, that is a good link, as Weiss and Baez said: it depends on what you mean by "energy", and what you mean by "conserved".


In general in GR it is energy-momentum that is conserved and not energy. Energy, as well as energy-momentum, is conserved if there is no space-time curvature, that is why in the spherically symmetric case energy conservation only works "at inifinity", and then only when gravitational waves have been eliminated from the equation, which is why it has to be a "null inifinity".

In the cosmological case the requirement for homogeneity and isotropy regains the conservation of the total energy of particles but cosmological expansion excludes the conservation of energy of photons, hence energy is simply 'lost' from the CMB.

The problem is that to define energy you have to specify the frame of reference in which it is measured, as energy is a frame dependent quantity. You cannot then translate that frame (parallel transport it) to a different part of curved space-time, such as a later time in an expanding universe, unless there is a time independent Killing Vector. These do not in general exist, although Robertson-Walker spacetimes admit a conformal Killing vector normal to the spacelike homogeneous hypersurfaces.

Garth

[Chronos]

Only a handful of people will grasp the meaning of that discussion. Garth, I think, missed the point. Specifically, there is a tensor thing he might have overlooked.