Conservation Principle Related to Expanding Space?

[jrdev]

Could someone comment on this:

Since space has been expanding since the Big Bang, does this mean there is a conservation principle such that some other property of the universe must be declining? If so, what is that principle and what would be declining?

Thanks.

 

[Nereid]

Welcome to Physics Forums jrdev!

1) Pick your theory, and ye shall be answered!
2) If it's GR, then the folk in the SR&GR section of PF will surely give you an answer in a Planck second! (so I'm moving this thread there).

 

[Crosson]

If energy is conserved, and space is expanding, than energy density is constantly decreasing (same amount of stuff spread out into a bigger space).

But energy is not conserved in an expanding universe, so oh well.

 

[Garth]

Energy-momentum is conserved in a GR understanding of the universe. If there is no pressure (a dust filled universe) then energy is conserved as measured in the cosmological comoving frame of reference.

The energy of the CMB is not conserved in such a universe as radiation pressure is equal to one third the radiation density.c².
Garth

 

[yogi]

In the zero energy universe, first suggested by Ed Tryon about 50 years ago, the positive energy is always balanced by the negative potential energy. This leads to a critical density universe, that is, the gravitational potential GM/r is always equal to the Kinetic Energy (1/2M)(v^2). There are however literally hundreds of models of the universe - in some energy is conserved - in others it is not.