# Where does the kinetic energy come from if galaxys are moving apart due to the expansion of the universe?

We know the universe is expanding because of the red shift of far away galaxies, the further away the galaxies are the faster they recede. We also know that the energy in the universe (mass and other forms) will slows the expansion. If there is enough energy we will end up with the big crunch and if not enough, the slow death. (Please correct me if there is a fundamental error in the above)

So it seems to me that if there was less mass energy (or at least less than currently) in the universe then the the size (volume) of the universe would have been greater than it is due to the (Mass) energy it currently has ( ie a larger volume, 13 billion years after the big bang.)

Now if space ("the vacuum") has energy, (ie so and so joules per cubic meter,) then a larger volume would have a larger energy content. (assuming the same energy density of course.)

Assuming that energy can't be created or destroyed so that the total energy of the universe is constant ( maybe a dodgy assumption ...but anyway..) Then the total vacuum energy of the universe is less due to the matter and stuff in the universe than the vacuum energy would be if there was less matter and stuff in the universe. (assuming the energy density of the vacuum is the same in the two scenarios.).

But all the galaxies are receding from each other, so they are all ncreasing their kinetic energy (ie moving faster).

So on the one hand we have more kinetic energy (moving galaxies) and on the otherhand less vacuum energy (smaller volume due to the effects of mass/gravity)

So my question is, could mass somehow be converting the vacuum energy into the kinetic energy?

ie Kind of sucking space into mass which would cause effects similar to gravity , and of course, the moving space would give rise to different geometries of the space much like the different geometry of water flowing down a plughole vs when the plug is in place and as such "bends space" to give gravity ala general relativity.

• Read Dark energy here at en.wikipedia.org/wiki/Dark_energy – SchrodingersCat Oct 7 '15 at 16:15
• – John Rennie Oct 7 '15 at 16:29
• So Dark enregy is causes the expansion, but we have no idea what it is which is why it is dark, so that doesn't really explain anything, and if energy isn't conserved in an expanding universe, then where does the "new" energy come from.... Perhaps you can quote the experiment that "proves new energy is being created all the time int he expanding universe"? – andy Oct 8 '15 at 12:04
• And "dark energy" creates or destroys energy in the universe so energy is not conserved? Are you trying to tell me this is a scientific proven fact.. Is it not just a proposition, because nobody understands what we are seeing. – andy Oct 8 '15 at 12:16
• Conservation of energy is a local phenomenon in general relativity. It cannot be applied to a collection of different inertial frames. – Rob Jeffries Dec 24 '16 at 20:23