Originally posted by hellfire
This may be a simple question but I didn¡¯t found anything in my references: the vacuum energy density is a constant value, which depends on the virtual excitations of the quantum fields. If the universe expands, increases its volume, and the vacuum energy density remains is constant, does this mean (asuming energy conservation) that an additional input of energy is needed to maintain the vacuum energy density?
Thanks.

Ok I need to place this here:
http://uk.arxiv.org/PS_cache/hepth/...12/0312099.pdf
Just found this today 16th Dec.
Just an extract from the paper:For IR ¡_ H0, today¡¯s Hubble scale, we have −1 UV ¡_ 1000 km, which is much larger
than the distances to which we have probed gravity. Roughly speaking, this happens because gravity is being coupled to a sector that is sick in the limit as gravity is turned off (MPl ¡æ ¡Ä). The ¥ð¡¯s have no kinetic term in this limit, even though they
have cubic and higher order selfinteractions. The theory only becomes tenuously healthy due to the coupling with gravity: mixing with gravity generates small kinetic terms for ¥ð of the correct sign (in flat space), but leads to strong coupling physics at
low energies. This does not necessarily mean that massive gravity or the DGP model cannot describe the real world, only that it appears necessary to make nontrivial assumptions about the UV completion of the theory.
I have to go over the paper again, as I am inquiring to the meaning of this quote:The ¥ð¡¯s have no kinetic term in this limit, even though they have cubic and higher order selfinteractions. The theory only becomes tenuously healthy due to the coupling with gravity: mixing with gravity generates small kinetic terms for ¥ð of the correct sign (in flat space), but leads to strong coupling physics at low energies.
I do not know of the status of the Authors, but at first reading the paper seems to be a robust and very interesting paper, I am going to delve into this for some time.