  Due to the Hubble redshift, photon energy, chiefly in the form of CMBR
photons, is currently disappearing from the universe at the rate of nearly
10^55 erg s^-1. An ongoing problem in cosmology concerns the fate of this
energy. In one interpretation it is irretrievably lost, i.e., energy is not
conserved on the cosmic scale. Here we consider a different possibility which
retains universal energy conservation. If gravitational energy is redshifted in
the same manner as photons, then it can be shown that the cosmic redshift
removes gravitational energy from space at about the same rate as photon
energy. Treating gravitational potential energy conventionally as negative
energy, it is proposed that the Hubble shift 'flips' positive energy (photons)
to negative energy (gravitons) and vice versa. The lost photon energy would
thus be directed towards gravitation, making gravitational energy wells more
negative. Conversely, within astrophysical bodies of sufficient size, the
flipping of gravitons to photons would give rise to a 'Hubble luminosity' of
magnitude -UH, where U is the internal gravitational potential energy of the
object and H the Hubble constant. Evidence of such an energy release is
presented in bodies ranging from planets, white dwarfs and neutron stars to
supermassive black holes and the visible universe.
