On the stellar luminosity of the universe
  It has been noted at times that the rate of energy release in the most violent explosive events in the Universe, such as supernovae and gamma-ray bursts, rivals the stellar luminosity of the observable universe, Lstar. The underlying reason turns out to be that both can be scaled to c^5/G = Lgrav, albeit that for the explosions Lstar/Lgrav follows from first principles, whereas for Lstar the scaling involves quantities too complex to derive from elementary considerations at the present time. Under fairly general circumstances, Lstar is dominated by stars whose age is similar to the Hubble time. 
Ralph A.M.J. Wijers