Strong limits on the possible decay of the vacuum energy into CDM or CMB   photons
  We investigate models that suggest that the vacuum energy decays into cold dark matter (CDM) and into a homogeneous distribution of a thermalized cosmic microwave background (CMB), which is characteristic of an adiabatic vacuum energy decay into photons. We show that the density fluctuation spectrum obtained from the CMB and galaxy distribution data agreement put strong limits on the rate of decay of the vacuum energy. A vacuum energy decaying into CDM increases its total density \rho, diluting (\delta \rho/\rho)^2. The observed temperature fluctuations of the CMB photons (\delta T/T)^2 are approximately proportional to CDM density fluctuations (\delta \rho/\rho)^2. In both case, when evaluating (\delta \rho/\rho)^2 at the recombination era, its present measured value must be increased by a factor F. Since the (\delta \rho/\rho)^2 derived from the CMB and galaxy distribution data agree to \sim 10%, the maximum value for F is F_{\rm max}\aprox 1.1. Our results indicate that the rate of the decay of the vacuum energy into CDM and CMB photons are extremely small. 
Reuven Opher, Ana Pelinson