  We examine the possibility of the decay of the vacuum energy into cosmic
microwave background (CMB) photons. It is shown that observations of the
primordial density fluctuation spectrum put strong limits on the possible decay
rate. When photon creation due to the vacuum energy decay takes place, the
standard linear temperature dependence, T(z)=T_0(1+z), where T_0 is the present
CMB temperature, is modified. The decay is described by a generic temperature
dependence, T(z)=T_0(1+z)^{1-\beta}, of the CMB photons. A strong limit on the
maximum value of the decay rate is obtained by placing a maximum value on the
\beta parameter: \beta_{max}\approx 3\times 10^{-3}.
