ANISOTROPIES OF THE COSMIC BACKGROUND-RADIATION IN A REIONIZED UNIVERSE

We trace the evolution of cosmic microwave background photons propagating through a model universe filled with ''hot'' dark matter and a reionized intergalactic medium (IGM). The reionization of the intergalactic medium is achieved by UV photons emitted from the decaying ''hot'' dark matter neutrinos with a mass of 29 eV and lifetime of 1.5 x 10(23) s. The model universe is represented by a comoving cube of present size 529 Mpc through which we propagate photons and subject them to the Sachs-Wolfe effect and Thomson scattering off the ionized fraction of the IGM, beginning at z = 1900 just before the epoch of decoupling until the present z = 0. The simulation follows the evolving matter inhomogeneities and microwave background photons into the present nonlinear regime and yields temperature maps of the microwave sky as a comoving observer would detect. Our temperature maps display anisotropies on scales ranging from 2 degrees to 8 degrees with rms amplitudes ranging from Delta T/T-rms = 2.8 to 3.4 x 10(-5) for Ohm(0) = 1 and h = 0.55 without reionization. If we allow the neutrinos to decay and subsequently reionize the IGM, the level of the temperature anisotropies is reduced to Delta T/T-rms = 0.8 to 1.1 x 10(-5). This includes the Doppler shifts suffered by the CMB photons as they scatter off the moving ionized fraction of the IGM. If we turn off the contribution to Delta T/T-rms from these Doppler shifts, the temperature anisotropies are reduced to Delta T/T-rms = 0.7 to 0.8 x 10(-5) for Ohm(0) = 1 and h = 0.55 in the reionized model. If, however, reionization is caused by a late and sudden energy input into the IGM, we find that some reduction in Delta T/T occurs, but not enough to make this scenario consistent with observations. Including the contribution to Delta T/T-rms from Doppler shifts at the surface of last scattering, the level of anisotropies in this scenario is usually at the same level and sometimes even higher than the original primordial fluctuations. We find that reionization via the decaying neutrino hypothesis is more efficient at reducing primordial anisotropies and generates smaller Doppler shift anisotropies than a model in which the universe is reionized via a late and sudden energy input into the IGM. In the case the universe is not reionized the contribution to Delta T/T-rms from Doppler shifts at decoupling is small compared to the Sachs-Wolfe effect at large angular scales.