Anisotropies of the Cosmic Background Radiation in a Reionized Universe

Abstract

We trace the evolution of cosmic microwave background photons propagating through a reionized model universe. The reionization of the intergalactic medium is achieved by UV photons emitted from the decaying `hot' dark matter neutrinos. The model universe is represented by a comoving cube 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 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 to 8 degrees with rms amplitudes ranging from DT/T_{rms} =2.8 to 3.4 times 10^{-5} without reionization. If we allow the neutrinos to decay and subsequently reionize the IGM, the level of the temperature anisotropies is reduced to DT/T=0.8 to 1.1 times 10^{-5}. This includes the Doppler shifts suffered by the CMB photons as they scatter off the moving ionized fraction of the IGM. 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. We find that the decaying neutrino hypothesis is more efficient at reducing primordial anisotropies and generates smaller Doppler anisotropies than a model reionized via a late and sudden energy input into the IGM.