We discuss a cosmology in which cold dark matter particles decay into relativistic particles. We argue that such decays could lead naturally to a bulk viscosity in the cosmic fluid. For decay lifetimes comparable to the present Hubble age, this bulk viscosity enters the cosmic energy equation as an effective negative pressure. We investigate whether this negative pressure is of sufficient magnitude to account for the observed cosmic acceleration. We show that a single decaying species in a Λ=0, flat, dark-matter dominated cosmology can not reproduce the observed magnitude-redshift relation from Type Ia supernovae. However, a delayed bulk viscosity, possibly due to a cascade of decaying particles may be able to account for a significant fraction of the apparent cosmic acceleration. Possible candidate nonrelativistic particles for this scenario include sterile neutrinos or gauge-mediated decaying supersymmetric particles. © 2007 The American Physical Society.

We study the early evolution of the electron fraction (or, alternatively, the neutron-to-proton ratio) in the region above the hot proto-neutron star formed after a supernova explosion. We study the way in which the electron fraction in this environment is set by a competition between lepton (electron, positron, neutrino, and antineutrino) capture processes on free neutrons and protons and nuclei. Our calculations take explicit account of the effect of nuclear composition changes, such as formation of alpha particles (the alpha effect) and the shifting of nuclear abundances in nuclear statistical equilibrium associated with cooling in near-adiabatic outflow. We take detailed account of the process of weak interaction freeze-out in conjunction with these nuclear composition changes. Our detailed treatment shows that the alpha effect can cause significant increases in the electron fraction, while neutrino and antineutrino capture on heavy nuclei tends to have a buffering effect on this quantity. We also examine the effect on weak rates and the electron fraction of fluctuations in time in the neutrino and antineutrino energy spectra arising from hydrodynamic waves. Our analysis is guided by the Mayle & Wilson supernova code numerical results for the neutrino energy spectra and density and velocity profiles.

We point out that a vμ or vτ neutrino with a cosmologically significant mass (10-100 eV) and a small mixing (vacuum mixing angle θ > 10-4) with a light ve would result in a matter-enhanced Mikheyev-Smirnov-Wolfenstein resonant transformation between these species in a region above the neutrino sphere but below the stalled shock during the reheating phase of a Type II supernova. The neutrino heating behind the shock is due to charged-current ve and v̄e captures. Since the vμ and vτ have considerably higher average energies than do the ve, neutrino flavor mixing would result in higher effective ve energies behind the shock and a concomitant increase in the heating rate. Our numerical calculations suggest that this effect results in a 60% increase in the supernova explosion energy, possibly helping to solve the energy problem of delayed-mechanism supernova models.

We use heavy element nucleosynthesis from supernovae to probe the mixing of νe with ντ (or νμ) possessing cosmologically significant masses (1 to 100 eV). We conclude that the ντ(νμ)-νe vacuum mixing angle must satisfy sin22θ<10-5, in order to ensure that r-process heavy elements can be produced in neutrino-heated supernova ejecta. Mixing at a level exceeding this limit precludes r-process nucleosynthesis in this site. © 1993 The American Physical Society.

We suggest a new method for detection of neutrino bursts generated by distant supernovas in our galaxy and in the local group of galaxies. This new method is based on the detection of neutrons produced in the inelastic scattering of νμ and ντ neutrinos with nuclei via the neutral current channel. The advantages of such a detector are (1) the enhancement of neutrino scattering cross sections due to nuclear collective effects, (2) the selective detection of νμ and ντ supernova neutrinos, and (3) the existence of large geological deposits of suitable neutrino detector materials in nature. © 1990.