Nonequilibrium Corrections to the Spectra of Massless Neutrinos in the Early Universe

Distortion of the equilibrium spectra of cosmic neutrinos due to interaction with hotter electrons and positrons in the primeval cosmic plasma is considered. The set of integro-differential kinetic equations for neutrinos is accurately numerically solved. The relative corrections to neutrino energy densities are approximately 0.9% for $\nu_e$ and 0.4% for $\nu_\mu$ and $\nu_\tau$. This effect results in $1.4 \cdot 10^{-4}$ increase in the primordial $^4 He$ abundance.Comment: 28 pages including 6 figures. Latex

Bose Einstein condensation at reheating

We discuss the possibility that a perturbative reheating stage after inflation produces a scalar particle gas in a Bose condensate state, emphasizing the possible cosmological role of this phenomenon for symmetry restoration.Comment: 4 pages, 4 figures. Revised version, with an improved analysis of the condensate formatio

Standard Model Neutrinos as Warm Dark Matter

Standard Model neutrinos are not usually considered plausible dark matter candidates because the usual treatment of their decoupling in the early universe implies that their mass must be sufficiently small to make them ``hot'' dark matter. In this paper we show that decoupling of Standard Model neutrinos in low reheat models may result in neutrino densities very much less than usually assumed, and thus their mass may be in the keV range. Standard Model neutrinos may therefore be warm dark matter candidates.Comment: 5 pages, 5 figures, LaTeX file uses revtex packag

Gamma-Ray Constraints on Maximum Cosmogenic Neutrino Fluxes and UHECR Source Evolution Models

The dip model assumes that the ultra-high energy cosmic rays (UHECRs) above 10$^{18}$ eV consist exclusively of protons and is consistent with the spectrum and composition measure by HiRes. Here we present the range of cosmogenic neutrino fluxes in the dip-model which are compatible with a recent determination of the extragalactic very high energy (VHE) gamma-ray diffuse background derived from 2.5 years of Fermi/LAT data. We show that the largest fluxes predicted in the dip model would be detectable by IceCube in about 10 years of observation and are within the reach of a few years of observation with the ARA project. In the incomplete UHECR model in which protons are assumed to dominate only above 10$^{19}$ eV, the cosmogenic neutrino fluxes could be a factor of 2 or 3 larger. Any fraction of heavier nuclei in the UHECR at these energies would reduce the maximum cosmogenic neutrino fluxes. We also consider here special evolution models in which the UHECR sources are assumed to have the same evolution of either the star formation rate (SFR), or the gamma-ray burst (GRB) rate, or the active galactic nuclei (AGN) rate in the Universe and found that the last two are disfavored (and in the dip model rejected) by the new VHE gamma-ray background.Comment: 19 pages, 16 figures, JHEP3.cls needed to typese

Lepton asymmetry creation in the Early Universe

Oscillations of active to sterile neutrinos with a small mixing angle sin 2 \theta < 10^{-2} could generate a large lepton asymmetry in the Early Universe. The final order of magnitude of the lepton asymmetry \eta is mainly determined by its growth in the last stage of evolution, the so called power-law regime. There exist two contradictory results in the literature, \eta \propto T^{-1} and \eta \propto T^{-4}, where T is the background medium temperature. In the first case, the lepton asymmetry does not exceed values of 10^{-4} for |\delta m^2| < 1 eV^2, while in the second case it can become larger than 10^{-1}. In this work we analytically investigate the case \eta \propto T^{-1}, using a new approach to solve the kinetic equations. We find that the power-law solution \eta \propto T^{-1} is not self-consistent. Instead, we find the power law \eta \propto T^{-11/3} to be a good approximation, which leads to a large final asymmetry.Comment: 33 pp, 7 figure

Unstable massive tau-neutrinos and primordial nucleosynthesis

The impact of unstable Majorana tau neutrinos on primordial nucleosynthesis is considered. The mass and lifetime of nu_tau are taken in the intervals 0.1-20 MeV and 0.001-400 sec respectively. The studied decay modes are nu_tau -> nu_mu + phi and nu_tau -> nu_e + phi, where phi is a massless (or light) scalar. Integro-differential kinetic equations are solved numerically without any simplifying assumptions. Our results deviate rather strongly from earlier calculations. Depending on mass, lifetime, and decay channels of the nu_tau, the number of effective neutrino species (found from He4), in addition to the 3 standard ones, varies from -2 to +2.5. The abundances of H2 and Li7 are also calculated.Comment: 36 pages including 28 figures, minor change

Heavy sterile neutrinos: Bounds from big-bang nucleosynthesis and SN 1987A

Cosmological and astrophysical effects of heavy (10 - 200 MeV) sterile Dirac neutrinos, mixed with the active ones, are considered. The bounds on mass and mixing angle from both supernovae and big-bang nucleosynthesis are presented.Comment: 19 pages, 5 figures, 1 table. Some references adde

Neutrino oscillations in the early universe: How large lepton asymmetry can be generated?

The lepton asymmetry that could be generated in the early universe through oscillations of active to sterile neutrinos is calculated (almost) analytically for small mixing angles, sin 2\theta < 10^{-2}. It is shown that for a mass squared difference, \delta m^2=-1 eV^2 it may rise at most by 6 orders of magnitude from the initial ``normal'' value of 10^{-10}, since the back-reaction from the refraction index terminates this rise while the asymmetry is still small. Only for very large mass differences, \delta m^2 about 10^9 eV^2, the lepton asymmetry could reach a significant magnitude exceeding 0.1.Comment: 25 pages, LaTeX (5 graphs

Thermalization of an impurity cloud in a Bose-Einstein condensate

We study the thermalization dynamics of an impurity cloud inside a Bose-Einstein condensate at finite temperature, introducing a suitable Boltzmann equation. Some values of the temperature and of the initial impurity energy are considered. We find that, below the Landau critical velocity, the macroscopic population of the initial impurity state reduces its depletion rate. For sufficiently high velocities the opposite effect occurs. For appropriate parameters the collisions cool the condensate. The maximum cooling per impurity atom is obtained with multiple collisions.Comment: 4 pages 6 figure

Impact of massive tau-neutrinos on primordial nucleosynthesis. Exact calculations

The influence of a massive Majorana tau-neutrino on primordial nucleosynthesis is rigorously calculated. The system of three integro-differential kinetic equations is solved numerically for the tau-neutrino mass in the interval from 0 to 20 MeV. It is found that the usual assumption of kinetic equilibrium is strongly violated and non-equilibrium corrections considerably amplify the effect. Even a very weak restriction from nucleosynthesis, allowing for one extra massless neutrino species, permits to conclude that m_\nu_\tau < 1 MeV. For a stricter bound, e.g. for dN < 0.3, the limit is m_\nu_\tau < 0.35 MeV.Comment: 30 pages, 9 figures, 2 tables, revised figures, minor change