3,400 research outputs found
Evaporation of charged bosonic condensate in cosmology
Cosmological evolution of equilibrium plasma with a condensate of
U(1)-charged bosonic field is considered. It is shown that the evaporation of
the condensate is very much different from naive expectations, discussed in the
literature, as well as from evaporation of non-equilibrium neutral condensate.
The charged condensate evaporates much slower than the decay of the
corresponding bosons. The evaporation rate is close to that of the cosmological
expansion. The plasma temperature, in contrast, drops much faster than usually,
namely as the third power of the cosmological scale factor. As a result the
universe becomes very cold and the cosmological charge asymmetry reaches a huge
value.Comment: 18 pages, 9 figure
Heavy sterile neutrinos - what they can be and what they can't
We review current astrophysical bounds on MeV sterile neutrinos, and then we
discuss why a sterile keV neutrino is a natural warm dark matter candidate.Comment: 4 pages, 2 figures, talk given at Frontiers in Particle Astrophysics
and Cosmology held at San Feli
Matter-antimatter domains in the universe
A possible existence of cosmologically large domains of antimatter or
astronomical ``anti-objects'' is discussed. A brief review of different
scenarios of baryogenesis predicting a noticeable amount of antimatter is
given. Though both theory and observations indicate that the universe is most
possibly uniformly charge asymmetric without any noticeable amount of
antimatter, several natural scenarios are possible that allow for
cosmologically (astronomically) interesting objects in close vicinity to us.
The latter may be discovered by observation of cosmic ray antinuclei.Comment: Invited Talk at EuroConference on Frontiers in Particle Astrophysics
and Cosmology, San Feliu de Guixols, Spain, 30 September - 5 October 2000; 7
pages; latex; two references are adde
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
Nonequilibrium corrections to energy spectra of massive particles in expanding universe
Deviations from kinetic equilibrium of massive particles caused by the
universe expansion are calculated analytically in the Boltzmann approximation.
For the case of an energy independent amplitude of elastic scattering, an exact
partial differential equation is derived instead of the usual
integro-differential one. A simple perturbative solution of the former is
found. For the case of an energy-dependent amplitude the problem cannot be
reduced to the differential equation but the solution of the original
integro-differential equation can be found in terms of the Taylor expansion,
which in the case of aconstant amplitude shows a perfect agreement with the
perturbative solution of the differential equation. Corrections to the spectrum
of (possibly) massive tau-neutrinos are calculated. The method may be of more
general interest and can be applied to the calculation of spectrum distortion
in other (not necessarily cosmological) nonequilibrium processes.Comment: 14 pages, latex twice; ps-files for figures are available upon
reques
An improved cosmological bound on the tau-neutrino mass
We consider the influence of non-equilibrium electronic neutrinos (and
anti-neutrinos) on the neutron-to-proton ratio. These neutrinos would come from
massive annihilations . For sufficiently large masses this new effect would strongly
enhance the (n/p)-ratio, leading to a very stringent bound on the
mass, even adopting a rather weak upper bound on the effective number on
neutrino species during nucleosynthesis.Comment: 10 pages, LaTex file + 1 figure compressed using uufile
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
Gravitational particle production in braneworld cosmology
Gravitational particle production in time variable metric of an expanding
universe is efficient only when the Hubble parameter is not too small in
comparison with the particle mass. In standard cosmology, the huge value of the
Planck mass makes the mechanism phenomenologically irrelevant. On the
other hand, in braneworld cosmology the expansion rate of the early universe
can be much faster and many weakly interacting particles can be abundantly
created. Cosmological implications are discussed.Comment: 4 pages, 1 figure, v3 with new definition of and minor text
modification
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