7,887 research outputs found
Solving Cosmological Problems of Supersymmetric Axion Models in an Inflationary Universe
We revisit inflationary cosmology of axion models in the light of recent
developments on the inflaton decay in supergravity. We find that all the
cosmological difficulties, including gravitino, axino overproduction and
axionic isocurvature fluctuation, can be avoided if the saxion field has large
initial amplitude during inflation and decays before big-bang nucleosynthesis.Comment: 19 pages, 4 figure
Updated constraint on a primordial magnetic field during big bang nucleosynthesis and a formulation of field effects
A new upper limit on the amplitude of primordial magnetic field (PMF) is
derived by a comparison between a calculation of elemental abundances in big
bang nucleosynthesis (BBN) model and the latest observational constraints on
the abundances. Updated nuclear reaction rates are adopted in the calculation.
Effects of PMF on the abundances are consistently taken into account in the
numerical calculation with the precise formulation of changes in physical
variables. We find that abundances of 3He and 6Li increase while that of 7Li
decreases when the PMF amplitude increases, in the case of the baryon-to-photon
ratio determined from the measurement of cosmic microwave background radiation.
We derive a constraint on the present amplitude of PMF, i.e., B(0)<1.5 micro G
[corresponding to the amplitude less than 2.0x10^{11} G at BBN temperature of
T=10^9 K] based on the rigorous calculation.Comment: 26 pages, 4 figures, new observation of D/H ratio adopted, tighter
constraint derived, Sec. IV modified, accepted for publication in PR
General treatment of isocurvature perturbations and non-Gaussianities
We present a general formalism that provides a systematic computation of the
linear and non-linear perturbations for an arbitrary number of cosmological
fluids in the early Universe going through various transitions, in particular
the decay of some species (such as a curvaton or a modulus). Using this
formalism, we revisit the question of isocurvature non-Gaussianities in the
mixed inflaton-curvaton scenario and show that one can obtain significant
non-Gaussianities dominated by the isocurvature mode while satisfying the
present constraints on the isocurvature contribution in the observed power
spectrum. We also study two-curvaton scenarios, taking into account the
production of dark matter, and investigate in which cases significant
non-Gaussianities can be produced.Comment: Substantial improvements with respect to the first version. In
particular, we added a discussion on the confrontation of the models with
future observational data. This version is accepted for publication in JCA
Cosmological Constraints on Late-time Entropy Production
We investigate cosmological effects concerning the late-time entropy
production due to the decay of non-relativistic massive particles. The
thermalization process of neutrinos after the entropy production is properly
solved by using the Boltzmann equation. If a large entropy production takes
place at late time t 1 sec, it is found that a large fraction of
neutrinos cannot be thermalized. This fact loosens the tight constraint on the
reheating temperature T_R from the big bang nucleosynthesis and T_R could be as
low as 0.5 MeV. The influence on the large scale structure formation and cosmic
microwave background anisotropies is also discussed.Comment: 4 pages, using RevTeX and five postscript figures, comments added, to
appear in Phys. Rev. Let
Spectrum of Background X-rays from Moduli Dark Matter
We examine the -ray spectrum from the decay of the dark-matter moduli with
mass keV, in particular, paying attention to the line
spectrum from the moduli trapped in the halo of our galaxy. It is found that
with the energy resolution of the current experiments (%) the line
intensity is about twice stronger than that of the continuum spectrum from the
moduli that spread in the whole universe. Therefore, in the future experiments
with higher energy resolutions it may be possible to detect such line photons.
We also investigate the -ray spectrum emitted from the decay of the
multi-GeV moduli. It is shown that the emitted photons may form MeV-bump in the
-ray spectrum. We also find that if the modulus mass is of the order of
10 GeV, the emitted photons at the peak of the continuum spectrum loses their
energy by the scattering and the shape of the spectrum is significantly
changed, which makes the constraint weaker than that obtained in the previous
works.Comment: 14 pages (RevTeX file) including four postscript figures, reviced
version to be published in Physical Review
Reheat temperature in supersymmetric hybrid inflation models
The allowed range of parameters for supersymmetric hybrid inflation and its
extensions are investigated. The lower bound on the reheat temperature T_r in
these models with hierarchical right handed neutrinos is found to be 3*10^7
GeV. (T_r as low as 100 GeV is possible for quasi degenerate right handed
neutrinos.) We also present revised estimates for the scalar spectral index and
the symmetry breaking scale associated with inflation.Comment: 15 pages, 8 figures, revtex4. v2: minor changes, matches published
versio
Isocurvature perturbations in extra radiation
Recent cosmological observations, including measurements of the CMB
anisotropy and the primordial helium abundance, indicate the existence of an
extra radiation component in the Universe beyond the standard three neutrino
species. In this paper we explore the possibility that the extra radiation has
isocurvatrue fluctuations. A general formalism to evaluate isocurvature
perturbations in the extra radiation is provided in the mixed inflaton-curvaton
system, where the extra radiation is produced by the decay of both scalar
fields. We also derive constraints on the abundance of the extra radiation and
the amount of its isocurvature perturbation. Current observational data favors
the existence of an extra radiation component, but does not indicate its having
isocurvature perturbation. These constraints are applied to some particle
physics motivated models. If future observations detect isocurvature
perturbations in the extra radiation, it will give us a hint to the origin of
the extra radiation.Comment: 41 pages, 8 figures; version accepted for publication in JCA
Reheating of the Universe and Population III
We note that current observational evidence strongly favors a conventional
recombination of ionized matter subsequent to redshift z=1200, followed by
reionization prior to redshift z=5 and compute how this would have occurred in
a standard scenario for the growth of structure. Extending prior semi-analytic
work, we show by direct, high-resolution numerical simulations (of a COBE
normalized CDM+Lambda model) that reheating, will occur in the interval 15>z>7,
followed by reionization and accompanied by a significant increase in the Jeans
mass. However, the evolution of the Jeans mass does not significantly affect
star formation in dense, self-shielded clumps of gas, which are detached from
the thermal evolution of the rest of the universe. On average, the growth of
the Jeans mass tracks the growth of the nonlinear mass scale, a result we
suspect is due to nonlinear feedback effects. Cooling on molecular hydrogen
leads to a burst of star formation prior to reheating which produces Population
III stars with Omega_* reaching 10^{-5.5} and Z/Z_sun reaching 10^{-3.7} by
z=14. Star formation subsequently slows down as molecular hydrogen is depleted
by photo-destruction and the rise of the temperature. At later times, z<10,
when the characteristic virial temperature of gas clumps reach 10,000 degrees,
star formation increases again as hydrogen line cooling become efficient.
Objects containing Pop III stars accrete mass with time and, as soon as they
reach 10,000 K virial temperature, they engage in renewed star formation and
turn into normal Pop II objects having an old Pop III metal poor component.Comment: six postscript figures included, submitted to ApJ
Hunting for Isocurvature Modes in the CMB non-Gaussianities
We investigate new shapes of local primordial non-Gaussianities in the CMB.
Allowing for a primordial isocurvature mode along with the main adiabatic one,
the angular bispectrum is in general a superposition of six distinct shapes:
the usual adiabatic term, a purely isocurvature component and four additional
components that arise from correlations between the adiabatic and isocurvature
modes. We present a class of early Universe models in which various hierarchies
between these six components can be obtained, while satisfying the present
upper bound on the isocurvature fraction in the power spectrum. Remarkably,
even with this constraint, detectable non-Gaussianity could be produced by
isocurvature modes. We finally discuss the prospects of detecting these new
shapes with the Planck satellite.Comment: 9 pages, 2 figure
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