445 research outputs found
Thermal and Non-Thermal Production of Gravitinos in the Early Universe
The excessive production of gravitinos in the early universe destroys the
successful predictions of nucleosynthesis. The thermal generation of gravitinos
after inflation leads to the bound on the reheating temperature, T_{RH}< 10^9
GeV. However, it has been recently realized that the non-thermal generation of
gravitinos in the early universe can be extremely efficient and overcome the
thermal production by several orders of magnitude, leading to much tighter
constraints on the reheating temperature. In this paper, we first investigate
some aspects of the thermal production of gravitinos, taking into account that
in fact reheating is not instantaneous and inflation is likely to be followed
by a prolonged stage of coherent oscillations of the inflaton field. We then
proceed by further investigating the non-thermal generation of gravitinos,
providing the necessary tools to study this process in a generic time-dependent
background with any number of superfields. We also present the first numerical
results regarding the non-thermal generation of gravitinos in particular
supersymmetric models.Comment: 31 pages, 7 Postscript figures. New references adde
The Cosmological Moduli Problem and Preheating
Many models of supersymmetry breaking, in the context of either supergravity
or superstring theories, predict the presence of particles with
Planck-suppressed couplings and masses around the weak scale. These particles
are generically called moduli. The excessive production of moduli in the early
Universe jeopardizes the successful predictions of nucleosynthesis. In this
paper we show that the efficient generation of these dangerous relics is an
unescapable consequence of a wide variety of inflationary models which have a
preheating stage. Moduli are generated as coherent states in a novel way which
differs from the usual production mechanism during parametric resonance. The
corresponding limits on the reheating temperature are often very tight and more
severe than the bound of 10^9 GeV coming from the production of moduli via
thermal scatterings during reheating.Comment: 17 pages, 5 Postscript figures, corrected some typo
Non-Thermal Production of Dangerous Relics in the Early Universe
Many models of supersymmetry breaking, in the context of either supergravity
or superstring theories, predict the presence of particles with weak scale
masses and Planck-suppressed couplings. Typical examples are the scalar moduli
and the gravitino. Excessive production of such particles in the early Universe
destroys the successful predictions of nucleosynthesis. In particular, the
thermal production of these relics after inflation leads to a bound on the
reheating temperature, T_{RH} < 10^9 GeV. In this paper we show that the
non-thermal generation of these dangerous relics may be much more efficient
than the thermal production after inflation. Scalar moduli fields may be
copiously created by the classical gravitational effects on the vacuum state.
Consequently, the new upper bound on the reheating temperature is shown to be,
in some cases, as low as 100 GeV. We also study the non-thermal production of
gravitinos in the early Universe, which can be extremely efficient and overcome
the thermal production by several orders of magnitude, in realistic
supersymmetric inflationary models.Comment: 21 pages, 4 Postscript figure
Primordial Black Holes from Inflation and non-Gaussianity
Primordial black holes may owe their origin to the small-scale enhancement of
the comoving curvature perturbation generated during inflation. Their mass
fraction at formation is markedly sensitive to possible non-Gaussianities in
such large, but rare fluctuations. We discuss a path-integral formulation which
provides the exact mass fraction of primordial black holes at formation in the
presence of non-Gaussianity. Through a couple of classes of models, one based
on single-field inflation and the other on spectator fields, we show that
restricting to a Gaussian statistics may lead to severe inaccuracies in the
estimate of the mass fraction as well as on the clustering properties of the
primordial black holes.Comment: 21 pages, 2 figures, v2: matching published versio
CMB temperature anisotropies from third order gravitational perturbations
In this paper we present a complete computation of the Cosmic Microwave
Background (CMB) anisotropies up to third order from gravitational
perturbations accounting for scalar, vector and tensor perturbations. We then
specify our results to the large scale limit, providing the evolution of the
gravitational potentials in a flat universe filled with matter and cosmological
constant which characterizes the Integrated Sachs-Wolfe effect. As a byproduct
in the large scale approximation we are able to give non-perturbative solutions
for the photon geodesic equations. Our results are the first step to provide a
complete theoretical prediction for cubic non-linearities which are
particularly relevant for characterizing the level of non-Gaussianity in the
CMB through the detection of the four-point angular connected correlation
function (trispectrum). For this purpose we also allow for generic initial
conditions due to primordial non-Gaussianity.Comment: 19 pages, LateX file; typos corrected; some corrections made and
several consistency checks performed regarding Eqs.(2.18); (2.28)-(2.29) and
Eqs.(3.8)-(3.24) and Eq.(4.2). Version accepted for publication in JCA
Production of Massive Fermions at Preheating and Leptogenesis
We present a complete computation of the inflaton decay into very massive
fermions during preheating. We show that heavy fermions are produced very
efficiently up to masses of order 10^{17}-10^{18} GeV; the accessible mass
range is thus even broader than the one for heavy bosons. We apply our findings
to the leptogenesis scenario, proposing a new version of it, in which the
massive right-handed neutrinos, responsible for the generation of the baryon
asymmetry, are produced during preheating. We also discuss other production
mechanisms of right-handed neutrinos in the early Universe, identifying the
neutrino mass parameters for which the observed baryon asymmetry is reproduced.Comment: 29 pages, 4 figure
A Monte Carlo study of Inverse Symmetry Breaking
We make a Monte Carlo study of the coupled two-scalar
model in four dimensions at finite temperature. We
find no trace of Inverse Symmetry Breaking for values of the renormalized
parameters for which perturbation theory predicts this phenomenon.Comment: 4 pages, revtex, 3 figures include
Can sterile neutrinos be ruled out as warm dark matter candidates?
We present constraints on the mass of warm dark matter (WDM) particles from a
combined analysis of the matter power spectrum inferred from the Sloan Digital
Sky Survey \lya flux power spectrum at 2.2<z<4.2, cosmic microwave background
data, and the galaxy power spectrum. We obtain a lower limit of m~10 keV (2
sigma) if the WDM consists of sterile neutrinos and m~2 keV (2 sigma) for early
decoupled thermal relics. If we combine this bound with the constraint derived
from x-ray flux observations in the Coma cluster, we find that the allowed
sterile neutrino mass is ~10 keV (in the standard production scenario). Adding
constraints based on x-ray fluxes from the Andromeda galaxy, we find that dark
matter particles cannot be sterile neutrinos, unless they are produced by a
nonstandard mechanism (resonant oscillations, coupling with the inflaton) or
get diluted by some large entropy release.Comment: 4 pages, 3 figures, matches published versio
On the Catalysis of the Electroweak Vacuum Decay by Black Holes at High Temperature
We study the effect of primordial black holes on the classical rate of
nucleation of AdS regions within the standard electroweak vacuum at high
temperature. We find that the energy barrier for transitions to the new vacuum,
which determines the exponential suppression of the nucleation rate, can be
reduced significantly, or even eliminated completely, in the black-hole
background if the Standard Model Higgs is coupled to gravity through the
renormalizable term .Comment: LaTeX file, 15 pages, 5 figure
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