83 research outputs found
Cosmological Consequences of String-forming Open Inflation Models
We present a study of open inflation cosmological scenarios in which cosmic
strings form betwen the two inflationary epochs. It is shown that in these
models strings are stretched outside the horizon due to the inflationary
expansion but must necessarily re-enter the horizon before the epoch of equal
matter and radiation densities. We determine the power spectrum of cold dark
matter perturbations in these hybrid models, finding good agreement with
observations for values of and comparable
contributions from the active and passive sources to the CMB. Finally, we
briefly discuss other cosmological consequences of these models.Comment: 11 LaTeX pages with 3 eps figure
Viability of primordial black holes as short period gamma-ray bursts
It has been proposed that the short period gamma-ray bursts, which occur at a
rate of , may be evaporating primordial black holes
(PBHs). Calculations of the present PBH evaporation rate have traditionally
assumed that the PBH mass function varies as . This mass
function only arises if the density perturbations from which the PBHs form have
a scale invariant power spectrum. It is now known that for a scale invariant
power spectrum, normalised to COBE on large scales, the PBH density is
completely negligible, so that this mass function is cosmologically irrelevant.
For non-scale-invariant power spectra, if all PBHs which form at given epoch
have a fixed mass then the PBH mass function is sharply peaked around that
mass, whilst if the PBH mass depends on the size of the density perturbation
from which it forms, as is expected when critical phenomena are taken into
account, then the PBH mass function will be far broader than . In this paper we calculate the present day PBH evaporation rate,
using constraints from the diffuse gamma-ray background, for both of these mass
functions. If the PBH mass function has significant finite width, as recent
numerical simulations suggest, then it is not possible to produce a present day
PBH evaporation rate comparable with the observed short period gamma-ray burst
rate. This could also have implications for other attempts to detect
evaporating PBHs.Comment: 5 pages, 2 figures, version to appear in Phys. Rev. D with additional
reference
Constraints on diffuse neutrino background from primordial black holes
We calculated the energy spectra and the fluxes of electron neutrino emitted
in the process of evaporation of primordial black holes (PBHs) in the early
universe. It was assumed that PBHs are formed by a blue power-law spectrum of
primordial density fluctuations. We obtained the bounds on the spectral index
of density fluctuations assuming validity of the standard picture of
gravitational collapse and using the available data of several experiments with
atmospheric and solar neutrinos. The comparison of our results with the
previous constraints (which had been obtained using diffuse photon background
data) shows that such bounds are quite sensitive to an assumed form of the
initial PBH mass function.Comment: 18 pages,(with 7 figures
Chaotic Inflationary Universe on Brane
The chaotic inflationary model of the early universe, proposed by Linde is
explored in the brane world considering matter described by a minimally coupled
self interacting scalar field. We obtain cosmological solutions which admit
evolution of a universe either from a singularity or without a singularity. It
is found that a very weakly coupled self-interacting scalar field is necessary
for a quartic type potential in the brane world model compared to that
necessary in general relativity. In the brane world sufficient inflation may be
obtained even with an initial scalar field having value less than the Planck
scale. It is found that if the universe is kinetic energy dominated to begin
with, it transits to an inflationary stage subsequently.Comment: 13 pages, no fig., accepted in Physical Review
An Isocurvature Mechanism for Structure Formation
We examine a novel mechanism for structure formation involving initial number
density fluctuations between relativistic species, one of which then undergoes
a temporary downward variation in its equation of state and generates
superhorizon-scale density fluctuations. Isocurvature decaying dark matter
models (iDDM) provide concrete examples. This mechanism solves the
phenomenological problems of traditional isocurvature models, allowing iDDM
models to fit the current CMB and large-scale structure data, while still
providing novel behavior. We characterize the decaying dark matter and its
decay products as a single component of ``generalized dark matter''. This
simplifies calculations in decaying dark matter models and others that utilize
this mechanism for structure formation.Comment: 4 pages, 3 figures, submitted to PRD (rapid communications
Harmonic E/B decomposition for CMB polarization maps
The full sky cosmic microwave background polarization field can be decomposed
into 'electric' (E) and 'magnetic' (B) components that are signatures of
distinct physical processes. We give a general construction that achieves
separation of E and B modes on arbitrary sections of the sky at the expense of
increasing the noise. When E modes are present on all scales the separation of
all of the B signal is no longer possible: there are inevitably ambiguous modes
that cannot be separated. We discuss the practicality of performing E/B
decomposition on large scales with realistic non-symmetric sky-cuts, and show
that separation on large scales is possible by retaining only the well
supported modes. The large scale modes potentially contain a great deal of
useful information, and E/B separation at the level of the map is essential for
clean detection of B without confusion from cosmic variance due to the E
signal. We give simple matrix manipulations for creating pure E and B maps of
the large scale signal for general sky cuts. We demonstrate that the method
works well in a realistic case and give estimates of the performance with data
from the Planck satellite. In the appendix we discuss the simple analytic case
of an azimuthally symmetric cut, and show that exact E/B separation is possible
on an azimuthally symmetric cut with a finite number of non-intersecting
circular cuts around foreground sources.Comment: Fixed numerical bug in tensor C_l: Planck detection probability
results updated (supersedes PRD version). Sample code and additional examples
available at http://cosmologist.info/polar
Dynamics of coupled bosonic systems with applications to preheating
Coupled, multi-field models of inflation can provide several attractive
features unavailable in the case of a single inflaton field. These models have
a rich dynamical structure resulting from the interaction of the fields and
their associated fluctuations. We present a formalism to study the
nonequilibrium dynamics of coupled scalar fields. This formalism solves the
problem of renormalizing interacting models in a transparent way using
dimensional regularization. The evolution is generated by a renormalized
effective Lagrangian which incorporates the dynamics of the mean fields and
their associated fluctuations at one-loop order. We apply our method to two
problems of physical interest: (i) a simple two-field model which exemplifies
applications to reheating in inflation, and (ii) a supersymmetric hybrid
inflation model. This second case is interesting because inflation terminates
via a smooth phase transition which gives rise to a spinodal instability in one
of the fields. We study the evolution of the zero mode of the fields and the
energy density transfer to the fluctuations from the mean fields. We conclude
that back reaction effects can be significant over a wide parameter range. In
particular for the supersymmetric hybrid model we find that particle production
can be suppressed due to these effects.Comment: 23 pages, 16 eps-figures, minor changes in the text, references
added, accepted for publication in PR
Cosmic Numbers: A Physical Classification for Cosmological Models
We introduce the notion of the cosmic numbers of a cosmological model, and
discuss how they can be used to naturally classify models according to their
ability to solve some of the problems of the standard cosmological model.Comment: 3 pages, no figures. v2: Two references added, cosmetic changes.
Version to appear in Phys. Rev. D (Brief reports
Observational constraints on an inflation model with a running mass
We explore a model of inflation where the inflaton mass-squared is generated
at a high scale by gravity-mediated soft supersymmetry breaking, and runs at
lower scales to the small value required for slow-roll inflation. The running
is supposed to come from the coupling of the inflaton to a non-Abelian gauge
field. In contrast with earlier work, we do not constrain the magnitude of the
supersymmetry breaking scale, and we find that the model might work even if
squark and slepton masses come from gauge-mediated supersymmetry breaking. With
the inflaton and gaugino masses in the expected range, and
in the range to (all at the high scale) the model can give
the observed cosmic microwave anisotropy, and a spectral index in the observed
range. The latter has significant variation with scale, which can confirm or
rule out the model in the forseeable future.Comment: Latex, 19 pages, 14 figures, uses epsf.st
String Imprints from a Pre-inflationary Era
We derive the equations governing the dynamics of cosmic strings in a flat
anisotropic universe of Bianchi type I and study the evolution of simple cosmic
string loop solutions. We show that the anisotropy of the background can have a
characteristic effect in the loop motion. We discuss some cosmological
consequences of these findings and, by extrapolating our results to cosmic
string networks, we comment on their ability to survive an inflationary epoch,
and hence be a possible fossil remnant (still visible today) of an anisotropic
phase in the very early universe.Comment: 5 pages, 3 figure
- …