795 research outputs found
Bounds on the cosmological abundance of primordial black holes from diffuse sky brightness: single mass spectra
We constrain the mass abundance of unclustered primordial black holes (PBHs),
formed with a simple mass distribution and subject to the Hawking evaporation
and particle absorption from the environment. Since the radiative flux is
proportional to the numerical density, an upper bound is obtained by comparing
the calculated and observed diffuse background values, (similarly to the Olbers
paradox in which point sources are considered) for finite bandwidths. For a
significative range of formation redshifts the bounds are better than several
values obtained by other arguments ; and they apply
to PBHs which are evaporating today.Comment: 20 pages, 5 figures, to appear in PR
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
Bounds from Primordial Black Holes with a Near Critical Collapse Initial Mass Function
Recent numerical evidence suggests that a mass spectrum of primordial black
holes (PBHs) is produced as a consequence of near critical gravitational
collapse. Assuming that these holes formed from the initial density
perturbations seeded by inflation, we calculate model independent upper bounds
on the mass variance at the reheating temperature by requiring the mass density
not exceed the critical density and the photon emission not exceed current
diffuse gamma-ray measurements. We then translate these results into bounds on
the spectral index n by utilizing the COBE data to normalize the mass variance
at large scales, assuming a constant power law, then scaling this result to the
reheating temperature. We find that our bounds on n differ substantially
(\delta n > 0.05) from those calculated using initial mass functions derived
under the assumption that the black hole mass is proportional to the horizon
mass at the collapse epoch. We also find a change in the shape of the diffuse
gamma-ray spectrum which results from the Hawking radiation. Finally, we study
the impact of a nonzero cosmological constant and find that the bounds on n are
strengthened considerably if the universe is indeed vacuum-energy dominated
today.Comment: 24 pages, REVTeX, 5 figures; minor typos fixed, two refs added,
version to be published in PR
Primordial black holes in braneworld cosmologies: Formation, cosmological evolution and evaporation
We consider the population evolution and evaporation of primordial black
holes in the simplest braneworld cosmology, Randall-Sundrum type II. We
demonstrate that black holes forming during the high-energy phase of this
theory (where the expansion rate is proportional to the density) have a
modified evaporation law, resulting in a longer lifetime and lower temperature
at evaporation, while those forming in the standard regime behave essentially
as in the standard cosmology. For sufficiently large values of the AdS radius,
the high-energy regime can be the one relevant for primordial black holes
evaporating at key epochs such as nucleosynthesis and the present. We examine
the formation epochs of such black holes, and delimit the parameter regimes
where the standard scenario is significantly modified.Comment: 9 pages RevTeX4 file with four figures incorporated, minor changes to
match published versio
Brans-Dicke Theory and primordial black holes in Early Matter-Dominated Era
We show that primordial black holes can be formed in the matter-dominated era
with gravity described by the Brans-Dicke theory. Considering an early
matter-dominated era between inflation and reheating, we found that the
primordial black holes formed during that era evaporate at a quicker than those
of early radiation-dominated era. Thus, in comparison with latter case, less
number of primordial black holes could exist today. Again the constraints on
primordial black hole formation tend towards the larger value than their
radiation-dominated era counterparts indicating a significant enhancement in
the formation of primordial black holes during the matter-dominaed era.Comment: 9 page
Primordial black hole production due to preheating
During the preheating process at the end of inflation the amplification of
field fluctuations can lead to the amplification of curvature perturbations. If
the curvature perturbations on small scales are sufficiently large, primordial
black holes (PBHs) will be overproduced. In this paper we study PBH production
in the two-field preheating model with quadratic inflaton potential. We show
that for many values of the inflaton mass m, and coupling g, small scale
perturbations will be amplified sufficiently, before backreaction can shut off
preheating, so that PBHs will be overproduced during the subsequent radiation
dominated era.Comment: 5 pages, 3 eps figures. Minor changes to match version to appear in
PRD as a rapid communicatio
Black hole formation in the Friedmann universe: Formulation and computation in numerical relativity
We study formation of black holes in the Friedmann universe. We present a
formulation of the Einstein equations under the constant mean curvature
time-slicing condition. Our formalism not only gives us the analytic solution
of the perturbation equations for non-linear density and metric fluctuations on
superhorizon scales, but also allows us to carry out a numerical relativity
simulation for black hole formation after the scale of the density fluctuations
is well within the Hubble horizon scale. We perform a numerical simulation of
spherically symmetric black hole formation in the radiation-dominated,
spatially flat background universe for a realistic initial condition supplied
from the analytic solution. It is found that the initial metric perturbation
has to be non-linear (the maximum value of 3D conformal factor at
should be larger than ) for a black hole to be formed, but the
threshold amplitude for black hole formation and the final black hole mass
considerably depend on the initial density (or metric) profile of the
perturbation: The threshold value of at for formation of a black
hole is smaller for a high density peak surrounded by a low density region than
for that surrounded by the average density region of the flat universe. This
suggests that it is necessary to take into account the spatial correlation of
density fluctuations in the study of primordial black hole formation.Comment: 12 pages, to be published in Phys. Rev.
Domain Walls in Two-Component Dynamical Lattices
We introduce domain-wall (DW) states in the bimodal discrete nonlinear
Schr{\"{o}}dinger equation, in which the modes are coupled by cross phase
modulation (XPM). By means of continuation from various initial patterns taken
in the anti-continuum (AC) limit, we find a number of different solutions of
the DW type, for which different stability scenarios are identified. In the
case of strong XPM coupling, DW configurations contain a single mode at each
end of the chain. The most fundamental solution of this type is found to be
always stable. Another solution, which is generated by a different AC pattern,
demonstrates behavior which is unusual for nonlinear dynamical lattices: it is
unstable for small values of the coupling constant (which measures the
ratio of the nonlinearity and coupling lengths), and becomes stable at larger
. Stable bound states of DWs are also found. DW configurations generated by
more sophisticated AC patterns are identified as well, but they are either
completely unstable, or are stable only at small values of . In the case of
weak XPM, a natural DW solution is the one which contains a combination of both
polarizations, with the phase difference between them 0 and at the
opposite ends of the lattice. This solution is unstable at all values of ,
but the instability is very weak for large , indicating stabilization as the
continuum limit is approached. The stability of DWs is also verified by direct
simulations, and the evolution of unstable DWs is simulated too; in particular,
it is found that, in the weak-XPM system, the instability may give rise to a
moving DW.Comment: 14 pages, 14 figures, Phys. Rev. E (in press
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
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