15,196 research outputs found
Growth of primordial black holes in a universe containing a massless scalar field
The evolution of primordial black holes in a flat Friedmann universe with a
massless scalar field is investigated in fully general relativistic numerical
relativity. A primordial black hole is expected to form with a scale comparable
to the cosmological apparent horizon, in which case it may go through an
initial phase with significant accretion. However, if it is very close to the
cosmological apparent horizon size, the accretion is suppressed due to general
relativistic effects. In any case, it soon gets smaller than the cosmological
horizon and thereafter it can be approximated as an isolated vacuum solution
with decaying mass accretion. In this situation the dynamical and inhomogeneous
scalar field is typically equivalent to a perfect fluid with a stiff equation
of state . The black hole mass never increases by more than a factor of
two, despite recent claims that primordial black holes might grow substantially
through accreting quintessence. It is found that the gravitational memory
scenario, proposed for primordial black holes in Brans-Dicke and scalar-tensor
theories of gravity, is highly unphysical.Comment: 24 pages, accepted for publication in Physical Review
Self-similar cosmological solutions with dark energy. II: black holes, naked singularities and wormholes
We use a combination of numerical and analytical methods, exploiting the
equations derived in a preceding paper, to classify all spherically symmetric
self-similar solutions which are asymptotically Friedmann at large distances
and contain a perfect fluid with equation of state with
. The expansion of the Friedmann universe is accelerated in this
case. We find a one-parameter family of self-similar solutions representing a
black hole embedded in a Friedmann background. This suggests that, in contrast
to the positive pressure case, black holes in a universe with dark energy can
grow as fast as the Hubble horizon if they are not too large. There are also
self-similar solutions which contain a central naked singularity with negative
mass and solutions which represent a Friedmann universe connected to either
another Friedmann universe or some other cosmological model. The latter are
interpreted as self-similar cosmological white hole or wormhole solutions. The
throats of these wormholes are defined as two-dimensional spheres with minimal
area on a spacelike hypersurface and they are all non-traversable because of
the absence of a past null infinity.Comment: 12 pages, 19 figures, 1 table, final version to appear in Physical
Review
Limits of sympathetic cooling of fermions: The role of the heat capacity of the coolant
The sympathetic cooling of an initially degenerate Fermi gas by either an
ideal Bose gas below or an ideal Boltzmann gas is investigated. It is
shown that the efficiency of cooling by a Bose gas below is by no means
reduced when its heat capacity becomes much less than that of the Fermi gas,
where efficiency is measured by the decrease in the temperature of the Fermi
gas per number of particles evaporated from the coolant. This contradicts the
intuitive idea that an efficient coolant must have a large heat capacity. In
contrast, for a Boltzmann gas a minimal value of the ratio of the heat
capacities is indeed necessary to achieve T=0 and all of the particles must be
evaporated.Comment: 5 pages, 3 figure
Bose-Einstein condensates in standing waves: The cubic nonlinear Schroedinger equation with a periodic potential
We present a new family of stationary solutions to the cubic nonlinear
Schroedinger equation with a Jacobian elliptic function potential. In the limit
of a sinusoidal potential our solutions model a dilute gas Bose-Einstein
condensate trapped in a standing light wave. Provided the ratio of the height
of the variations of the condensate to its DC offset is small enough, both
trivial phase and nontrivial phase solutions are shown to be stable. Numerical
simulations suggest such stationary states are experimentally observable.Comment: 4 pages, 4 figure
Stability criterion for self-similar solutions with a scalar field and those with a stiff fluid in general relativity
A stability criterion is derived in general relativity for self-similar
solutions with a scalar field and those with a stiff fluid, which is a perfect
fluid with the equation of state . A wide class of self-similar
solutions turn out to be unstable against kink mode perturbation. According to
the criterion, the Evans-Coleman stiff-fluid solution is unstable and cannot be
a critical solution for the spherical collapse of a stiff fluid if we allow
sufficiently small discontinuity in the density gradient field in the initial
data sets. The self-similar scalar-field solution, which was recently found
numerically by Brady {\it et al.} (2002 {\it Class. Quantum. Grav.} {\bf 19}
6359), is also unstable. Both the flat Friedmann universe with a scalar field
and that with a stiff fluid suffer from kink instability at the particle
horizon scale.Comment: 15 pages, accepted for publication in Classical and Quantum Gravity,
typos correcte
Holes in the walls: primordial black holes as a solution to the cosmological domain wall problem
We propose a scenario in which the cosmological domain wall and monopole
problems are solved without any fine tuning of the initial conditions or
parameters in the Lagrangian of an underlying filed theory. In this scenario
domain walls sweep out (unwind) the monopoles from the early universe, then the
fast primordial black holes perforate the domain walls, change their topology
and destroy them. We find further that the (old vacuum) energy density released
from the domain walls could alleviate but not solve the cosmological flatness
problem.Comment: References added; Published in Phys. Rev.
Evolution of primordial black holes in Jordan-Brans-Dicke cosmology
We consider the evolution of primordial black holes in a generalyzed
Jordan-Brans-Dicke cosmological model where both the Brans-Dicke scalar field
and its coupling to gravity are dynamical functions determined from the
evolution equations. The evaporation rate for the black holes changes compared
to that in standard cosmology. We show that accretion of radiation can proceed
effectively in the radiation dominated era. The black hole lifetime shortens
for low initial mass, but increases for high initial mass, and is thus
considerably modified compared to the case of standard cosmology. We derive a
cut-off value for the initial black hole mass, below which primordial black
holes evaporate out in the radiation dominated era, and above which they
survive beyond the present era.Comment: 5 pages, Latex; uses MNRAS stylefiles; minor changes; accepted for
publication in MNRA
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