8,456 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
Asymptotically Friedmann self-similar scalar field solutions with potential
We investigate self-similar solutions which are asymptotic to the Friedmann
universe at spatial infinity and contain a scalar field with potential. The
potential is required to be exponential by self-similarity. It is found that
there are two distinct one-parameter families of asymptotic solutions,one is
asymptotic to the proper Friedmann universe, while the other is asymptotic to
the quasi-Friedmann universe, i.e., the Friedmann universe with anomalous solid
angle. The asymptotically proper Friedmann solution is possible only if the
universe is accelerated or the potential is negative. If the potential is
positive, the density perturbation in the asymptotically proper Friedmann
solution rapidly falls off at spatial infinity, while the mass perturbation is
compensated. In the asymptotically quasi-Friedmann solution, the density
perturbation falls off only in proportion to the inverse square of the areal
radius and the relative mass perturbation approaches a nonzero constant at
spatial infinity. The present result shows that a necessary condition holds in
order that a self-gravitating body grows self-similarly due to the constant
accretion of quintessence in an accelerating universe.Comment: accepted for publication in Physical Review D, minor correction,
typos correcte
Accretion with back reaction
We calculate analytically a back reaction of the stationary spherical
accretion flow near the event horizon and near the inner Cauchy horizon of the
charged black hole. It is shown that corresponding back-reaction corrections to
the black hole metric depend only on the fluid accretion rate and diverge in
the case of an extremely charged black hole. In result, the test fluid
approximation for stationary accretion is violated for extreme black holes.
This behavior of the accreting black hole is in accordance with the third law
of black hole thermodynamics, forbidding the practical attainability of the
extreme state.Comment: 5 pages, 2 figures; new figure and references adde
The state space and physical interpretation of self-similar spherically symmetric perfect-fluid models
The purpose of this paper is to further investigate the solution space of
self-similar spherically symmetric perfect-fluid models and gain deeper
understanding of the physical aspects of these solutions. We achieve this by
combining the state space description of the homothetic approach with the use
of the physically interesting quantities arising in the comoving approach. We
focus on three types of models. First, we consider models that are natural
inhomogeneous generalizations of the Friedmann Universe; such models are
asymptotically Friedmann in their past and evolve fluctuations in the energy
density at later times. Second, we consider so-called quasi-static models. This
class includes models that undergo self-similar gravitational collapse and is
important for studying the formation of naked singularities. If naked
singularities do form, they have profound implications for the predictability
of general relativity as a theory. Third, we consider a new class of
asymptotically Minkowski self-similar spacetimes, emphasizing that some of them
are associated with the self-similar solutions associated with the critical
behaviour observed in recent gravitational collapse calculations.Comment: 24 pages, 12 figure
Timelike self-similar spherically symmetric perfect-fluid models
Einstein's field equations for timelike self-similar spherically symmetric
perfect-fluid models are investigated. The field equations are rewritten as a
first-order system of autonomous differential equations. Dimensionless
variables are chosen in such a way that the number of equations in the coupled
system is reduced as far as possible and so that the reduced phase space
becomes compact and regular. The system is subsequently analysed qualitatively
using the theory of dynamical systems.Comment: 23 pages, 6 eps-figure
Are black holes over-produced during preheating?
We provide a simple but robust argument that primordial black hole (PBH)
production generically does {\em not} exceed astrophysical bounds during the
resonant preheating phase after inflation. This conclusion is supported by
fully nonlinear lattice simulations of various models in two and three
dimensions which include rescattering but neglect metric perturbations. We
examine the degree to which preheating amplifies density perturbations at the
Hubble scale and show that at the end of the parametric resonance, power
spectra are universal, with no memory of the power spectrum at the end of
inflation. In addition we show how the probability distribution of density
perturbations changes from exponential on very small scales to Gaussian when
smoothed over the Hubble scale -- the crucial length for studies of primordial
black hole formation -- hence justifying the standard assumption of
Gaussianity.Comment: 12 pages, 8 figures, revtex, added references for section
A complete classification of spherically symmetric perfect fluid similarity solutions
We classify all spherically symmetric perfect fluid solutions of Einstein's
equations with equation of state p/mu=a which are self-similar in the sense
that all dimensionless variables depend only upon z=r/t. For a given value of
a, such solutions are described by two parameters and they can be classified in
terms of their behaviour at large and small distances from the origin; this
usually corresponds to large and small values of z but (due to a coordinate
anomaly) it may also correspond to finite z. We base our analysis on the
demonstration that all similarity solutions must be asymptotic to solutions
which depend on either powers of z or powers of lnz. We show that there are
only three similarity solutions which have an exact power-law dependence on z:
the flat Friedmann solution, a static solution and a Kantowski-Sachs solution
(although the latter is probably only physical for a1/5, there are
also two families of solutions which are asymptotically (but not exactly)
Minkowski: the first is asymptotically Minkowski as z tends to infinity and is
described by one parameter; the second is asymptotically Minkowski at a finite
value of z and is described by two parameters. A complete analysis of the dust
solutions is given, since these can be written down explicitly and elucidate
the link between the z>0 and z<0 solutions. Solutions with pressure are then
discussed in detail; these share many of the characteristics of the dust
solutions but they also exhibit new features.Comment: 63 pages. To appear in Physical Review
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