57 research outputs found
Hydrodynamics of primordial black hole formation
The hydrodynamic picture of the formation of primordial black holes (PBH) at the early stages of expansion of the Universe is considered. It is assumed that close to singularity, expansion occurs in a quasi-isotropic way. Using an EVM, a spherically symmetrical nonlinear problem of the evolution of primary strong deviation from the Fridman solution was solved. What these deviations must be, so that the formation of PBH occurred was clarified. Attention was devoted to the role of pressure gradients. It is pointed out that at the moment of formation of PBH, only a small part of matter enters into it, primarily the component of perturbation. It is also pointed out that at this moment, the mass of PBH essentially is smaller than the mass considered within the cosmic horizon. The possibility of changing the mass of the PBH as a result of accretion is analyzed
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
Primordial Black Hole: Mass and Angular Momentum Evolution
The evolution of the primordial low mass black hole (PBH) in hot universe is
considered. Increase of mass and decrease of PBH spin due to the accretion of
radiation dominated matter are estimated with using of results of numerical
simulation of PBH formation and approximate relations for accretion to a
rotating black hole.Comment: Gravitation and Cosmology, accepted, 3 pages, Talk presented at the
russian summer school-seminar "Modern theoretical problems of gravitation and
cosmology" (GRACOS-2007), September 9-16, 2007, Kazan-Yalchik, Russi
Primordial black hole formation in the radiative era: investigation of the critical nature of the collapse
Following on after two previous papers discussing the formation of primordial
black holes in the early universe, we present here results from an in-depth
investigation of the extent to which primordial black hole formation in the
radiative era can be considered as an example of the critical collapse
phenomenon. We focus on initial supra-horizon-scale perturbations of a type
which could have come from inflation, with only a growing component and no
decaying component. In order to study perturbations with amplitudes extremely
close to the supposed critical limit, we have modified our previous computer
code with the introduction of an adaptive mesh refinement scheme. This has
allowed us to follow black hole formation from perturbations whose amplitudes
are up to eight orders of magnitude closer to the threshold than we could do
before. We find that scaling-law behaviour continues down to the smallest black
hole masses that we are able to follow and we see no evidence of shock
production such as has been reported in some previous studies and which led
there to a breaking of the scaling-law behaviour at small black-hole masses. We
attribute this difference to the different initial conditions used. In addition
to the scaling law, we also present other features of the results which are
characteristic of critical collapse in this context.Comment: 21 pages, 7 figures, the present version is updated with some changes
and two new appendix. Accepted for pubblication in Classical and Quantum
Gravit
Separate Universes Do Not Constrain Primordial Black Hole Formation
Carr and Hawking showed that the proper size of a spherical overdense region
surrounded by a flat FRW universe cannot be arbitrarily large as otherwise the
region would close up on itself and become a separate universe. From this
result they derived a condition connecting size and density of the overdense
region ensuring that it is part of our universe. Carr used this condition to
obtain an upper bound for the density fluctuation amplitude with the property
that for smaller amplitudes the formation of a primordial black hole is
possible, while larger ones indicate a separate universe. In contrast, we find
that the appearance of a maximum is not a consequence of avoiding separate
universes but arises naturally from the geometry of the chosen slicing. Using
instead of density a volume fluctuation variable reveals that a fluctuation is
a separate universe iff this variable diverges on superhorizon scales. Hence
Carr's and Hawking's condition does not pose a physical constraint on density
fluctuations. The dynamics of primordial black hole formation with an initial
curvature fluctuation amplitude larger than the one corresponding to the
maximum density fluctuation amplitude was previously not considered in detail
and so we compare it to the well-known case where the amplitude is smaller by
presenting embedding and conformal diagrams of both types in dust spacetimes.Comment: Updated version corresponds to the published version
10.1103/PhysRevD.83.124025, 22 pages, 22 figure
Cosmology vs. Holography
The most radical version of the holographic principle asserts that all
information about physical processes in the world can be stored on its surface.
This formulation is at odds with inflationary cosmology, which implies that
physical processes in our part of the universe do not depend on the boundary
conditions. Also, there are some indications that the radical version of the
holographic theory in the context of cosmology may have problems with unitarity
and causality. Another formulation of the holographic principle, due to
Fischler and Susskind, implies that the entropy of matter inside the
post-inflationary particle horizon must be smaller than the area of the
horizon. Their conjecture was very successful for a wide class of open and flat
universes, but it did not apply to closed universes. Bak and Rey proposed a
different holographic bound on entropy which was valid for closed universes of
a certain type. However, as we will show, neither proposal applies to open,
flat and closed universes with matter and a small negative cosmological
constant. We will argue, in agreement with Easther, Lowe, and Veneziano, that
whenever the holographic constraint on the entropy inside the horizon is valid,
it follows from the Bekenstein-Hawking bound on the black hole entropy. These
constraints do not allow one to rule out closed universes and other universes
which may experience gravitational collapse, and do not impose any constraints
on inflationary cosmology.Comment: 8 pages, we added one reference and comments about possible problems
with unitarity and causality of the holographic theory in cosmolog
Upper limits on the size of a primordial black hole
We provide precise constraints on the size of any black holes forming in the
early Universe for a variety of formation scenarios. In particular, we prove
that the size of the apparent horizon of a primordial black hole formed by
causal processes in a flat Friedmann universe is considerably smaller than the
cosmological apparent horizon size for an equation of state
(). This also applies for a stiff equation of state () or for a
massless scalar field. The apparent horizon of a primordial black hole formed
through hydrodynamical processes is also considerably smaller than the
cosmological apparent horizon for . We derive an expression for the
maximum size which an overdense region can have without being a separate closed
universe rather than part of our own. Newtonian argument shows that a black
hole smaller than the cosmological horizon can never accrete much.Comment: 15 pages, accepted for publication in Physical Review
Thermal X-ray Emission and Cosmic Ray Production in Young Supernova Remnants
We have developed a simple model to investigate the modifications of the
hydrodynamics and non-equilibrium ionization X-ray emission in young supernova
remnants due to nonlinear particle acceleration. In nonlinear, diffusive shock
acceleration, the heating of the gas to X-ray emitting temperatures is strongly
coupled to the acceleration of cosmic ray ions. If the acceleration is
efficient and a significant fraction of the shock ram energy ends up in cosmic
rays, compression ratios will be higher and the shocked temperature lower than
test-particle, Rankine-Hugoniot relations predict. We illustrate how particle
acceleration impacts the interpretation of X-ray data using the X-ray spectra
of Kepler's remnant, observed by ASCA and RXTE. The thermal X-ray emission
provides important constraints on the efficiency of particle acceleration, in
complement to nonthermal emission. X-ray data from Chandra and XMM Newton, plus
radio observations, will be essential to quantify nonlinear effects.Comment: 4 pages, 3 figures, accepted in ApJ Letter
Dynamics and Radiation of Young Type-Ia Supernova Remnants: Important Physical Processes
We examine and analyze the physical processes that should be taken into
account when modeling young type-Ia SNRs, with ages of several hundred years.
It is shown, that energy losses in the metal-rich ejecta can be essential for
remnants already at this stage of evolution. The influence of electron thermal
conduction and the rate of the energy exchange between electrons and ions on
the temperature distribution and the X-radiation from such remnants is studied.
The data for Tycho SNR from the XMM-Newton X-ray telescope have been employed
for the comparison of calculations with observations.Comment: 19 pages, 8 figure
Probability of primordial black hole formation and its dependence on the radial profile of initial configurations
In this paper we derive the probability of the radial profiles of spherically
symmetric inhomogeneities in order to provide an improved estimation of the
number density of primordial black holes (PBHs). We demonstrate that the
probability of PBH formation depends sensitively on the radial profile of the
initial configuration. We do this by characterising this profile with two
parameters chosen heuristically: the amplitude of the inhomogeneity and the
second radial derivative, both evaluated at the centre of the configuration. We
calculate the joint probability of initial cosmological inhomogeneities as a
function of these two parameters and then find a correspondence between these
parameters and those used in numerical computations of PBH formation. Finally,
we extend our heuristic study to evaluate the probability of PBH formation
taking into account for the first time the radial profile of curvature
inhomogeneities.Comment: Version 2 with corrections from referees included, changes mostly
improve the presentatio
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