22,071 research outputs found
Persistence of black holes through a cosmological bounce
We discuss whether black holes could persist in a universe which recollapses
and then bounces into a new expansion phase. Whether the bounce is of classical
or quantum gravitational origin, such cosmological models are of great current
interest. In particular, we investigate the mass range in which black holes
might survive a bounce and ways of differentiating observationally between
black holes formed just after and just before the last bounce. We also discuss
the consequences of the universe going through a sequence of dimensional
changes as it passes through a bounce.Comment: 8 pages, 1 figur
Facilities for meteorological research at NASA Goddard/Wallops Flight Facility
The technical characteristics of the Atmospheric Sciences Research Facility, the improvements being made to the instrumentation there which will enhance its usefulness in atmospheric research, and several of the on-going research programs are described. Among the area of atmospheric research discussed are clouds and precipitation, lightning, ozone, wind, and storms. Meteorological instruments including Doppler radar, spectrophotometers, and ozone sensors are mentioned. Atmospheric research relevant to aircraft design and COMSTAR communication satellites is briefly discussed
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
Evolution of a Primordial Black Hole Population
We reconsider in this work the effects of an energy absorption term in the
evolution of primordial black holes (hereafter PBHs) in the several epochs of
the Universe. A critical mass is introduced as a boundary between the accreting
and evaporating regimes of the PBHs. We show that the growth of PBHs is
negligible in the Radiation-dominated Era due to scarcity of energy density
supply from the expanding background, in agreement with a previous analysis by
Carr and Hawking, but that nevertheless the absorption term is large enough for
black holes above the critical mass to preclude their evaporation until the
universe has cooled sufficiently. The effects of PBH motion are also discussed:
the Doppler effect may give rise to energy accretion in black-holes with large
peculiar motions relative to background. We discuss how cosmological
constraints are modified by the introduction of the critical mass since that
PBHs above it do not disturb the CMBR. We show that there is a large range of
admissible masses for PBHs above the critical mass but well below the
cosmological horizon. Finally we outline a minimal kinetic formalism, solved in
some limiting cases, to deal with more complicated cases of PBH populationsComment: RevTex file, 8 pp., 3 .ps figures available upon request from
[email protected]
Near-Critical Gravitational Collapse and the Initial Mass Function of Primordial Black Holes
The recent discovery of critical phenomena arising in gravitational collapse
near the threshold of black hole formation is used to estimate the initial mass
function of primordial black holes (PBHs). It is argued that the universal
scaling relation between black hole mass and initial perturbation found for a
variety of collapsing space-times also applies to PBH formation, indicating the
possibility of the formation of PBHs with masses much smaller than one horizon
mass. Owing to the natural fine-tuning of initial conditions by the exponential
decline of the probability distribution for primordial density fluctuations,
sub-horizon mass PBHs are expected to form at all epochs. This result suggests
that the constraints on the primordial fluctuation spectrum based on the
abundance of PBHs at different mass scales may have to be revisited.Comment: 4 pages, uses revtex, also available at
http://bigwhirl.uchicago.edu/jcn/pub_pbh.html . To appear in Phys. Rev. Let
Who Should be Interviewed in Surveys of Household Income?
This study tests the null hypothesis that it is sufficient to interview only the household head to obtain accurate information on household income. Results show that using a husband’s estimate of his wife’s income does not produce statistically reliable results for poverty analysis. Estimates of the wife’s income provided by the husband and wife are in agreement in only six percent of households. While limiting interviews to one person has the advantage of reducing the time and expense of household surveys, this appears detrimental in terms of accuracy, and may lead to incorrect conclusions on the determinants of poverty.Africa, gender, household dynamics, household surveys, Malawi, poverty, Agricultural Finance,
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An atlas of CO2 storage potential in the nearshore waters of the Texas coast – American Recovery and Reinvestment Act – “Gulf of Mexico Miocene CO2 site characterization mega-transect”
Bureau of Economic Geolog
Semi-Static Hedging Based on a Generalized Reflection Principle on a Multi Dimensional Brownian Motion
On a multi-assets Black-Scholes economy, we introduce a class of barrier
options. In this model we apply a generalized reflection principle in a context
of the finite reflection group acting on a Euclidean space to give a valuation
formula and the semi-static hedge.Comment: Asia-Pacific Financial Markets, online firs
Calculation of two-dimensional turbulent flow fields
Navier-Stokes equation solutions for two- dimensional turbulent flow fields of compressible viscous flui
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