15,324 research outputs found
Gravitational entropy of Kerr black holes
Classical invariants of General Relativity can be used to approximate the
entropy of the gravitational field. In this work, we study two proposed
estimators based on scalars constructed out from the Weyl tensor, in Kerr
spacetime. In order to evaluate Clifton, Ellis and Tavakol's proposal, we
calculate the gravitational energy density, gravitational temperature, and
gravitational entropy of the Kerr spacetime. We find that in the frame we
consider, Clifton et al.'s estimator does not reproduce the Bekenstein-Hawking
entropy of a Kerr black hole. The results are compared with previous estimates
obtained by the authors using the Rudjord-Grn-Hervik approach. We
conclude that the latter represents better the expected behaviour of the
gravitational entropy of black holes.Comment: 12 pages, 7 figures, accepted for publication in General Relativity
and Gravitatio
Gravitational entropy of black holes and wormholes
Pure thermodynamical considerations to describe the entropic evolution of the
universe seem to violate the Second Law of Thermodynamics. This suggests that
the gravitational field itself has entropy. In this paper we expand recent work
done by Rudjord, Gr{\O}n and Sigbj{\O}rn where they suggested a method to
calculate the gravitational entropy in black holes based on the so-called `Weyl
curvature conjecture'. We study the formulation of an estimator for the
gravitational entropy of Reissner-Nordstr\"om, Kerr, Kerr-Newman black holes,
and a simple case of wormhole. We calculate in each case the entropy for both
horizons and the interior entropy density. Then, we analyse whether the
functions obtained have the expected behaviour for an appropriate description
of the gravitational entropy density.Comment: 11 pages, 11 figures, accepted for publication in International
Journal of Theoretical Physic
Financial Restrictions, Personal Income Tax (PIT) and Demand for a Permanent Home in a Dynamic Model. An analysis with Panel Data for Spain
This paper analyzes the way in which income tax and liquidity determine the purchase or rental of a permanent home in Spain. To do this, we have developed a theoretical dynamic model based on Euler’s equation. This model is verified using a sample from the 1991-1995 Panel of income taxpayers. Results suggest that the degree of financial restriction is the most relevant variable when determining the possibility of purchasing a home, while tax incentives increase their relative weighting once this asset has been acquired. Incentives for renting a home are relatively insignificant particularly for taxpayers who habitually rent their homes.personal income tax, liquidity, permanent home, tax incentives
High-energy signatures of binary systems of supermassive black holes
Context. Binary systems of supermassive black holes are expected to be strong
sources of long gravitational waves prior to merging. These systems are good
candidates to be observed with forthcoming space-borne detectors. Only a few of
these systems, however, have been firmly identified to date.
Aims. We aim at providing a criterion for the identification of some
supermassive black hole binaries based on the characteristics of the
high-energy emission of a putative relativistic jet launched from the most
massive of the two black holes.
Methods. We study supermassive black hole binaries where the less massive
black hole has carved an annular gap in the circumbinary disk, but nevertheless
there is a steady mass flow across its orbit. Such a perturbed disk is hotter
and more luminous than a standard thin disk in some regions. Assuming that the
jet contains relativistic electrons, we calculate its broadband spectral energy
distribution focusing on the inverse Compton up-scattering of the disk photons.
We also compute the opacity to the gamma rays produced in the jet by photon
annihilation with the disk radiation and take into account the effects of the
anisotropy of the target photon field as seen from the jet.
Results. We find that the excess of low-energy photons radiated by the
perturbed disk causes an increment in the external Compton emission from the
jet in the X-ray band, and a deep absorption feature at energies of tens of
TeVs for some sets of parameters. According to our results, observations with
Cherenkov telescopes might help in the identification of supermassive black
hole binaries, especially those black hole binaries that host primaries from
tens to hundreds of million of solar masses.Comment: 12 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
Financial Restrictions, Personal Income Tax (PIT) and Demand for a Permanent Home in a Dynamic Model. An analysis with Panel Data for Spain
This paper analyzes the way in which income tax and liquidity determine the purchase or rental of a permanent home in Spain. To do this, we have developed a theoretical dynamic model based on Euler’s equation. This model is verified using a sample from the 1991-1995 Panel of income taxpayers. Results suggest that the degree of financial restriction is the most relevant variable when determining the possibility of purchasing a home, while tax incentives increase their relative weighting once this asset has been acquired. Incentives for renting a home are relatively insignificant particularly for taxpayers who habitually rent their homes.personal income tax, liquidity, permanent home, tax incentives
Accretion disks around black holes in modified strong gravity
Stellar-mass black holes offer what is perhaps the best scenario to test
theories of gravity in the strong-field regime. In particular, f(R) theories,
which have been widely discuss in a cosmological context, can be constrained
through realistic astrophysical models of phenomena around black holes. We aim
at building radiative models of thin accretion disks for both Schwarzschild and
Kerr black holes in f(R) gravity. We study particle motion in
f(R)-Schwarzschild and Kerr space-times. We present the spectral energy
distribution of the accretion disk around constant Ricci scalar f(R) black
holes, and constrain specific f(R) prescriptions using features of these
systems. A precise determination of both the spin and accretion rate onto black
holes along with X-ray observations of their thermal spectrum might allow to
identify deviations of gravity from General Relativity. We use recent data on
the high-mass X-ray binary Cygnus X-1 to restrict the values of the parameters
of a class of f(R) models.Comment: 16 pages, 20 figures, accepted for publication in Astronomy &
Astrophysic
Cosmological black holes and the direction of time
Macroscopic irreversible processes emerge from fundamental physical laws of
reversible character. The source of the local irreversibility seems to be not
in the laws themselves but in the initial and boundary conditions of the
equations that represent the laws. In this work we propose that the screening
of currents by black hole event horizons determines, locally, a preferred
direction for the flux of electromagnetic energy. We study the growth of black
hole event horizons due to the cosmological expansion and accretion of cosmic
microwave background radiation, for different cosmological models. We propose
generalized McVittie co-moving metrics and integrate the rate of accretion of
cosmic microwave background radiation onto a supermassive black hole over
cosmic time. We find that for flat, open, and closed Friedmann cosmological
models, the ratio of the total area of the black hole event horizons with
respect to the area of a radial co-moving space-like hypersurface always
increases. Since accretion of cosmic radiation sets an absolute lower limit to
the total matter accreted by black holes, this implies that the causal past and
future are not mirror symmetric for any spacetime event. The asymmetry causes a
net Poynting flux in the global future direction; the latter is in turn related
to the ever increasing thermodynamic entropy. Thus, we expose a connection
between four different "time arrows": cosmological, electromagnetic,
gravitational, and thermodynamic.Comment: 13 pages, 2 figures in Foundations of Science (2017
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