7,880 research outputs found
Capture of non-relativistic particles in eccentric orbits by a Kerr black hole
We obtain approximate analytic expressions for the critical value of the
total angular momentum of a non-relativistic test particle moving in the Kerr
geometry, such that it will be captured by the black hole. The expressions
apply to arbitrary orbital inclinations, and are accurate over the entire range
of angular momentum for the Kerr black hole. The expressions can be easily
implemented in N-body simulations of the evolution of star clusters around
massive galactic black holes, where such captures play an important role.Comment: 8 pages, 1 figure, published versio
Post-Newtonian constraints on f(R) cosmologies in metric formalism
We compute the complete post-Newtonian limit of the metric form of f(R)
gravities using a scalar-tensor representation. By comparing the predictions of
these theories with laboratory and solar system experiments, we find a set of
inequalities that any lagrangian f(R) must satisfy. The constraints imposed by
those inequalities allow us to find explicit bounds to the possible nonlinear
terms of the lagrangian. We conclude that the lagrangian f(R) must be almost
linear in R and that corrections that grow at low curvatures are incompatible
with observations. This result shows that modifications of gravity at very low
cosmic densities cannot be responsible for the observed cosmic speed-up.Comment: 10 pages, no figures, revtex
Galaxy peculiar velocities and evolution-bias
Galaxy bias can be split into two components: a formation-bias based on the
locations of galaxy creation, and an evolution-bias that details their
subsequent evolution. In this letter we consider evolution-bias in the peaks
model. In this model, galaxy formation takes place at local maxima in the
density field, and we analyse the subsequent peculiar motion of these galaxies
in a linear model of structure formation. The peak restriction yields
differences in the velocity distribution and correlation between the galaxy and
the dark matter fields, which causes the evolution-bias component of the total
bias to evolve in a scale-dependent way. This mechanism naturally gives rise to
a change in shape between galaxy and matter correlation functions that depends
on the mean age of the galaxy population. This model predicts that older
galaxies would be more strongly biased on large scales compared to younger
galaxies. Our arguments are supported by a Monte-Carlo simulation of galaxy
pairs propagated using the Zel'dovich-approximation for describing linear
peculiar galaxy motion.Comment: 5 pages, 4 figures, MNRAS accepte
The gravity lagrangian according to solar system experiments
In this work we show that the gravity lagrangian f(R) at relatively low
curvatures in both metric and Palatini formalisms is a bounded function that
can only depart from the linearity within the limits defined by well known
functions. We obtain those functions by analysing a set of inequalities that
any f(R) theory must satisfy in order to be compatible with laboratory and
solar system observational constraints. This result implies that the recently
suggested f(R) gravity theories with nonlinear terms that dominate at low
curvatures are incompatible with observations and, therefore, cannot represent
a valid mechanism to justify the cosmic speed-up.Comment: 4 pages, revtex
Figures of merit and constraints from testing General Relativity using the latest cosmological data sets including refined COSMOS 3D weak lensing
We use cosmological constraints from current data sets and a figure of merit
(FoM) approach to probe any deviations from general relativity (GR) at
cosmological scales. The FoM approach is used to study the constraining power
of various combinations of data sets on modified gravity (MG) parameters. We
use recently refined HST-COSMOS weak-lensing tomography data, ISW-galaxy cross
correlations from 2MASS and SDSS LRG surveys, matter power spectrum from
SDSS-DR7 (MPK), WMAP7 temperature and polarization spectra, BAO from 2DF and
SDSS-DR7, and Union2 compilation of supernovae, in addition to other bounds
from H_0 measurements and BBN. We use 3 parametrizations of MG parameters that
enter the perturbed field equations. In order to allow for variations with
redshift and scale, the first 2 parametrizations use recently suggested
functional forms while the third is based on binning methods. Using the first
parametrization, we find that CMB + ISW + WL provides the strongest constraints
on MG parameters followed by CMB+WL or CMB+MPK+ISW. Using the second
parametrization or binning methods, CMB+MPK+ISW consistently provides some of
the strongest constraints. This shows that the constraints are parametrization
dependent. We find that adding up current data sets does not improve
consistently uncertainties on MG parameters due to tensions between best-fit MG
parameters preferred by different data sets. Furthermore, some functional forms
imposed by the parametrizations can lead to an exacerbation of these tensions.
Next, unlike some studies that used the CFHTLS lensing data, we do not find any
deviation from GR using the refined HST-COSMOS data, confirming previous claims
in those studies that their result may have been due to some systematic effect.
Finally, we find in all cases that the values corresponding to GR are within
the 95% confidence level contours for all data set combinations. (abridged)Comment: 18 pages, 6 figures, matches version published in PR
Multimetric extension of the PPN formalism: experimental consistency of repulsive gravity
Recently we discussed a multimetric gravity theory containing several copies
of standard model matter each of which couples to its own metric tensor. This
construction contained dark matter sectors interacting repulsively with the
visible matter sector, and was shown to lead to cosmological late-time
acceleration. In order to test the theory with high-precision experiments
within the solar system we here construct a simple extension of the
parametrized post-Newtonian (PPN) formalism for multimetric gravitational
backgrounds. We show that a simplified version of this extended formalism
allows the computation of a subset of the PPN parameters from the linearized
field equations. Applying the simplified formalism we find that the PPN
parameters of our theory do not agree with the observed values, but we are able
to improve the theory so that it becomes consistent with experiments of
post-Newtonian gravity and still features its promising cosmological
properties.Comment: 19 pages, no figures, journal versio
The Newtonian limit at intermediate energies
We study the metric solutions for the gravitational equations in Modified
Gravity Models (MGMs). In models with negative powers of the scalar curvature,
we show that the Newtonian Limit (NL) is well defined as a limit at
intermediate energies, in contrast with the usual low energy interpretation.
Indeed, we show that the gravitational interaction is modified at low densities
or low curvatures.Comment: 4 pages, REVTeX 4; minor typos corrected, one reference adde
Constraints on a New Post-General Relativity Cosmological Parameter
A new cosmological variable is introduced which characterizes the degree of
departure from Einstein's General Relativity (GR) with a cosmological constant.
The new parameter, \varpi, is the cosmological analog of \gamma, the
parametrized post-Newtonian variable which measures the amount of spacetime
curvature per unit mass. In the cosmological context, \varpi measures the
difference between the Newtonian and longitudinal potentials in response to the
same matter sources, as occurs in certain scalar-tensor theories of gravity.
Equivalently, \varpi measures the scalar shear fluctuation in a dark energy
component. In the context of a "vanilla" LCDM background cosmology, a non-zero
\varpi signals a departure from GR or a fluctuating cosmological constant.
Using a phenomenological model for the time evolution \varpi=\varpi_0
\rho_{DE}/\rho_{M} which depends on the ratio of energy density in the
cosmological constant to the matter density at each epoch, it is shown that the
observed cosmic microwave background (CMB) temperature anisotropies limit the
overall normalization constant to be -0.4 < \varpi_0 < 0.1 at the 95%
confidence level. Existing measurements of the cross-correlations of the CMB
with large-scale structure further limit \varpi_0 > -0.2 at the 95% CL. In the
future, integrated Sachs-Wolfe and weak lensing measurements can more tightly
constrain \varpi_0, providing a valuable clue to the nature of dark energy and
the validity of GR.Comment: 9 pages, 7 figures; added reference
Variational Integrators for the Gravitational N-Body Problem
This paper describes a fourth-order integration algorithm for the
gravitational N-body problem based on discrete Lagrangian mechanics. When used
with shared timesteps, the algorithm is momentum conserving and symplectic. We
generalize the algorithm to handle individual time steps; this introduces
fifth-order errors in angular momentum conservation and symplecticity. We show
that using adaptive block power of two timesteps does not increase the error in
symplecticity. In contrast to other high-order, symplectic, individual
timestep, momentum-preserving algorithms, the algorithm takes only forward
timesteps. We compare a code integrating an N-body system using the algorithm
with a direct-summation force calculation to standard stellar cluster
simulation codes. We find that our algorithm has about 1.5 orders of magnitude
better symplecticity and momentum conservation errors than standard algorithms
for equivalent numbers of force evaluations and equivalent energy conservation
errors.Comment: 31 pages, 8 figures. v2: Revised individual-timestepping description,
expanded comparison with other methods, corrected error in predictor
equation. ApJ, in pres
A Metric for Rapidly Spinning Black Holes Suitable for Strong-Field Tests of the No-Hair Theorem
According to the no-hair theorem, astrophysical black holes are uniquely
characterized by their masses and spins and are described by the Kerr metric.
Several parametric deviations from the Kerr metric have been suggested to study
observational signatures in both the electromagnetic and gravitational-wave
spectra that differ from the expected Kerr signals. Due to the no-hair theorem,
however, such spacetimes cannot be regular everywhere outside the event
horizons, if they are solutions to the Einstein field equations; they are often
characterized by naked singularities or closed time-like loops in the regions
of the spacetime that are accessible to an external observer. For observational
tests of the no-hair theorem that involve phenomena in the vicinity of the
circular photon orbit or the innermost stable circular orbit around a black
hole, these pathologies limit the applicability of the metrics only to compact
objects that do not spin rapidly. In this paper, we construct a Kerr-like
metric which depends on a set of free parameters in addition to its mass and
spin and which is regular everywhere outside of the event horizon. We derive
expressions for the energy and angular momentum of a particle on a circular
equatorial orbit around the black hole and compute the locations of the
innermost stable circular orbit and the circular photon orbit. We demonstrate
that these orbits change significantly for even moderate deviations from the
Kerr metric. The properties of our metric make it an ideally suited spacetime
to carry out strong-field tests of the no-hair theorem in the electromagnetic
spectrum using the properties of accretion flows around astrophysical black
holes of arbitrary spin.Comment: 11 pages, 7 figures, accepted for publication in PR
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