6,762 research outputs found
Cosmological Constraints on Horndeski Gravity in Light of GW170817
The discovery of the electromagnetic counterpart to GW170817 severely
constrains the tensor mode propagation speed, eliminating a large model space
of Horndeski theory. We use the cosmic microwave background data from Planck
and the joint analysis of the BICEP2/Keck Array and Planck, galaxy clustering
data from the SDSS LRG survey, BOSS baryon acoustic oscillation data, and
redshift space distortion measurements to place constraints on the remaining
Horndeski parameters. We evolve the Horndeski parameters as power laws with
both the amplitude and power law index free. We find a 95% CL upper bound on
the present-day coefficient of the Hubble friction term in the cosmological
propagation of gravitational waves is 2.38, whereas General Relativity gives 2
at all times. While an enhanced friction suppresses the amplitude of the
reionization bump of the primordial B-mode power spectrum at , our
result limits the suppression to be less than 0.8%. This constraint is
primarily due to the scalar integrated Sachs-Wolfe effect in temperature
fluctuations at low multipoles.Comment: 23 pages, 10 figures. Version accepted for publication in JCA
Non-Gaussianities from isocurvature modes
This contribution discusses isocurvature modes, in particular the
non-Gaussianities of local type generated by these modes. Since the
isocurvature transfer functions differ from the adiabatic one, the coexistence
of a primordial isocurvature mode with the usual adiabatic mode leads to a rich
structure of the angular bispectrum, which can be decomposed into six
elementary bispectra. Future analysis of the CMB data will enable to measure
their relative weights, or at least constrain them. Non-Gaussianity thus
provides a new window on isocurvature modes. This is particularly relevant for
some scenarios, such as those presented here, which generate isocurvature modes
whose contribution in the power spectrum is suppressed, as required by present
data, but whose contribution in the non-Gaussianities could be dominant and
measurable.Comment: 8 pages, 2 figures; to appear in the Proceedings of COSGRAV-2012
(International Conference on Modern Perspectives of Cosmology and
Gravitation), Indian Statistical Institute, Kolkata, India, February 7-11,
201
Oscillations and instabilities of fast and differentially rotating relativistic stars
We study non-axisymmetric oscillations of rapidly and differentially rotating
relativistic stars in the Cowling approximation. Our equilibrium models are
sequences of relativistic polytropes, where the differential rotation is
described by the relativistic -constant law. We show that a small degree of
differential rotation raises the critical rotation value for which the
quadrupolar f-mode becomes prone to the CFS instability, while the critical
value of at the mass-shedding limit is raised even more. For softer
equations of state these effects are even more pronounced. When increasing
differential rotation further to a high degree, the neutral point of the CFS
instability first reaches a local maximum and is lowered afterwards. For stars
with a rather high compactness we find that for a high degree of differential
rotation the absolute value of the critical is below the corresponding
value for rigid rotation. We conclude that the parameter space where the CFS
instability is able to drive the neutron star unstable is increased for a small
degree of differential rotation and for a large degree at least in stars with a
higher compactness.Comment: 16 pages, 11 figures; paper accepted for publication in Phys. Rev. D
(81.084019
Cosmic String Power Spectrum, Bispectrum and Trispectrum
We use analytic calculations of the post-recombination gravitational effects
of cosmic strings to estimate the resulting CMB power spectrum, bispectrum and
trispectrum. We place a particular emphasis on multipole regimes relevant for
forthcoming CMB experiments, notably the Planck satellite. These calculations
use a flat sky approximation, generalising previous work by integrating string
contributions from last scattering to the present day, finding the dominant
contributions to the correlators for multipoles l > 50. We find a well-behaved
shape for the string bispectrum (without divergences) which is easily
distinguishable from the inflationary bispectra which possess significant
acoustic peaks. We estimate that the nonlinearity parameter characterising the
bispectrum is approximately f_NL \sim -20 (given present string constraints
from the CMB power spectrum. We also apply these unequal time correlator
methods to calculate the trispectrum for parrallelogram configurations, again
valid over a large range of angular scales relevant for WMAP and Planck, as
well as on very small angular scales. We find that, unlike the bispectrum which
is suppressed by symmetry considerations, the trispectrum for cosmic strings is
large. Our current estimate for the trispectrum parameter is tau_NL \sim 10^5,
which may provide one of the strongest constraints on the string model as
estimators for the trispectrum are developed
Improved simulation of non-Gaussian temperature and polarization CMB maps
We describe an algorithm to generate temperature and polarization maps of the
cosmic microwave background radiation containing non-Gaussianity of arbitrary
local type. We apply an optimized quadrature scheme that allows us to predict
and control integration accuracy, speed up the calculations, and reduce memory
consumption by an order of magnitude. We generate 1000 non-Gaussian CMB
temperature and polarization maps up to a multipole moment of l_max = 1024. We
validate the method and code using the power spectrum and the fast cubic
(bispectrum) estimator and find consistent results. The simulations are
provided to the community.Comment: 18 pages, 19 figures. Accepted for publication in ApJS. Simulations
can be obtained at http://planck.mpa-garching.mpg.de/cmb/fnl-simulation
Nonaxisymmetric Neutral Modes in Rotating Relativistic Stars
We study nonaxisymmetric perturbations of rotating relativistic stars.
modeled as perfect-fluid equilibria. Instability to a mode with angular
dependence sets in when the frequency of the mode vanishes. The
locations of these zero-frequency modes along sequences of rotating stars are
computed in the framework of general relativity. We consider models of
uniformly rotating stars with polytropic equations of state, finding that the
relativistic models are unstable to nonaxisymmetric modes at significantly
smaller values of rotation than in the Newtonian limit. Most strikingly, the
m=2 bar mode can become unstable even for soft polytropes of index , while in Newtonian theory it becomes unstable only for stiff polytropes
of index . If rapidly rotating neutron stars are formed by the
accretion-induced collapse of white dwarfs, instability associated with these
nonaxisymmetric, gravitational-wave driven modes may set an upper limit on
neutron-star rotation. Consideration is restricted to perturbations that
correspond to polar perturbations of a spherical star. A study of axial
perturbations is in progress.Comment: 57 pages, 9 figure
Effects of Differential Rotation on the Maximum Mass of Neutron Stars
The merger of binary neutron stars is likely to lead to differentially
rotating remnants. In this paper we numerically construct models of
differentially rotating neutron stars in general relativity and determine their
maximum allowed mass. We model the stars adopting a polytropic equation of
state and tabulate maximum allowed masses as a function of differential
rotation and stiffness of the equation of state. We also provide a crude
argument that yields a qualitative estimate of the effect of stiffness and
differential rotation on the maximum allowed mass.Comment: 6 pages, to appear in Ap
Kink Solution in a Fluid Model of Traffic Flows
Traffic jam in a fluid model of traffic flows proposed by Kerner and
Konh\"auser (B. S. Kerner and P. Konh\"auser, Phys. Rev. E 52 (1995), 5574.) is
analyzed. An analytic scaling solution is presented near the critical point of
the hetero-clinic bifurcation. The validity of the solution has been confirmed
from the comparison with the simulation of the model.Comment: RevTeX v3.1, 6 pages, and 2 figure
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