853 research outputs found
A Multi-Parameter Investigation of Gravitational Slip
A detailed analysis of gravitational slip, a new post-general relativity
cosmological parameter characterizing the degree of departure of the laws of
gravitation from general relativity on cosmological scales, is presented. This
phenomenological approach assumes that cosmic acceleration is due to new
gravitational effects; the amount of spacetime curvature produced per unit mass
is changed in such a way that a universe containing only matter and radiation
begins to accelerate as if under the influence of a cosmological constant.
Changes in the law of gravitation are further manifest in the behavior of the
inhomogeneous gravitational field, as reflected in the cosmic microwave
background, weak lensing, and evolution of large-scale structure. The new
parameter, , is naively expected to be of order unity. However, a
multiparameter analysis, allowing for variation of all the standard
cosmological parameters, finds that
where corresponds to a CDM universe under general
relativity. Future probes of the cosmic microwave background (Planck) and
large-scale structure (Euclid) may improve the limits by a factor of four.Comment: 7 pages, 9 figures, colo
Weak Lensing of the CMB: Cumulants of the Probability Distribution Function
We discuss the real-space moments of temperature anisotropies in the cosmic
microwave background (CMB) due to weak gravitational lensing by intervening
large-scale structure. We show that if the probability distribution function of
primordial temperature anisotropies is Gaussian, then it remains unchanged
after gravitational lensing. With finite resolution, however, non-zero
higher-order cumulants are generated both by lensing autocorrelations and by
cross-correlations between the lensing potential and secondary anisotropies in
the CMB such as the Sunayev-Zel'dovich (SZ) effect. Skewness is produced by
these lensing-SZ correlations, while kurtosis receives contributions from both
lensing alone and lensing-SZ correlations. We show that if the projected
lensing potential is Gaussian, all cumulants of higher-order than the kurtosis
vanish. While recent results raise the possibility of detection of the skewness
in upcoming data, the kurtosis will likely remain undetected.Comment: 11 pages, 4 figures, submitted to PR
LISA Measurement of Gravitational Wave Background Anisotropy: Hexadecapole Moment via a Correlation Analysis
We discuss spatial fluctuations in the gravitational wave background arising
from unresolved Galactic binary sources, such as close white dwarf binaries,
due to the fact the galactic binary source distribution is anisotropic. We
introduce a correlation analysis of the two data streams of the Laser
Interferometer Space Antenna (LISA) to extract spherical harmonic coefficients,
in an independent manner, of the hexadecapole moment () related to the
projected two-dimensional density distribution of the binary source population.
The proposed technique complements and improves over previous suggestions in
the literature to measure the gravitational wave background anisotropy based on
the time modulation of data as LISA orbits around the Sun. Such techniques,
however, are restricted only to certain combinations of spherical harmonic
coefficients of the galaxy with no ability to separate them individually. With
LISA, and 4 coefficients of the hexadecapole () can be measured
with signal-to-noise ratios at the level of 10 and above in a certain
coordinate system. In addition to the hexadecapole coefficients, when combined
with the time modulation analysis, the correlation study can also be used, in
principle, to measure quadrupole coefficients of the binary distribution.Comment: 8 pages, 2 figure
Scintillation Caustics in Planetary Occultation Light Curves
We revisit the GSC5249-01240 light curve obtained during its occultation by
Saturn's North polar region. In addition to refractive scintillations, the
power spectrum of intensity fluctuations shows an enhancement of power between
refractive and diffractive regimes. We identify this excess power as due to
high amplitude spikes in the light curve and suggest that these spikes are due
to caustics associated with ray crossing situations. The flux variation in
individual spikes follows the expected caustic behavior, including diffraction
fringes which we have observed for the first time in a planetary occultation
light curve. The presence of caustics in scintillation light curves require an
inner scale cut off to the power spectrum of underlying density fluctuations
associated with turbulence. Another possibility is the presence of gravity
waves in the atmosphere. While occultation light curves previously showed the
existence of refractive scintillations, a combination of small projected
stellar size and a low relative velocity during the event have allowed us to
identify caustics in this occultation. This has led us to re-examine previous
data sets, in which we have also found likely examples of caustics.Comment: 4 pages, 3 figures; ApJL submitte
Heating of the IGM
Using the cosmic virial theorem, Press-Schechter analysis and numerical
simulations, we compute the expected X-ray background (XRB) from the diffuse
IGM with the clumping factor expected from gravitational shock heating. The
predicted fluxes and temperatures are excluded from the observed XRB. The
predicted clumping can be reduced by entropy injection. The required energy is
computed from the two-point correlation function, as well as from
Press-Schechter formalisms. The minimal energy injection of 1 keV/nucleon
excludes radiative or gravitational heating as a primary energy source. We
argue that the intergalactic medium (IGM) must have been heated through violent
processes such as massive supernova bursts. If the heating proceeded through
supernova explosions, it likely proceeded in bursts which may be observable in
high redshift supernova searches. Within our model we reproduce the observed
cluster luminosity-temperature relation with energy injection of 1 keV/nucleon
if this injection is assumed to be uncorrelated with the local density. These
parameters predict that the diffuse IGM soft XRB has a temperature of ~1 keV
with a flux near 10 keV/cm^2 s str keV, which may be detectable in the near
future.Comment: to appear in ApJ Lett., 11 pages incl 1 figur
Weak Lensing by Large-Scale Structure: A Dark Matter Halo Approach
Weak gravitational lensing observations probe the spectrum and evolution of
density fluctuations and the cosmological parameters which govern them but are
currently limited to small fields and subject to selection biases. We show how
the expected signal from large-scale structure arises from the contributions
from and correlations between individual halos. We determine the convergence
power spectrum as a function of the maximum halo mass and so provide the means
to interpret results from surveys that lack high mass halos either through
selection criteria or small fields. Since shot noise from rare massive halos is
mainly responsible for the sample variance below 10', our method should aid our
ability to extract cosmological information from small fields.Comment: 4 ApJ pages, 3 figures; submitted to ApJ Letter
Second Order Corrections to Weak Lensing by Large-Scale Structure
We calculate corrections to the power spectrum predictions of weak lensing by
large scale structure due to higher order effects in the gravitational
potential. Using a perturbative approach to third order in transverse
displacements, we calculate a second order correction to the angular power
spectra of E and B mode shear and convergence resulting from dropping the
so-called Born approximation, where one integrates along the unperturbed photon
path. We also consider a correction to the power spectra from the coupling
between lenses at different redshifts. Both effects generate B-mode shear and
the latter also causes a net rotation of the background galaxy images. We show
all these corrections are at least two orders of magnitude below the
convergence or E-mode power and hence relevant only to future ultra high
precision measurements. These analytical calculations are consistent with
previous numerical estimates and validate the use of current large scale
structure weak lensing predictions for cosmological studies and future use of
B-modes as a monitor of systematic effects.Comment: 4 pages, 1 figure, submitted to ApJ
Large Scale Structure as a Probe of Gravitational Slip
A new time-dependent, scale-independent parameter, Ï, is employed in a phenomenological model of the deviation from general relativity in which the Newtonian and longitudinal gravitational potentials slip apart on cosmological scales as dark energy, assumed to be arising from a new theory of gravitation, appears to dominate the Universe. A comparison is presented between Ï and other parametrized post-Friedmannian models in the literature. The effect of Ï on the cosmic microwave background anisotropy spectrum, the growth of large-scale structure, the galaxy weak-lensing correlation function, and cross correlations of cosmic microwave background anisotropy with galaxy clustering are illustrated. Cosmological models with conventional maximum likelihood parameters are shown to find agreement with a narrow range of gravitational slip
Lensed Cosmic Microwave Background Constraints on Post-General Relativity Parameters
The constraints on departures from general relativity (GR) at cosmological
length scales due to cosmic microwave background (CMB) data are discussed. The
departure from GR is measured by the ratio, parameterized as , between the gravitational potentials conventionally appearing in the
geodesic equation and the Poisson equation. Current CMB data indicate
at the 2 confidence level, while
remains unconstrained. The departure from GR affects the lensing conversion of
E-mode into B-mode polarization. Hence, the lensing measurements from a future
CMBpol experiment should be able to improve the constraints to
for a fiducial model and independent of .Comment: 4 pages, 2 figure
Power Spectrum Covariance of Weak Gravitational Lensing
Weak gravitational lensing observations probe the spectrum and evolution of
density fluctuations and the cosmological parameters which govern them. At low
redshifts, the non-linear gravitational evolution of large scale structure
produces a non-Gaussian covariance in the shear power spectrum measurements
that affects their translation into cosmological parameters. Using the dark
matter halo approach, we study the covariance of binned band power spectrum
estimates and the four point function of the dark matter density field that
underlies it. We compare this semi-analytic estimate to results from N-body
numerical simulations and find good agreement. We find that for a survey out to
z ~ 1, the power spectrum covariance increases the errors on cosmological
parameters determined under the Gaussian assumption by about 15%.Comment: 11 ApJ pages, 6 figures; submitted to Ap
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