20 research outputs found

### Large Angular Scale Polarization of the Cosmic Microwave Background and the Feasibility of its Detection

In addition to its spectrum and temperature anisotropy, the 2.7K Cosmic
Microwave Background is also expected to exhibit a low level of polarization.
The spatial power spectrum of the polarization can provide details about the
formation of structure in the universe as well as its ionization history. Here
we calculate the magnitude of the CMB polarization in various cosmological
scenarios, with both an analytic and a numerical method. We then outline the
fundemental challenges to measuring these signals and focus on two of them:
achieving adequate sensitivity and removing contamination from foreground
sources. We then describe the design of a ground based instrument (POLAR) that
could detect polarization of the CMB at large angular scales in the new few
years.Comment: 40 pages, 7 figures, accepted for publication in the Astrophysical
Journa

### Propagation of Light in the Field of Stationary and Radiative Gravitational Multipoles

Extremely high precision of near-future radio/optical interferometric observatories like SKA, Gaia, SIM and the unparalleled sensitivity of LIGO/LISA gravitational-wave detectors demands more deep theoretical treatment of relativistic effects in the propagation of electromagnetic signals through variable gravitational fields of the solar system, oscillating and precessing neutron stars, coalescing binary systems, exploding supernova, and colliding galaxies. Especially important for future gravitational-wave observatories is the problem of propagation of light rays in the field of multipolar gravitational waves emitted by a localized source of gravitational radiation. Present paper suggests physically-adequate and consistent mathematical solution of this problem in the first post-Minkowskian approximation of General Relativity which accounts for all time-dependent multipole moments of an isolated astronomical system

### The Polarization of the Cosmic Microwave Background Due to Primordial Gravitational Waves

We review current observational constraints on the polarization of the Cosmic
Microwave Background (CMB), with a particular emphasis on detecting the
signature of primordial gravitational waves. We present an analytic solution to
the Polanarev approximation for CMB polarization produced by primordial
gravitational waves. This simplifies the calculation of the curl, or B-mode
power spectrum associated with gravitational waves during the epoch of
cosmological inflation. We compare our analytic method to existing numerical
methods and also make predictions for the sensitivity of upcoming CMB
polarization observations to the inflationary gravitational wave background. We
show that upcoming experiments should be able either detect the relic
gravitational wave background or completely rule out whole classes of
inflationary models.Comment: 25 pages, 4 figures, review published in IJMP

### Gravimagnetic effect of the barycentric motion of the Sun and determination of the post-Newtonian parameter gamma in the Cassini experiment

The most precise test of the post-Newtonian gamma parameter in the solar system has been achieved in measurement of the frequency shift of radio waves to and from the Cassini spacecraft as they passed near the Sun. The test relies upon the JPL model of radiowave propagation that includes, but does not explicitly parametrize, the impact of the non-stationary component of the gravitational field of the Sun, generated by its barycentric orbital motion, on the Shapiro delay. This non-stationary gravitational field of the Sun is associated with the Lorentz transformation of the metric tensor and the affine connection from the heliocentric to the barycentric frame of the solar system and can be treated as gravimagnetic field. The gravimagnetic field perturbs the propagation of a radio wave and contributes to its frequency shift at the level up to 4 10^{-13} that may affect the precise measurement of the parameter gamma in the Cassini experiment to about one part in 10,000. Our analysis suggests that the translational gravimagnetic field of the Sun can be extracted from the Cassini data, and its effect is separable from the space curvature characterized by the parameter gamma

### Primordial black hole formation in the radiative era: investigation of the critical nature of the collapse

Following on after two previous papers discussing the formation of primordial
black holes in the early universe, we present here results from an in-depth
investigation of the extent to which primordial black hole formation in the
radiative era can be considered as an example of the critical collapse
phenomenon. We focus on initial supra-horizon-scale perturbations of a type
which could have come from inflation, with only a growing component and no
decaying component. In order to study perturbations with amplitudes extremely
close to the supposed critical limit, we have modified our previous computer
code with the introduction of an adaptive mesh refinement scheme. This has
allowed us to follow black hole formation from perturbations whose amplitudes
are up to eight orders of magnitude closer to the threshold than we could do
before. We find that scaling-law behaviour continues down to the smallest black
hole masses that we are able to follow and we see no evidence of shock
production such as has been reported in some previous studies and which led
there to a breaking of the scaling-law behaviour at small black-hole masses. We
attribute this difference to the different initial conditions used. In addition
to the scaling law, we also present other features of the results which are
characteristic of critical collapse in this context.Comment: 21 pages, 7 figures, the present version is updated with some changes
and two new appendix. Accepted for pubblication in Classical and Quantum
Gravit

### Propagation of Light in the Field of Stationary and Radiative Gravitational Multipoles

Extremely high precision of near-future radio/optical interferometric
observatories like SKA, Gaia, SIM and the unparalleled sensitivity of LIGO/LISA
gravitational-wave detectors demands more deep theoretical treatment of
relativistic effects in the propagation of electromagnetic signals through
variable gravitational fields of the solar system, oscillating and precessing
neutron stars, coalescing binary systems, exploding supernova, and colliding
galaxies. Especially important for future gravitational-wave observatories is
the problem of propagation of light rays in the field of multipolar
gravitational waves emitted by a localized source of gravitational radiation.
Present paper suggests physically-adequate and consistent mathematical solution
of this problem in the first post-Minkowskian approximation of General
Relativity which accounts for all time-dependent multipole moments of an
isolated astronomical system.Comment: 36 pages, no figure

### $CPT$ Violating Electrodynamics and Chern-Simons Modified Gravity

The electrodynamics with a Chern-Simons term
$p_{\mu}A_{\nu}\widetilde{F}^{\mu\nu}$ violates Lorentz and $CPT$ symmetries
with a non-vanishing $p_{\mu}$. For a fixed vector $p_{\mu}$, in this paper we
point out that the energy-momentum tensor of this theory coupled to the gravity
minimally is symmetric but not divergence free, which consequently makes the
gravitational field equation inconsistent. To preserve the consistency, we
introduce a Chern-Simons term in the gravity sector with the coefficient
determined by the Lorentz and $CPT$ violating term in the electromagnetic
field. Further we study the phenomenologies of the model on the cosmic
microwave background radiation and the relic gravitational waves.Comment: 11 pages, 1 figure, the version to appear in Physics Letters

### Formation of Black Holes from Collapsed Cosmic String Loops

The fraction of cosmic string loops which collapse to form black holes is
estimated using a set of realistic loops generated by loop fragmentation. The
smallest radius sphere into which each cosmic string loop may fit is obtained
by monitoring the loop through one period of oscillation. For a loop with
invariant length $L$ which contracts to within a sphere of radius $R$, the
minimum mass-per-unit length $\mu_{\rm min}$ necessary for the cosmic string
loop to form a black hole according to the hoop conjecture is $\mu_{\rm min} =
R /(2 G L)$. Analyzing $25,576$ loops, we obtain the empirical estimate $f_{\rm
BH} = 10^{4.9\pm 0.2} (G\mu)^{4.1 \pm 0.1}$ for the fraction of cosmic string
loops which collapse to form black holes as a function of the mass-per-unit
length $\mu$ in the range $10^{-3} \lesssim G\mu \lesssim 3 \times 10^{-2}$. We
use this power law to extrapolate to $G\mu \sim 10^{-6}$, obtaining the
fraction $f_{\rm BH}$ of physically interesting cosmic string loops which
collapse to form black holes within one oscillation period of formation.
Comparing this fraction with the observational bounds on a population of
evaporating black holes, we obtain the limit $G\mu \le 3.1 (\pm 0.7) \times
10^{-6}$ on the cosmic string mass-per-unit-length. This limit is consistent
with all other observational bounds.Comment: uuencoded, compressed postscript; 20 pages including 7 figure

### Black Holes from Nucleating Strings

We evaluate the probability that a loop of string that has spontaneously
nucleated during inflation will form a black hole upon collapse, after the end
of inflation. We then use the observational bounds on the density of primordial
black holes to put constraints on the parameters of the model. Other
constraints from the distortions of the microwave background and emission of
gravitational radiation by the loops are considered. Also, observational
constraints on domain wall nucleation and monopole pair production during
inflation are briefly discussed.Comment: 27 pages, tutp-92-