36 research outputs found
Bayesian Analysis of the Polarization of Distant Radio Sources: Limits on Cosmological Birefringence
A recent study of the rotation of the plane of polarization of light from 160
cosmological sources claims to find significant evidence for cosmological
anisotropy. We point out methodological weaknesses of that study, and reanalyze
the same data using Bayesian methods that overcome these problems. We find that
the data always favor isotropic models for the distribution of observed
polarizations over counterparts that have a cosmological anisotropy of the type
advocated in the earlier study. Although anisotropic models are not completely
ruled out, the data put strong lower limits on the length scale (in
units of the Hubble length) associated with the anisotropy; the lower limits of
95% credible regions for lie between 0.43 and 0.62 in all anisotropic
models we studied, values several times larger than the best-fit value of
found in the earlier study. The length scale is not
constrained from above. The vast majority of sources in the data are at
distances closer than 0.4 Hubble lengths (corresponding to a redshift of
0.8); the results are thus consistent with there being no significant
anisotropy on the length scale probed by these data.Comment: 8 pages, 3 figures; submitted to Phys. Rev.
Dynamical Lorentz simmetry breaking from 3+1 Axion-Wess-Zumino model
We study the renormalizable abelian vector-field models in the presence of
the Wess-Zumino interaction with the pseudoscalar matter. The renormalizability
is achieved by supplementing the standard kinetic term of vector fields with
higher derivatives. The appearance of fourth power of momentum in the
vector-field propagator leads to the super-renormalizable theory in which the
-function, the vector-field renormalization constant and the anomalous
mass dimension are calculated exactly. It is shown that this model has the
infrared stable fixed point and its low-energy limit is non-trivial. The
modified effective potential for the pseudoscalar matter leads to the possible
occurrence of dynamical breaking of the Lorentz symmetry. This phenomenon is
related to the modification of Electrodynamics by means of the Chern-Simons
(CS) interaction polarized along a constant CS vector. Its presence makes the
vacuum optically active that has been recently estimated from astrophysical
data. We examine two possibilities for the CS vector to be time-like or
space-like, under the assumption that it originates from v.e.v. of some
pseudoscalar matter and show that only the latter one is consistent in the
framework of the AWZ model, because a time-like CS vector makes the vacuum
unstable under pairs creation of tachyonic photon modes with the finite vacuum
decay rate.Comment: 33 pages, no Figures, Plain TeX, submitted to Phys. Rev.
Cosmic optical activity in the spacetime of a scalar-tensor screwed cosmic string
Measurements of radio emission from distant galaxies and quasars verify that
the polarization vectors of these radiations are not randomly oriented as
naturally expected. This peculiar phenomenon suggests that the spacetime
intervening between the source and observer may be exhibiting some sort of
optical activity, the origin of which is not known. In the present paper we
provide a plausible explanation to this phenomenon by investigating the r\^ole
played by a Chern-Simons-like term in the background of an ordinary or
superconducting screwed cosmic string in a scalar-tensor gravity. We discuss
the possibility that the excess in polarization of the light from
radio-galaxies and quasars can be understood as if the electromagnetic waves
emitted by these cosmic objects interact with a scalar-tensor screwed cosmic
string through a Chern-Simons coupling. We use current astronomical data to
constrain possible values for the coupling constant of this theory, and show
that it turns out to be: eV, which is two orders of
magnitude larger than in string-inspired theories.Comment: Revised version, to appear in Phys. Rev.
Cosmological Birefringence: an Astrophysical test of Fundamental Physics
We review the methods used to test for the existence of cosmological
birefringence, i.e. a rotation of the plane of linear polarization for
electromagnetic radiation traveling over cosmological distances, which might
arise in a number of important contexts involving the violation of fundamental
physical principles. The main methods use: (1) the radio polarization of radio
galaxies and quasars, (2) the ultraviolet polarization of radio galaxies, and
(3) the cosmic microwave background polarization. We discuss the main results
obtained so far, the advantages and disadvantages of each method, and future
prospects.Comment: To appear in the Proceedings of the JENAM 2010 Symposium "From
Varying Couplings to Fundamental Physics", held in Lisbon, 6-10 Sept. 201
Kalb-Ramond excitations in a thick-brane scenario with dilaton
We compute the full spectrum and eigenstates of the Kalb-Ramond field in a
warped non-compact Randall-Sundrum -type five-dimensional spacetime in which
the ordinary four-dimensional braneworld is represented by a sine-Gordon
soliton. This 3-brane solution is fully consistent with both the warped
gravitational field and bulk dilaton configurations. In such a background we
embed a bulk antisymmetric tensor field and obtain, after reduction, an
infinite tower of normalizable Kaluza-Klein massive components along with a
zero-mode. The low lying mass eigenstates of the Kalb-Ramond field may be
related to the axion pseudoscalar. This yields phenomenological implications on
the space of parameters, particularly on the dilaton coupling constant. Both
analytical and numerical results are given.Comment: 10 pages, 13 figures, and 2 tables. Final version to appear in The
European Physical Journal
Geometrodynamics of Variable-Speed-of-Light Cosmologies
This paper is dedicated to the memory of Dennis Sciama.
Variable-Speed-of-Light (VSL) cosmologies are currently attracting interest
as an alternative to inflation. We investigate the fundamental geometrodynamic
aspects of VSL cosmologies and provide several implementations which do not
explicitly break Lorentz invariance (no "hard" breaking). These "soft"
implementations of Lorentz symmetry breaking provide particularly clean answers
to the question "VSL with respect to what?". The class of VSL cosmologies we
consider are compatible with both classical Einstein gravity and low-energy
particle physics. These models solve the "kinematic" puzzles of cosmology as
well as inflation does, but cannot by themselves solve the flatness problem,
since in their purest form no violation of the strong energy condition occurs.
We also consider a heterotic model (VSL plus inflation) which provides a number
of observational implications for the low-redshift universe if chi contributes
to the "dark energy" either as CDM or quintessence. These implications include
modified gravitational lensing, birefringence, variation of fundamental
constants and rotation of the plane of polarization of light from distant
sources.Comment: 19 pages, latex 209, revtex 3.1; To appear in Physical Review D;
Numerous small changes of presentation and emphasis; new section on the
entropy problem; references updated; central results unaffecte
Rotational Perturbations in Neveu-Schwarz-Neveu-Schwarz String Cosmology
First order rotational perturbations of the flat Friedmann-Robertson-Walker
(FRW) metric are considered in the framework of four dimensional
Neveu-Schwarz-Neveu-Schwarz (NS-NS) string cosmological models coupled with
dilaton and axion fields. The decay rate of rotation depends mainly upon the
dilaton field potential U. The equation for rotation imposes strong limitations
upon the functional form of U, restricting the allowed potentials to two: the
trivial case U=0 and a generalized exponential type potential. In these two
models the metric rotation function can be obtained in an exact analytic form
in both Einstein and string frames. In the potential-free case the decay of
rotational perturbations is governed by an arbitrary function of time while in
the presence of a potential the rotation tends rapidly to zero in both Einstein
and string frames.Comment: 13 pages, 8 figure
Is there evidence for cosmic anisotropy in the polarization of distant radio sources?
Measurements of the polarization angle and orientation of cosmological radio
sources may be used to search for unusual effects in the propagation of light
through the universe. Recently, Nodland and Ralston (astro-ph/9704196) have
claimed to find evidence for a redshift- and direction-dependent rotation
effect in existing data. We re-examine these data and argue that there is no
statistically significant signal present. We are able to place stringent limits
on hypothetical chiral interactions of photons propagating through spacetime.Comment: 17 pages, including 7 figures. (Arithmetic error corrected, fixing
value of chi^2
Rotation of the Universe and the angular momenta of celestial bodies
We discuss the equation of motion of the rotating homogenous and isotropic
model of the Universe. We show that the model predicts the presence of a
minimum in the relation between the mass of an astronomical object and its
angular momentum. We show that this relation appears to be universal, and we
predict the masses of structures with minimal angular momenta in agreement with
observations. In such a manner we suggest the possibility at acquirement of
angular momenta of celestial bodies during their formation from the global
rotation of the Universe.Comment: 10 pages 2 figure
Optical Activity of a Neutrino Gas
For photons that propagate through a gas of neutrinos with a non-zero
chemical potential, the left-handed and right-handed polarization modes acquire
different dispersion relations. This is due to the CP and CPT-odd terms induced
by such a background on the photon self-energy. We present a detailed
calculation of this effect, which does not depend on any physical assumptions
beyond those of the standard electroweak model. Some possible cosmological and
astrophysical implications of our results are considered in several contexts,
including the recent discussions regarding the rotation of the plane of
polarization of electromagnetic waves over cosmological distances.Comment: latex, axodra