168 research outputs found
DSR as an explanation of cosmological structure
Deformed special relativity (DSR) is one of the possible realizations of a
varying speed of light (VSL). It deforms the usual quadratic dispersion
relations so that the speed of light becomes energy dependent, with preferred
frames avoided by postulating a non-linear representation of the Lorentz group.
The theory may be used to induce a varying speed of sound capable of generating
(near) scale-invariant density fluctuations, as discussed in a recent Letter.
We identify the non-linear representation of the Lorentz group that leads to
scale-invariance, finding a universal result. We also examine the higher order
field theory that could be set up to represent it
Non-Gaussian CMBR angular power spectra
In this paper we show how the prediction of CMBR angular power spectra
in non-Gaussian theories is affected by a cosmic covariance problem, that is
correlations impart features on any observed spectrum
which are absent from the average spectrum. Therefore the average
spectrum is rendered a bad observational prediction, and two new prediction
strategies, better adjusted to these theories, are proposed. In one we search
for hidden random indices conditional to which the theory is released from the
correlations. Contact with experiment can then be made in the form of the
conditional power spectra plus the random index distribution. In another
approach we apply to the problem a principal component analysis. We discuss the
effect of correlations on the predictivity of non-Gaussian theories. We finish
by showing how correlations may be crucial in delineating the borderline
between predictions made by non-Gaussian and Gaussian theories. In fact, in
some particular theories, correlations may act as powerful non-Gaussianity
indicators
A Bayesian estimate of the skewness of the Cosmic Microwave Background
We propose a formalism for estimating the skewness and angular power spectrum
of a general Cosmic Microwave Background data set. We use the Edgeworth
Expansion to define a non-Gaussian likelihood function that takes into account
the anisotropic nature of the noise and the incompleteness of the sky coverage.
The formalism is then applied to estimate the skewness of the publicly
available 4 year Cosmic Background Explorer (COBE) Differential Microwave
Radiometer data. We find that the data is consistent with a Gaussian skewness,
and with isotropy. Inclusion of non Gaussian degrees of freedom has essentially
no effect on estimates of the power spectrum, if each is regarded as a
separate parameter or if the angular power spectrum is parametrized in terms of
an amplitude (Q) and spectral index (n). Fixing the value of the angular power
spectrum at its maxiumum likelihood estimate, the best fit skewness is
S=6.5\pm6.0\times10^4(\muK)^3; marginalizing over Q the estimate of the
skewness is S=6.5\pm8.4\times10^4(\muK)^3 and marginalizing over n one has
S=6.5\pm8.5\times10^4(\muK)^3.Comment: submitted to Astrophysical Journal Letter
Analytical modeling of large-angle CMBR anisotropies from textures
We propose an analytic method for predicting the large angle CMBR temperature
fluctuations induced by model textures. The model makes use of only a small
number of phenomenological parameters which ought to be measured from simple
simulations. We derive semi-analytically the -spectrum for together with its associated non-Gaussian cosmic variance error bars. A
slightly tilted spectrum with an extra suppression at low is found, and we
investigate the dependence of the tilt on the parameters of the model. We also
produce a prediction for the two point correlation function. We find a high
level of cosmic confusion between texture scenarios and standard inflationary
theories in any of these quantities. However, we discover that a distinctive
non-Gaussian signal ought to be expected at low , reflecting the prominent
effect of the last texture in these multipoles
Testable polarization predictions for models of CMB isotropy anomalies
Anomalies in the large-scale CMB temperature sky measured by WMAP have been
suggested as possible evidence for a violation of statistical isotropy on large
scales. In any physical model for broken isotropy, there are testable
consequences for the CMB polarization field. We develop simulation tools for
predicting the polarization field in models that break statistical isotropy
locally through a modulation field. We study two different models: dipolar
modulation, invoked to explain the asymmetry in power between northern and
southern ecliptic hemispheres, and quadrupolar modulation, posited to explain
the alignments between the quadrupole and octopole. For the dipolar case, we
show that predictions for the correlation between the first 10 multipoles of
the temperature and polarization fields can typically be tested at better than
the 98% CL. For the quadrupolar case, we show that the polarization quadrupole
and octopole should be moderately aligned. Such an alignment is a generic
prediction of explanations which involve the temperature field at recombination
and thus discriminate against explanations involving foregrounds or local
secondary anisotropy. Predicted correlations between temperature and
polarization multipoles out to l = 5 provide tests at the ~ 99% CL or stronger
for quadrupolar models that make the temperature alignment more than a few
percent likely. As predictions of anomaly models, polarization statistics move
beyond the a posteriori inferences that currently dominate the field.Comment: 17 pages, 15 figures; published in PRD; references adde
Bouncing Universes with Varying Constants
We investigate the behaviour of exact closed bouncing Friedmann universes in
theories with varying constants. We show that the simplest BSBM varying-alpha
theory leads to a bouncing universe. The value of alpha increases
monotonically, remaining approximately constant during most of each cycle, but
increasing significantly around each bounce. When dissipation is introduced we
show that in each new cycle the universe expands for longer and to a larger
size. We find a similar effect for closed bouncing universes in Brans-Dicke
theory, where also varies monotonically in time from cycle to cycle.
Similar behaviour occurs also in varying speed of light theories
Observing the temperature of the Big Bang through large scale structure
It is widely accepted that the Universe underwent a period of thermal
equilibrium at very early times. One expects a residue of this primordial state
to be imprinted on the large scale structure of space time. In this paper we
study the morphology of this thermal residue in a universe whose early dynamics
is governed by a scalar field. We calculate the amplitude of fluctuations on
large scales and compare it to the imprint of vacuum fluctuations. We then use
the observed power spectrum of fluctuations on the cosmic microwave background
to place a constraint on the temperature of the Universe before and during
inflation. We also present an alternative scenario where the fluctuations are
predominantly thermal and near scale-invariant
Noncommutative fields in three dimensions and mass generation
We apply the noncommutative fields method for gauge theory in three
dimensions where the Chern-Simons term is generated in the three-dimensional
electrodynamics. Under the same procedure, the Chern-Simons term is shown to be
cancelled in the Maxwell-Chern-Simons theory for the appropriate value of the
noncommutativity parameter. Hence the mutual interchange between
Maxwell-Chern-Simons theory and pure Maxwell theory turns out to be generated
within this method.Comment: Comments 5 pages, epl, version accepted for publication in
Europhysics Letter
Generating non-Gaussian maps with a given power spectrum and bispectrum
We propose two methods for generating non-Gaussian maps with fixed power
spectrum and bispectrum. The first makes use of a recently proposed rigorous,
non-perturbative, Bayesian framework for generating non-Gaussian distributions.
The second uses a simple superposition of Gaussian distributions. The former is
best suited for generating mildly non-Gaussian maps, and we discuss in detail
the limitations of this method. The latter is better suited for the opposite
situation, i.e. generating strongly non-Gaussian maps. The ensembles produced
are isotropic and the power spectrum can be jointly fixed; however we cannot
set to zero all other higher order cumulants (an unavoidable mathematical
obstruction). We briefly quantify the leakage into higher order moments present
in our method. We finally present an implementation of our code within the
HEALPIX packageComment: 22 pages submitted to PRD, astro-ph version only includes low
resolution map
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