507 research outputs found
Helical Magnetic Fields from Inflation
We analyze the generation of seed magnetic fields during de Sitter inflation
considering a non-invariant conformal term in the electromagnetic Lagrangian of
the form , where
is a pseudoscalar function of a non-trivial background field .
In particular, we consider a toy model, that could be realized owing to the
coupling between the photon and either a (tachyonic) massive pseudoscalar field
and a massless pseudoscalar field non-minimally coupled to gravity, where
follows a simple power-law behavior during
inflation, while it is negligibly small subsequently. Here, is a positive
dimensionless constant, the wavenumber, the conformal time, and
a real positive number. We find that only when and astrophysically interesting fields can be produced as
excitation of the vacuum, and that they are maximally helical.Comment: 17 pages, 1 figure, subsection IIc and references added; accepted for
publication in IJMP
Cosmological quests in the CMB sky
Observational Cosmology has indeed made very rapid progress in recent years.
The ability to quantify the universe has largely improved due to observational
constraints coming from structure formation Measurements of CMB anisotropy and,
more recently, polarization have played a very important role. Besides precise
determination of various parameters of the `standard' cosmological model,
observations have also established some important basic tenets that underlie
models of cosmology and structure formation in the universe -- `acausally'
correlated initial perturbations in a flat, statistically isotropic universe,
adiabatic nature of primordial density perturbations. These are consistent with
the expectation of the paradigm of inflation and the generic prediction of the
simplest realization of inflationary scenario in the early universe. Further,
gravitational instability is the established mechanism for structure formation
from these initial perturbations. In the next decade, future experiments
promise to strengthen these deductions and uncover the remaining crucial
signature of inflation -- the primordial gravitational wave background.Comment: Plenary talk at the International Conference on Einstein's Legacy in
the New Millennium, December 15 - 22, 2005, Puri, India; to appear in the
Proceedings to be published in IJMPD; 18 pages, 7 figure
Reconstructing the Primordial Spectrum with CMB Temperature and Polarization
We develop a new method to reconstruct the power spectrum of primordial
curvature perturbations, , by using both the temperature and polarization
spectra of the cosmic microwave background (CMB). We test this method using
several mock primordial spectra having non-trivial features including the one
with an oscillatory component, and find that the spectrum can be reconstructed
with a few percent accuracy by an iterative procedure in an ideal situation in
which there is no observational error in the CMB data. In particular, although
the previous ``cosmic inversion'' method, which used only the temperature
fluctuations, suffered from large numerical errors around some specific values
of that correspond to nodes in a transfer function, these errors are found
to disappear almost completely in the new method.Comment: 18 pages, 17 figures, submitted to PR
Circular Polarization from Gamma-ray Burst Afterglows
We investigate the circular polarization (CP) from Gamma-Ray Burst (GRB)
afterglows. We show that a tangled magnetic field cannot generate CP without an
ordered magnetic field because there is always an oppositely directed field, so
that no handedness exists. This implies the observation of CP could be a useful
probe of an ordered field, which carries valuable information on the GRB
central engine. By solving the transfer equation of polarized radiation, we
find that the CP reaches 1% at radio frequencies and 0.01% at optical for the
forward shock, and 10-1% at radio and 0.1-0.01% at optical for the reverse
shock.Comment: 12 pages, 3 figure
Reconstructing the primordial power spectrum - a new algorithm
We propose an efficient and model independent method for reconstructing the
primordial power spectrum from Cosmic Microwave Background (CMB) and large
scale structure observations. The algorithm is based on a Monte Carlo principle
and therefore very simple to incorporate into existing codes such as Markov
Chain Monte Carlo. The algorithm has been used on present cosmological data to
test for features in the primordial power spectrum. No significant evidence for
features is found, although there is a slight preference for an overall bending
of the spectrum, as well as a decrease in power at very large scales. We have
also tested the algorithm on mock high precision CMB data, calculated from
models with non-scale invariant primordial spectra. The algorithm efficiently
extracts the underlying spectrum, as well as the other cosmological parameters
in each case. Finally we have used the algorithm on a model where an artificial
glitch in the CMB spectrum has been imposed, like the ones seen in the WMAP
data. In this case it is found that, although the underlying cosmological
parameters can be extracted, the recovered power spectrum can show significant
spurious features, such as bending, even if the true spectrum is scale
invariant.Comment: 22 pages, 12 figures, matches JCAP published versio
Primordial Power Spectrum Reconstruction
In order to reconstruct the initial conditions of the universe it is
important to devise a method that can efficiently constrain the shape of the
power spectrum of primordial matter density fluctuations in a model-independent
way from data. In an earlier paper we proposed a method based on the wavelet
expansion of the primordial power spectrum. The advantage of this method is
that the orthogonality and multiresolution properties of wavelet basis
functions enable information regarding the shape of to be
encoded in a small number of non-zero coefficients. Any deviation from
scale-invariance can then be easily picked out. Here we apply this method to
simulated data to demonstrate that it can accurately reconstruct an input
, and present a prescription for how this method should be used
on future data.Comment: 4 pages, 2 figures. JCAP accepted versio
A diffuse scattering model of ultracold neutrons on wavy surfaces
Metal tubes plated with nickel-phosphorus are used in many fundamental
physics experiments using ultracold neutrons (UCN) because of their ease of
fabrication. These tubes are usually polished to a average roughness of 25-150
nm. However, there is no scattering model that accurately describes UCN
scattering on such a rough guide surface with a mean-square roughness larger
than 5 nm. We therefore developed a scattering model for UCN in which
scattering from random surface waviness with a size larger than the UCN
wavelength is described by a microfacet Bidirectional Reflectance Distribution
Function model (mf-BRDF model), and scattering from smaller structures by the
Lambert's cosine law (Lambert model). For the surface waviness, we used the
statistical distribution of surface slope measured by an atomic force
microscope on a sample piece of guide tube as input of the model. This model
was used to describe UCN transmission experiments conducted at the pulsed UCN
source at J-PARC. In these experiments, a UCN beam collimated to a divergence
angle smaller than was directed into a guide tube with a
mean-square roughness of 6.4 nm to 17 nm at an oblique angle, and the UCN
transport performance and its time-of-flight distribution were measured while
changing the angle of incidence. The mf-BRDF model combined with the Lambert
model with scattering probability reproduced the
experimental results well. We have thus established a procedure to evaluate the
characteristics of UCN guide tubes with a surface roughness of approximately 10
nm.Comment: 15 pages, 11 figure
Natural Inflation, Planck Scale Physics and Oscillating Primordial Spectrum
In the ``natural inflation'' model, the inflaton potential is periodic. We
show that Planck scale physics may induce corrections to the inflaton
potential, which is also periodic with a greater frequency. Such high frequency
corrections produce oscillating features in the primordial fluctuation power
spectrum, which are not entirely excluded by the current observations and may
be detectable in high precision data of cosmic microwave background (CMB)
anisotropy and large scale structure (LSS) observations.Comment: 20 pages, 11 figures. To appear in Int J Mod. Phys.
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