494 research outputs found
Gravitational Wave Spectrum Induced by Primordial Scalar Perturbations
We derive the complete spectrum of gravitational waves induced by primordial
scalar perturbations ranging over all observable wavelengths. This
scalar-induced contribution can be computed directly from the observed scalar
perturbations and general relativity and is, in this sense, independent of the
cosmological model for generating the perturbations. The spectrum is
scale-invariant on small scales, but has an interesting scale-dependence on
large and intermediate scales, where scalar-induced gravitational waves do not
redshift and are hence enhanced relative to the background density of the
Universe. This contribution to the tensor spectrum is significantly different
in form from the direct model-dependent primordial tensor spectrum and,
although small in magnitude, it dominates the primordial signal for some
cosmological models. We confirm our analytical results by direct numerical
integration of the equations of motion.Comment: 19 pages, 5 figure
Cosmic microwave background bispectrum of tensor passive modes induced from primordial magnetic fields
If the seed magnetic fields exist in the early Universe, tensor components of
their anisotropic stresses are not compensated prior to neutrino decoupling and
the tensor metric perturbations generated from them survive passively.
Consequently, due to the decay of these metric perturbations after
recombination, the so-called integrated Sachs-Wolfe effect, the large-scale
fluctuations of CMB radiation are significantly boosted. This kind of CMB
anisotropy is called the "tensor passive mode." Because these fluctuations
deviate largely from the Gaussian statistics due to the quadratic dependence on
the strength of the Gaussian magnetic field, not only the power spectrum but
also the higher-order correlations have reasonable signals. With these motives,
we compute the CMB bispectrum induced by this mode. When the magnetic spectrum
obeys a nearly scale-invariant shape, we obtain an estimation of a typical
value of the normalized reduced bispectrum as depending on the energy scale of the magnetic field
production from GeV to GeV. Here, is the
strength of the primordial magnetic field smoothed on . From the
above estimation and the current observational constraint on the primordial
non-Gaussianity, we get a rough constraint on the magnetic field strength as
.Comment: 5 pages, 2 figures. Accepted for publication in PR
Probing the Nature of the Weakest Intergalactic Magnetic Fields with the High Energy Emission of Gamma-Ray Bursts
We investigate the delayed, secondary GeV-TeV emission of gamma-ray bursts
and its potential to probe the nature of intergalactic magnetic fields.
Geometrical effects are properly taken into account for the time delay between
primary high energy photons and secondary inverse Compton photons from
electron-positron pairs, which are produced in - interactions
with background radiation fields and deflected by intervening magnetic fields.
The time-dependent spectra of the delayed emission are evaluated for a wide
range of magnetic field strengths and redshifts. The typical flux and delay
time of secondary photons from bursts at are respectively GeV cm s and s if the field strengths are
G, as might be the case in intergalactic void regions. We find
crucial differences between the cases of coherent and tangled magnetic fields,
as well as dependences on the field coherence length.Comment: 19 pages, 9 figures, formulation revised, accepted for publication in
Ap
Effects of a primordial magnetic field with log-normal distribution on the cosmic microwave background
We study the effect of primordial magnetic fields (PMFs) on the anisotropies
of the cosmic microwave background (CMB). We assume the spectrum of PMFs is
described by log-normal distribution which has a characteristic scale, rather
than power-law spectrum. This scale is expected to reflect the generation
mechanisms and our analysis is complementary to previous studies with power-law
spectrum. We calculate power spectra of energy density and Lorentz force of the
log-normal PMFs, and then calculate CMB temperature and polarization angular
power spectra from scalar, vector, and tensor modes of perturbations generated
from such PMFs. By comparing these spectra with WMAP7, QUaD, CBI, Boomerang,
and ACBAR data sets, we find that the current CMB data set places the strongest
constraint at Mpc with the upper limit
nG.Comment: 14 pages, 6 figure
EoS of finite density QCD with Wilson fermions by Multi-Parameter Reweighting and Taylor expansion
The equation of state (EoS), quark number density and susceptibility at
nonzero quark chemical potential are studied in lattice QCD simulations
with a clover-improved Wilson fermion of 2-flavors and RG-improved gauge
action. To access nonzero , we employ two methods : a multi-parameter
reweighting (MPR) in and and Taylor expansion in . The use
of a reduction formula for the Wilson fermion determinant enables to study the
reweighting factor in MPR explicitly and heigher-order coefficients in Taylor
expansion free from errors of noise method, although calculations are limited
to small lattice size. As a consequence, we can study the reliability of the
thermodynamical quantities through the consistency of the two methods, each of
which has different origin of the application limit.
The thermodynamical quantities are obtained from simulations on a lattice with an intermediate quark mass(. The MPR
and Taylor expansion are consistent for the EoS and number density up to
and for the number susceptibility up to . This
implies within a given statistics that the overlap problem for the MPR and
truncation error for the Taylor expansion method are negligible in these
regions.
In order to make MPR methods work, the fluctuation of the reweighting factor
should be small. We derive the equation of the reweighting line where the
fluctuation is small, and show that the equation of the reweighting line is
consistent with the fluctuation minimum condition.Comment: 20 pages, 11 figures. Accepted to JHEP. Discussions are added.
Figures for Taylor coefficients (Fig. 7) are modifie
Exactly solvable model for cosmological perturbations in dilatonic brane worlds
We construct a model where cosmological perturbations are analytically solved
based on dilatonic brane worlds. A bulk scalar field has an exponential
potential in the bulk and an exponential coupling to the brane tension. The
bulk scalar field yields a power-law inflation on the brane. The exact
background metric can be found including the back-reaction of the scalar field.
Then exact solutions for cosmological perturbations which properly satisfy the
junction conditions on the brane are derived. These solutions provide us an
interesting model to understand the connection between the behavior of
cosmological perturbations on the brane and the geometry of the bulk. Using
these solutions, the behavior of an anisotropic stress induced on the
inflationary brane by bulk gravitational fields is investigated.Comment: 30 pages, typos corrected, reference adde
Effects of Neutrino Oscillation on the Supernova Neutrino Spectrum
The effects of three-flavor neutrino oscillation on the supernova neutrino
spectrum are studied. We calculate the expected event rate and energy spectra,
and their time evolution at the Superkamiokande (SK) and the Sudbury Neutrino
Observatory (SNO), by using a realistic neutrino burst model based on numerical
simulations of supernova explosions. We also employ a realistic density profile
based on a presupernova model for the calculation of neutrino conversion
probability in supernova envelopes. These realistic models and numerical
calculations allow us to quantitatively estimate the effects of neutrino
oscillation in a more realistic way than previous studies. We then found that
the degeneracy of the solutions of the solar neutrino problem can be broken by
the combination of the SK and SNO detections of a future Galactic supernova.Comment: 10 pages, 14 figures, corrected versio
Primordial magnetic fields generated by the non-adiabatic fluctuations at pre-recombination era
In the pre-recombination era, cosmological density fluctuations can naturally
generate magnetic fields through Thomson scatterings. In previous studies, only
the magnetic field generation from the initially-adiabatic fluctuations has
been considered. Here we investigate the generation of cosmological magnetic
fields sourced by the primordial non-adiabatic fluctuations based on the
cosmological perturbation theory, using the tight-coupling approximations
between photon and baryon fluids. It is found that the magnetic fields from the
non-adiabatic fluctuations can arise at the first-order expansion of the tight
coupling approximation. This result is in contrast to the case of adiabatic
initial fluctuations, where the magnetic fields can be generated only at the
second-order. In a general case where the primordial density perturbations
contain small non-adiabatic fluctuations on the top of the dominant adiabatic
ones, we show that the leading source of magnetic fields is given by the
second-order coupling of the adiabatic and non-adiabatic fluctuations. We
calculate the power spectrum of the generated magnetic fields when the
non-adiabatic fluctuations have a blue power spectrum, which has been suggested
by recent cosmological observations.Comment: 16 pages, 2 figures, minor corrections, references added, to be
published in JCA
Primordial fluctuations in bulk inflaton model
An inflationary brane model driven by a bulk inflaton with exponential
potential is proposed. We find a family of exact solutions that describe
power-law inflation on the brane. These solutions enable us to derive exact
solutions for metric perturbations analytically. By calculating scalar and
tensor perturbations, we obtain a spectrum of primordial fluctuations at the
end of the inflation. The amplitudes of scalar and tensor perturbations are
enhanced in the same way if the energy scale of the inflation is sufficiently
higher than the tension of the brane. Then the relative amplitude of scalar and
tensor perturbations is not suppressed even for high-energy inflation. This is
a distinguishable feature from the inflation model driven by inflaton on the
brane where tensor perturbations are suppressed for high-energy inflation. We
also point out that massive Kaluza-Klein modes are not negligible at
high-frequencies on 3-space of our brane.Comment: 16 pages, 3 figures, reference adde
Cosmology and two-body problem of D-branes
In this paper, we investigate the dynamics and the evolution of the scale
factor of a probe Dp-brane which move in the background of source Dp-branes.
Action of the probe brane is described by the Born-Infeld action and the
interaction with the background R-R field. When the probe brane moves away from
the source branes, it expands by power law, whose index depends on the
dimension of the brane. If the energy density of the gauge field on the brane
is subdominant, the expansion is decelerating irrespective of the dimension of
the brane. On the other hand, when the probe brane is a Nambu-Goto brane, the
energy density of the gauge field can be dominant, in which case accelerating
expansion occurs for . The accelerating expansion stops when the
brane has expanded sufficiently so that the energy density of the gauge field
become subdominant.Comment: 6 pages, 7 figures, reference added, accepted for publication in PR
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