3,126 research outputs found
Primordial Magnetic Fields from the Post-Inflationary Universe
We explore cosmological magnetogenesis in the post-inflationary universe,
when the inflaton oscillates around its potential minimum and the universe is
effectively dominated by cold matter. During this epoch prior to reheating,
large-scale magnetic fields can be significantly produced by the cosmological
background. By considering magnetogenesis both during and after inflation, we
demonstrate that magnetic fields stronger than 10^{-15} G can be generated on
Mpc scales without having strong couplings in the theory, or producing too
large electric fields that would dominate the universe.Comment: 30 pages, 6 figures, v2: published in JCA
Constraints on Primordial Magnetic Fields from Inflation
We present generic bounds on magnetic fields produced from cosmic inflation.
By investigating field bounds on the vector potential, we constrain both the
quantum mechanical production of magnetic fields and their classical growth in
a model independent way. For classical growth, we show that only if the
reheating temperature is as low as T_{reh} <~ 10^2 MeV can magnetic fields of
10^{-15} G be produced on Mpc scales in the present universe. For purely
quantum mechanical scenarios, even stronger constraints are derived. Our bounds
on classical and quantum mechanical scenarios apply to generic theories of
inflationary magnetogenesis with a two-derivative time kinetic term for the
vector potential. In both cases, the magnetic field strength is limited by the
gravitational back-reaction of the electric fields that are produced
simultaneously. As an example of quantum mechanical scenarios, we construct
vector field theories whose time diffeomorphisms are spontaneously broken, and
explore magnetic field generation in theories with a variable speed of light.
Transitions of quantum vector field fluctuations into classical fluctuations
are also analyzed in the examples.Comment: 26 pages, v2: published in JCA
General-affine invariants of plane curves and space curves
We present a fundamental theory of curves in the affine plane and the affine
space, equipped with the general-affine groups and ,
respectively. We define general-affine length parameter and curvatures and show
how such invariants determine the curve up to general-affine motions. We then
study the extremal problem of the general-affine length functional and derive a
variational formula. We give several examples of curves and also discuss some
relations with equiaffine treatment and projective treatment of curves.Comment: 51 pages, 4 figures, to appear in Czechoslovak Mathematical Journal,
version2: typos are fixe
Runnings in the Curvaton
We investigate the scale-dependence, or the runnings, of linear and second
order density perturbations generated in various curvaton scenarios. We argue
that the second order perturbations, i.e. non-Gaussianity, can strongly depend
on the scale, even when the linear perturbations are nearly scale-invariant. We
present analytic formulae for the runnings from curvatons with general energy
potentials, and clarify the conditions under which fNL becomes strongly
scale-dependent. From the point of view of the fNL running, curvaton potentials
can be classified into roughly two categories by whether the potential flattens
or steepens compared to a quadratic one. As such examples, we study
pseudo-Nambu-Goldstone curvatons, and self-interacting curvatons, respectively.
The dynamics of non-quadratic curvatons and the behaviors of the resulting
density perturbations are clarified by analytical methods. Then we also study
models where multiple source can be responsible for density perturbations such
as the multi-curvaton, and mixed curvaton and inflaton models where the running
of fNL can also be large due to their multi-source nature. We make quantitative
analysis for each curvaton scenario and discuss in what cases the
scale-dependence, in particular, of fNL can be large enough to be probed with
future CMB experiments.Comment: 39 pages, many figures, v2: published in JCA
Schwinger Effect in 4D de Sitter Space and Constraints on Magnetogenesis in the Early Universe
We investigate pair creation by an electric field in four-dimensional de
Sitter space. The expectation value of the induced current is computed, using
the method of adiabatic regularization. Under strong electric fields the
behavior of the current is similar to that in flat space, while under weak
electric fields the current becomes inversely proportional to the mass squared
of the charged field. Thus we find that the de Sitter space obtains a large
conductivity under weak electric fields in the presence of a charged field with
a tiny mass. We then apply the results to constrain electromagnetic fields in
the early universe. In particular, we study cosmological scenarios for
generating large-scale magnetic fields during the inflationary era. Electric
fields generated along with the magnetic fields can induce sufficiently large
conductivity to terminate the phase of magnetogenesis. For inflationary
magnetogenesis models with a modified Maxwell kinetic term, the generated
magnetic fields cannot exceed 10^{-30} G on Mpc scales in the present epoch,
when a charged field carrying an elementary charge with mass of order the
Hubble scale or smaller exists in the Lagrangian. Similar constraints from the
Schwinger effect apply for other magnetogenesis mechanisms.Comment: 34 pages, 4 figures, v2: published in JHE
Rolling in the Modulated Reheating Scenario
In the modulated reheating scenario, the field that drives inflation has a
spatially varying decay rate, and the resulting inhomogeneous reheating process
generates adiabatic perturbations. We examine the statistical properties of the
density perturbations generated in this scenario. Unlike earlier analyses, we
include the dynamics of the field that determines the inflaton decay rate. We
show that the dynamics of this modulus field can significantly alter the
amplitude of the power spectrum and the bispectrum, even if the modulus field
has a simple potential and its effective mass is smaller than the Hubble rate.
In some cases, the evolution of the modulus amplifies the non-Gaussianity of
the perturbations to levels that are excluded by recent observations of the
cosmic microwave background. Therefore, a proper treatment of the modulus
dynamics is required to accurately calculate the statistical properties of the
perturbations generated by modulated reheating.Comment: 27 pages, 11 figures: minor changes made to match version in JCA
Is it true that insurers benefit from a catastrophic event? Market reactions to the 1995 Hanshin-Awaji earthquake
Previous studies, investigating how the market in general viewed the impact of a big earthquake (e.g., the 1989 Loma Prieta earthquake in the San Francisco Bay Area) on insurance firm values, found a positive reaction of insurers' stock prices. This "gaining from loss" may be caused by the subsequent increased demand for insurance coverage. This paper investigates the impact of the 1995 Hanshin-Awaji earthquake on Japanese insurers' value. Contrary to the results for U.S. earthquakes, we find significant negative stock price reactions. Furthermore, our results demonstrate that Japanese stock markets are considerably efficient in assessing the new information generated by the Hanshin-Awaji earthquake. Finally, we also find a negative relationship between stock price reaction and the extent to which an insurer wrote earthquake coverage in the damaged area.Insurance industry ; Japan
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