3,933 research outputs found
Gravitational Waves from Abelian Gauge Fields and Cosmic Strings at Preheating
Primordial gravitational waves provide a very important stochastic background
that could be detected soon with interferometric gravitational wave antennas or
indirectly via the induced patterns in the polarization anisotropies of the
cosmic microwave background. The detection of these waves will open a new
window into the early Universe, and therefore it is important to characterize
in detail all possible sources of primordial gravitational waves. In this paper
we develop theoretical and numerical methods to study the production of
gravitational waves from out-of-equilibrium gauge fields at preheating. We then
consider models of preheating after hybrid inflation, where the symmetry
breaking field is charged under a local U(1) symmetry. We analyze in detail the
dynamics of the system in both momentum and configuration space, and show that
gauge fields leave specific imprints in the resulting gravitational wave
spectra, mainly through the appearence of new peaks at characteristic
frequencies that are related to the mass scales in the problem. We also show
how these new features in the spectra correlate with string-like spatial
configurations in both the Higgs and gauge fields that arise due to the
appearance of topological winding numbers of the Higgs around Nielsen-Olesen
strings. We study in detail the time evolution of the spectrum of gauge fields
and gravitational waves as these strings evolve and decay before entering a
turbulent regime where the gravitational wave energy density saturates.Comment: This paper is dedicated to the memory of Lev Kofman. Added references
and comments in Sec. III.B. Version accepted in PR
A Gravitational Wave Background from Reheating after Hybrid Inflation
The reheating of the universe after hybrid inflation proceeds through the
nucleation and subsequent collision of large concentrations of energy density
in the form of bubble-like structures moving at relativistic speeds. This
generates a significant fraction of energy in the form of a stochastic
background of gravitational waves, whose time evolution is determined by the
successive stages of reheating: First, tachyonic preheating makes the amplitude
of gravity waves grow exponentially fast. Second, bubble collisions add a new
burst of gravitational radiation. Third, turbulent motions finally sets the end
of gravitational waves production. From then on, these waves propagate
unimpeded to us. We find that the fraction of energy density today in these
primordial gravitational waves could be significant for GUT-scale models of
inflation, although well beyond the frequency range sensitivity of
gravitational wave observatories like LIGO, LISA or BBO. However, low-scale
models could still produce a detectable signal at frequencies accessible to BBO
or DECIGO. For comparison, we have also computed the analogous gravitational
wave background from some chaotic inflation models and obtained results similar
to those found by other groups. The discovery of such a background would open a
new observational window into the very early universe, where the details of the
process of reheating, i.e. the Big Bang, could be explored. Moreover, it could
also serve in the future as a new experimental tool for testing the
Inflationary Paradigm.Comment: 22 pages, 18 figures, uses revtex
On the Transverse-Traceless Projection in Lattice Simulations of Gravitational Wave Production
It has recently been pointed out that the usual procedure employed in order
to obtain the transverse-traceless (TT) part of metric perturbations in lattice
simulations was inconsistent with the fact that those fields live in the
lattice and not in the continuum. It was claimed that this could lead to a
larger amplitude and a wrong shape for the gravitational wave (GW) spectra
obtained in numerical simulations of (p)reheating. In order to address this
issue, we have defined a consistent prescription in the lattice for extracting
the TT part of the metric perturbations. We demonstrate explicitly that the GW
spectra obtained with the old continuum-based TT projection only differ
marginally in amplitude and shape with respect to the new lattice-based ones.
We conclude that one can therefore trust the predictions appearing in the
literature on the spectra of GW produced during (p)reheating and similar
scenarios simulated on a lattice.Comment: 22 pages, 8 figures, Submitted to JCA
Local and average fields inside surface-disordered waveguides: Resonances in the one-dimensional Anderson localization regime
We investigate the one-dimensional propagation of waves in the Anderson
localization regime, for a single-mode, surface disordered waveguide. We make
use of both an analytical formulation and rigorous numerical simulation
calculations. The occurrence of anomalously large transmission coefficients for
given realizations and/or frequencies is studied, revealing huge field
intensity concentration inside the disordered waveguide. The analytically
predicted s-like dependence of the average intensity, being in good agreement
with the numerical results for moderately long systems, fails to explain the
intensity distribution observed deep in the localized regime. The average
contribution to the field intensity from the resonances that are above a
threshold transmission coefficient is a broad distribution with a large
maximum at/near mid-waveguide, depending universally (for given ) on the
ratio of the length of the disorder segment to the localization length,
. The same universality is observed in the spatial distribution of the
intensity inside typical (non-resonant with respect to the transmission
coefficient) realizations, presenting a s-like shape similar to that of the
total average intensity for close to 1, which decays faster the lower
is . Evidence is given of the self-averaging nature of the random
quantity . Higher-order moments of the intensity are
also shown.Comment: 9 pages, 9 figure
A fuzzy methodology for innovation management measurement
Innovation has been recognized as one of the main sources of competitive advantage for organizations and nations. The purpose of this study is to present an innovation management measurement approach applying fuzzy techniques to small and medium manufacturing enterprises. ..
Crossing the phantom divide in an interacting generalized Chaplygin gas
Unified generalized Chaplygin gas models assuming an interaction between dark
energy and dark matter fluids have been previously proposed. Following these
ideas, we consider a particular relation between dark densities, which allows
the possibility of a time varying equation of state for dark energy that
crosses the phantom divide at a recent epoch. Moreover, these densities decay
during all the evolution of the Universe, avoiding a Big Rip. We find also a
scaling solution, i.e. these densities are asymptotically proportional in the
future, which contributes to the solution of the coincidence problem.Comment: Improved version, 10 pages, 4 figures, References adde
The kinematics of white dwarfs from the SDSS DR12
We use the Sloan Digital Sky Survey Data Release 12, which is the largest availablewhite dwarf catalogue to date, to study the evolution of the kinematical properties of the pop-ulation of white dwarfs of the Galactic disk. We derive masses, ages, photometric distances andradial velocities for all white dwarfs with hydrogen-rich atmospheres. For those stars for whichproper motions from the USNO-B1 catalogue are available, the three-dimensional componentsof the velocity are obtained. This subset of the original sample comprises 20,247 stars, makingit the largest sample of white dwarfs with measured three-dimensional velocities. The volumeprobed by our sample is large, allowing us to obtain relevant kinematical information. In partic-ular, our sample extends from a Galactocentric radial distanceRG=7.8 to 9.3 kpc, and verticaldistances from the Galactic plane ranging fromZ=+0.5to–0.5kpc.Peer ReviewedPostprint (published version
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