43 research outputs found
Can the observed large scale magnetic fields be seeded by helical primordial fields?
Gravitational wave production induces a strong constraint on the amplitude of
a primordial magnetic field. It has been shown that the nucleosynthesis bound
for a stochastic gravitational wave background implies that causally generated
fields cannot have enough power on large scales to provide the seeds necessary
for the observed magnetic fields in galaxies and clusters, even by the most
optimistic dynamo amplification. Magnetic fields generated at inflation can
have high enough amplitude only if their spectrum is very red. Here we show
that helicity, which leads to an inverse cascade, can mitigate these limits. In
particular, we find that helical fields generated at the QCD phase transition
or at inflation with red spectrum are possible seeds for the dynamo. Helical
fields generated at the electroweak phase transition are instead excluded as
seeds at large scales. We also calculate the spectrum of gravitational waves
generated by helical magnetic fields.Comment: 41 pages, 12 figure
Interactions of cosmological gravitational waves and magnetic fields
The energy momentum tensor of a magnetic field always contains a spin-2
component in its anisotropic stress and therefore generates gravitational
waves. It has been argued in the literature (Caprini & Durrer \cite{CD}) that
this gravitational wave production can be very strong and that back-reaction
cannot be neglected. On the other hand, a gravitational wave background does
affect the evolution of magnetic fields. It has also been argued (Tsagas et al.
\cite{Tsagas:2001ak},\cite{Tsagas:2005ki}) that this can lead to very strong
amplification of a primordial magnetic field. In this paper we revisit these
claims and study back reaction to second order.Comment: Added references, accepted for publication in PR
The seed magnetic field generated during recombination
Non-linear dynamics creates vortical currents when the tight-coupling approximation between photons and baryons breaks down around the time of recombination. This generates a magnetic field at second order in cosmological perturbations, whose power spectrum is fixed by standard physics, without the need for any ad hoc assumptions. We present the fully general relativistic calculation of the magnetic power spectrum, including the effects of metric perturbations, second-order velocity and photon anisotropic stress, thus generalizing and correcting previous results. We also show that significant magnetogenesis continues to occur after recombination. The power spectrum decays as k4 on large scales, and grows as k0.5 on small scales, down to the limit of our numerical computations, ∼1 Mpc. On cluster scales, the created field has a strength of ∼3 × 10−29
Cosmic Microwave Background temperature and polarization anisotropies from the large-N limit of global defects
We determine the full C_l spectra and correlation functions of the
temperature and polarization anisotropies in the CMB, generated by a source
modeled by the large N limit of spontaneously broken global O(N)-theories. We
point out a problem in the standard approach of treating the radiation-matter
transition by interpolating the eigenvectors of the unequal-time correlators of
the source energy-momentum tensor. This affects the CMB predictions from all
type of cosmic defects. We propose a method to overcome this difficulty, and
find that in the large-N global model that we study, differences in the final
CMB power spectra amplitudes reach up to 25%, when compared to implementations
of the eigenvector interpolation technique. We discuss as well how to optimally
search for the contribution in the CMB from active sources such as cosmic
defects, in experiments like Planck, COrE and PRISM.Comment: 16+4 pages, 13 figures (Version 2: minor changes to match published
version in PRD
The local B-polarization of the CMB: a very sensitive probe of cosmic defects
We present a new and especially powerful signature of cosmic strings and
other topological or non-topological defects in the polarization of the cosmic
microwave background (CMB). We show that even if defects contribute 1% or less
in the CMB temperature anisotropy spectrum, their signature in the local
-polarization correlation function at angular scales of tens of arc
minutes is much larger than that due to gravitational waves from inflation,
even if the latter contribute with a ratio as big as to the
temperature anisotropies. We show that when going from non-local to local
-polarization, the ratio of the defect signal-to-noise with respect
to the inflationary value increases by about an order of magnitude. Proposed
B-polarization experiments, with a good sensitivity on arcminute scales, may
either detect a contribution from topological defects produced after inflation
or place stringent limits on them. Even Planck should be able to improve
present constraints on defect models by at least an order of magnitude, to the
level of \ep <10^{-7}. A future full-sky experiment like CMBpol, with
polarization sensitivities of the order of K-arcmin, will be able to
constrain the defect parameter \ep=Gv^2 to a few , depending
on the defect model.Comment: Version Published in Physics Letters
Gravitational waves from self-ordering scalar fields
Gravitational waves were copiously produced in the early Universe whenever
the processes taking place were sufficiently violent. The spectra of several of
these gravitational wave backgrounds on subhorizon scales have been extensively
studied in the literature. In this paper we analyze the shape and amplitude of
the gravitational wave spectrum on scales which are superhorizon at the time of
production. Such gravitational waves are expected from the self ordering of
randomly oriented scalar fields which can be present during a thermal phase
transition or during preheating after hybrid inflation. We find that, if the
gravitational wave source acts only during a small fraction of the Hubble time,
the gravitational wave spectrum at frequencies lower than the expansion rate at
the time of production behaves as with an
amplitude much too small to be observable by gravitational wave observatories
like LIGO, LISA or BBO. On the other hand, if the source is active for a much
longer time, until a given mode which is initially superhorizon (), enters the horizon, for , we find that the gravitational
wave energy density is frequency independent, i.e. scale invariant. Moreover,
its amplitude for a GUT scale scenario turns out to be within the range and
sensitivity of BBO and marginally detectable by LIGO and LISA. This new
gravitational wave background can compete with the one generated during
inflation, and distinguishing both may require extra information.Comment: 21 pages, 2 figures, added discussion about numerical integration and
a new figure to illustrate the scale-invariance of the GW power spectrum,
conclusions unchange
The seed magnetic field generated during recombination
Nonlinear dynamics creates vortical currents when the tight-coupling
approximation between photons and baryons breaks down around the time of
recombination. This generates a magnetic field at second order in cosmological
perturbations, whose power spectrum is fixed by standard physics, without the
need for any ad hoc assumptions. We present the fully relativistic calculation
of the magnetic power spectrum, including the effects of metric perturbations,
second-order velocity and the photon anisotropic stress, thus generalizing and
correcting previous results. We also show that significant magnetogenesis
continues to occur after recombination. The power spectrum
behaves as on large scales, and on small
scales, down to Mpc. On cluster scales, the created field has
strength Gauss.Comment: references added, version accepted by MNRA
A 1/24 degree resolution Mediterranean analysis and forecast modeling system for the Copernicus Marine Environment Monitoring Service
The Mediterranean Forecasting System (MFS) is a numerical ocean prediction system that operationally
produces analyses, reanalyses and short-term forecasts of the main physical parameters for the entire
Mediterranean Sea and its Atlantic Ocean adjacent areas. This work is specifically focused on the
description and evaluation of the analysis and forecast modeling system that covers the analysis of the
current situation and produces daily updates of the following 10 days forecast. The system has been
recently upgraded in the framework of the Copernicus Marine Environment Monitoring Service
(CMEMS) by increasing the grid resolution from 1/16o to 1/24o in the horizontal and from 72 to 141
vertical levels, by increasing the number of fresh water river inputs and by updating the data assimilation
scheme. The model has a non-linear explicit free surface and it is forced by surface pressure, interactive
heat, momentum and water fluxes at the air-sea interface. In order to validate the modeling system and to
estimate the accuracy of the model products, a quality assessment is regularly performed including both
pre-operational qualification and near real time (NRT) validation procedures. Pre-operational
qualification activities focus on testing the improvements of the quality of the new system with respect
to the previous version and relies on past simulation and historical data, while NRT validation activities
aim at routinely and on-line providing the skill assessment of the model analysis and forecasts and relies
on the NRT available observations. The focus of this work is to present the new operational modeling
system and the skill assessment including comparison with independent (insitu coastal moorings) and
quasi-independent (insitu vertical profiles and satellite) datasets.PublishedBergen, Norway3SR. AMBIENTE - Servizi e ricerca per la Societ
A 1/24° resolution Mediterranean physical analysis and forecasting system for the Copernicus Marine Environment Monitoring Service
This study describes a new model implementation for the Mediterranean Sea that has
been achieved in the framework of the Copernicus Marine Environment Monitoring
Service (CMEMS). The numerical ocean prediction system, that operationally produces
analyses and forecasts of the main physical parameters for the entire Mediterranean
Sea and its Atlantic Ocean adjacent areas, has been upgraded by increasing the grid
resolution from 1/16o to 1/24o in the horizontal and from 72 to 141 unevenly spaced
vertical levels, by increasing the number of fresh water river inputs and by updating
the data assimilation scheme. The model has a non-linear explicit free surface and it
is forced by surface pressure, interactive heat, momentum and water fluxes at the airsea
interface. The focus of this work is to present the new modelling system which
will become operational in the near future and the validation assessment including
the comparison with an independent non assimilated dataset (coastal moorings) and
quasi-independent (in situ vertical profiles and satellite) datasets. The results show
that the higher resolution model is capable of representing most of the variability
of the general circulation in the Mediterranean Sea, however some improvements
need to be implemented in order to enhance the model ability in reproducing specific
hydrodynamic features particularly the Sea Level Anomaly.PublishedBergen, Norway3SR. AMBIENTE - Servizi e ricerca per la Societ