2,273 research outputs found
CMB temperature anisotropy at large scales induced by a causal primordial magnetic field
We present an analytical derivation of the Sachs Wolfe effect sourced by a
primordial magnetic field. In order to consistently specify the initial
conditions, we assume that the magnetic field is generated by a causal process,
namely a first order phase transition in the early universe. As for the
topological defects case, we apply the general relativistic junction conditions
to match the perturbation variables before and after the phase transition which
generates the magnetic field, in such a way that the total energy momentum
tensor is conserved across the transition and Einstein's equations are
satisfied. We further solve the evolution equations for the metric and fluid
perturbations at large scales analytically including neutrinos, and derive the
magnetic Sachs Wolfe effect. We find that the relevant contribution to the
magnetic Sachs Wolfe effect comes from the metric perturbations at
next-to-leading order in the large scale limit. The leading order term is in
fact strongly suppressed due to the presence of free-streaming neutrinos. We
derive the neutrino compensation effect dynamically and confirm that the
magnetic Sachs Wolfe spectrum from a causal magnetic field behaves as
l(l+1)C_l^B \propto l^2 as found in the latest numerical analyses.Comment: 31 pages, 2 figures, minor changes, matches published versio
Magnetic fields from inflation: the transition to the radiation era
We compute the contribution to the scalar metric perturbations from
large-scale magnetic fields which are generated during inflation. We show that
apart from the usual passive and compensated modes, the magnetic fields also
contribute to the constant mode from inflation. This is different from the
causal (post inflationary) generation of magnetic fields where such a mode is
absent and it might lead to significant, non-Gaussian CMB anisotropies.Comment: 19 pages, no figures. v2: Substantially revised version with
different conclusions. v3: one reference added, matches version accepted for
publication in PR
Gravitational wave production: A strong constraint on primordial magnetic fields
We compute the gravity waves induced by anisotropic stresses of stochastic primordial magnetic fields. The nucleosynthesis bound on gravity waves is then used to derive a limit on the magnetic field amplitude as function of the spectral index. The obtained limits are extraordinarily strong: If the primordial magnetic field is produced by a causal process, leading to a spectral index on super horizon scales, galactic magnetic fields produced at the electroweak phase transition or earlier have to be weaker than B_\la \le 10^{-27}Gauss! If they are induced during an inflationary phase (reheating temperature GeV) with a spectral index , the magnetic field has to be weaker than B_\la \le 10^{-39}Gauss! Only very red magnetic field spectra, are not strongly constrained. We also find that a considerable amount of the magnetic field energy is converted into gravity waves. The gravity wave limit derived in this work rules out most of the proposed processes for primordial seeds for the large scale coherent magnetic fields observed in galaxies and clusters
Gravitational wave generation from bubble collisions in first-order phase transitions: an analytic approach
Gravitational wave production from bubble collisions was calculated in the
early nineties using numerical simulations. In this paper, we present an
alternative analytic estimate, relying on a different treatment of
stochasticity. In our approach, we provide a model for the bubble velocity
power spectrum, suitable for both detonations and deflagrations. From this, we
derive the anisotropic stress and analytically solve the gravitational wave
equation. We provide analytical formulae for the peak frequency and the shape
of the spectrum which we compare with numerical estimates. In contrast to the
previous analysis, we do not work in the envelope approximation. This paper
focuses on a particular source of gravitational waves from phase transitions.
In a companion article, we will add together the different sources of
gravitational wave signals from phase transitions: bubble collisions,
turbulence and magnetic fields and discuss the prospects for probing the
electroweak phase transition at LISA.Comment: 48 pages, 14 figures. v2 (PRD version): calculation refined; plots
redone starting from Fig. 4. Factor 2 in GW energy spectrum corrected. Main
conclusions unchanged. v3: Note added at the end of paper to comment on the
new results of 0901.166
Theory of unitarity bounds and low energy form factors
We present a general formalism for deriving bounds on the shape parameters of
the weak and electromagnetic form factors using as input correlators calculated
from perturbative QCD, and exploiting analyticity and unitarity. The values
resulting from the symmetries of QCD at low energies or from lattice
calculations at special points inside the analyticity domain can beincluded in
an exact way. We write down the general solution of the corresponding Meiman
problem for an arbitrary number of interior constraints and the integral
equations that allow one to include the phase of the form factor along a part
of the unitarity cut. A formalism that includes the phase and some information
on the modulus along a part of the cut is also given. For illustration we
present constraints on the slope and curvature of the K_l3 scalar form factor
and discuss our findings in some detail. The techniques are useful for checking
the consistency of various inputs and for controlling the parameterizations of
the form factors entering precision predictions in flavor physics.Comment: 11 pages latex using EPJ style files, 5 figures; v2 is version
accepted by EPJA in Tools section; sentences and figures improve
Gravitational Waves from Warped Spacetime
We argue that the RSI model can provide a strong signature in gravitational
waves. This signal is a relic stochastic background generated during the
cosmological phase transition from an AdS-Schwarschild phase to the RS1
geometry that should occur at a temperature in the TeV range. We estimate the
amplitude of the signal in terms of the parameters of the potential stabilizing
the radion and show that over much of the parameter region in which the phase
transition completes, a signal should be detectable at the planned space
interferometer, LISA.Comment: 18 pages, 15 figures; v2: discussion improved, in particular on the
justification of the thick wall approximation. 6 figures added. 4 pi factor
corrected in perturbativity bound. N-dependence displayed. Conclusions
unchanged. JHEP versio
Detection of gravitational waves from the QCD phase transition with pulsar timing arrays
If the cosmological QCD phase transition is strongly first order and lasts
sufficiently long, it generates a background of gravitational waves which may
be detected via pulsar timing experiments. We estimate the amplitude and the
spectral shape of such a background and we discuss its detectability prospects.Comment: 7 pages, 5 figs. Version accepted by PR
Ultrafast entropy production in pump-probe experiments
The ultrafast control of materials has opened the possibility to investigate
non-equilibrium states of matter with striking properties, such as transient
superconductivity and ferroelectricity, ultrafast magnetization and
demagnetization, as well as Floquet engineering. The characterization of the
ultrafast thermodynamic properties within the material is key for their control
and design. Here, we develop the ultrafast stochastic thermodynamics for
laser-excited phonons. We calculate the entropy production and heat absorbed
from experimental data for single phonon modes of driven materials from
time-resolved X-ray scattering experiments where the crystal is excited by a
laser pulse. The spectral entropy production is calculated for SrTiO and
KTaO for different temperatures and reveals a striking relation with the
power spectrum of the displacement-displacement correlation function by
inducing a broad peak beside the eigenmode-resonance.Comment: 11 pages, 6 figure
Integration of capillary and EWOD technologies for autonomous and low-power consumption micro-analytical systems
This work presents a miniaturized system combining, on the same microfluidic chip, capillarity and electrowetting-on-dielectric (EWOD) techniques for movement and control of fluids. The change in hydrophobicity occurring at the edge between a capillary channel and a hydrophobic layer is successfully exploited as a stop-and-go valve, whose operation is electronically controlled through the EWOD electrodes. Taking into account the variety of microfluidic operation resulting from the combination of the two handling techniques and their characteristic features, this work prompts the development of autonomous, compact and low-power consumption lab-on-chip systems
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