120 research outputs found
Primordial magnetic fields from metric perturbations
We study the amplification of electromagnetic vacuum fluctuations induced by
the evolution of scalar metric perturbations at the end of inflation. Such
perturbations break the conformal invariance of Maxwell equations in
Friedmann-Robertson-Walker backgrounds and allow the growth of magnetic fields
on super-Hubble scales. We relate the strength of the fields generated by this
mechanism with the power spectrum of scalar perturbations and estimate the
amplification on galactic scales for different values of the spectral index.
Finally we discuss the possible effects of finite conductivity during
reheating.Comment: 10 pages, 1 figure. Final version to appear in Phys. Rev.
Dark energy rest frame and the CMB dipole
If dark energy can be described as a perfect fluid, then, apart from its
equation of state relating energy density and pressure, we should also especify
the corresponding rest frame. Since dark energy is typically decoupled from the
rest of components of the universe, in principle such a frame could be
different from that of matter and radiation. In this work we consider the
potential observable effects of the motion of dark energy and the possibility
to measure the dark energy velocity relative to matter. In particular we
consider the modification of the usual interpretation of the CMB dipole and its
implications for the determination of matter bulk flows on very large scales.
We also comment on the possible origin of a dark energy flow and its evolution
in different models.Comment: 7 pages, 2 figures. Contribution to the Proceedings of the
International Workshop The Dark Side of the Universe, Madrid, June 20-24,
200
Gravitino production during preheating
We study the production of gravitinos during the preheating era after
inflation by means of the non-perturbative Bogolyubov technique. Considering
only the helicity states, the problem is reduced to the simpler Dirac
fermions case. We calculate the production in a particular supergravity model
in an expanding universe and obtain the spectrum and number density. Finally we
compare the results with the nucleosynthesis bounds and extract some
consequences.Comment: 10 pages, LaTeX, 2 figures. Talk given in the International Workshop
on Particles in Astrophysics and Cosmology, Valencia (Spain), May 3-8, 199
Dark energy in motion
Recent large-scale peculiar velocity surveys suggest that large matter
volumes could be moving with appreciable velocity with respect to the CMB rest
frame. If confirmed, such results could conflict with the Cosmological
Principle according to which the matter and CMB rest frames should converge on
very large scales. In this work we explore the possibility that such large
scale bulk flows are due, not to the motion of matter with respect to the CMB,
but to the flow of dark energy with respect to matter. Indeed, when dark energy
is moving, the usual definition of the CMB rest frame as that in which the CMB
dipole vanishes is not appropriate. We find instead that the dipole vanishes
for observers at rest with respect to the cosmic center of mass, i.e. in motion
with respect to the background radiation.Comment: 9 pages, 1 figure. Essay selected for "Honorable Mention" in the 2006
Awards for Essays on Gravitation (Gravity Research Foundation
On the tension between growth rate and CMB data
We analyze the claimed tension between redshift space distorsions
measurements of and the predictions of standard CDM
(Planck 2015 and 2018) cosmology. We consider a dataset consisting of 17 data
points extending up to redshift and corrected for the Alcock-Paczynski
effect. Thus, calculating the evolution of the growth factor in a CDM
cosmology, we find that the tension for the best fit parameters ,
and with respect to the Planck 2018 CDM parameters is below
in all the marginalized confidence regions.Comment: 6 pages, 4 figures. Final version to appear in Eur. Phys. J.
Dark energy: the absolute electric potential of the universe
Is there an absolute cosmic electric potential?. The recent discovery of the
accelerated expansion of the universe could be indicating that this is
certainly the case. In this essay we show that the consistency of the covariant
and gauge invariant theory of electromagnetism is truly questionable when
considered on cosmological scales. Out of the four components of the
electromagnetic field, Maxwell's theory only contains two physical degrees of
freedom. However, in the presence of gravity, one of the "unphysical" states
cannot be consistently eliminated, thus becoming real. This third polarization
state is completely decoupled from charged matter, but can be excited
gravitationally thus breaking gauge invariance. On large scales the new state
can be seen as a homogeneous cosmic electric potential, whose energy density
behaves as a cosmological constant.Comment: 9 pages, 2 figures. Essay selected for "Honorable Mention" in the
2009 Awards for Essays on Gravitation (Gravity Research Foundation
Cosmological magnetic fields from inflation in extended electromagnetism
In this work we consider an extended electromagnetic theory in which the
scalar state which is usually eliminated by means of the Lorenz condition is
allowed to propagate. This state has been shown to generate a small
cosmological constant in the context of standard inflationary cosmology. Here
we show that the usual Lorenz gauge-breaking term now plays the role of an
effective electromagnetic current. Such a current is generated during inflation
from quantum fluctuations and gives rise to a stochastic effective charge
density distribution. Due to the high electric conductivity of the cosmic
plasma after inflation, the electric charge density generates currents which
give rise to both vorticity and magnetic fields on sub-Hubble scales. Present
upper limits on vorticity coming from temperature anisotropies of the CMB are
translated into lower limits on the present value of cosmic magnetic fields. We
find that, for a nearly scale invariant vorticity spectrum, magnetic fields
G are typically generated with coherence lengths
ranging from sub-galactic scales up to the present Hubble radius. Those fields
could act as seeds for a galactic dynamo or even account for observations just
by collapse and differential rotation of the protogalactic cloud.Comment: 5 pages, 2 figures. Final version to appear in Phys. Rev.
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