10,276 research outputs found
Adiabatic regularization of the graviton stress-energy tensor in de Sitter space-time
We study the renormalized energy-momentum tensor of gravitons in a de Sitter
space-time. After canonically quantizing only the physical degrees of freedom,
we adopt the standard adiabatic subtraction used for massless minimally coupled
scalar fields as a regularization procedure and find that the energy density of
gravitons in the E(3) invariant vacuum is proportional to H^4, where H is the
Hubble parameter, but with a positive sign. According to this result the scalar
expansion rate, which is gauge invariant in de Sitter space-time, is increased
by the fluctuations. This implies that gravitons may then add to conformally
coupled matter in driving the Starobinsky model of inflation.Comment: 5 pages, revtex, final version accepted for publication in PR
An elusive radio halo in the merging cluster Abell 781?
Deep radio observations of the galaxy cluster Abell 781 have been carried out
using the Giant Metrewave Radio Telescope at 325 MHz and have been compared to
previous 610 MHz observations and to archival VLA 1.4 GHz data. The radio
emission from the cluster is dominated by a diffuse source located at the
outskirts of the X-ray emission, which we tentatively classify as a radio
relic. We detected residual diffuse emission at the cluster centre at the level
of S(325 MHz)~15-20 mJy. Our analysis disagrees with Govoni et al. (2011), and
on the basis of simple spectral considerations we do not support their claim of
a radio halo with flux density of 20-30 mJy at 1.4 GHz. Abell 781, a massive
and merging cluster, is an intriguing case. Assuming that the residual emission
is indicative of the presence of a radio halo barely detectable at our
sensitivity level, it could be a very steep spectrum source.Comment: 5 pages, 4 figures, 1 table - Accepted for publication on Monthly
Notices of the Royal Astronomical Society Letter
Stochastic growth of quantum fluctuations during slow-roll inflation
We compute the growth of the mean square of quantum fluctuations of test
fields with small effective mass during a slowly changing, nearly de Sitter
stage which took place in different inflationary models. We consider a
minimally coupled scalar with a small mass, a modulus with an effective mass (with as the Hubble parameter) and a massless non-minimally
coupled scalar in the test field approximation and compare the growth of their
relative mean square with the one of gauge-invariant inflaton fluctuations. We
find that in most of the single field inflationary models the mean square gauge
invariant inflaton fluctuation grows {\em faster} than any test field with a
non-negative effective mass. Hybrid inflationary models can be an exception:
the mean square of a test field can dominate over the gauge invariant inflaton
fluctuation one on suitably choosing parameters. We also compute the stochastic
growth of quantum fluctuation of a second field, relaxing the assumption of its
zero homogeneous value, in a generic inflationary model; as a main result, we
obtain that the equation of motion of a gauge invariant variable associated,
order by order, with a generic quantum scalar fluctuation during inflation can
be obtained only if we use the number of e-folds as the time variable in the
corresponding Langevin and Fokker-Planck equations for the stochastic approach.
We employ this approach to derive some bounds in the case of a model with two
massive fields.Comment: 9 pages, 4 figures. Added references, minor changes, matches the
version to be published in Phys. Rev.
Inflation and Reheating in Spontaneously Generated Gravity
Inflation is studied in the context of induced gravity (IG) , where is the Ricci scalar, a scalar field and a
dimensionless constant, and diverse symmetry-breaking potentials
are considered. In particular we compared the predictions for Landau-Ginzburg
(LG) and Coleman-Weinberg (CW) type potentials and their possible
generalizations with the most recent data. We find that large field inflation
generally leads to fewer constraints on the parameters and the shape of the
potential whereas small field inflation is more problematic and, if viable,
implies more constraints, in particular on the parameter . We also
examined the reheating phase and obtained an accurate analytical solution for
the dynamics of inflaton and the Hubble parameter by using a multiple scale
analysis (MSA). The solutions were then used to study the average expansion of
the Universe, the average equation of state for the scalar field and both the
perturbative and resonant decays of the inflaton field.Comment: 15 pages, 10 figures, to be published in Phys. Rev.
A note on the introduction of Hilbert’s Grundlagen der Geometrie
FAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOWe present and discuss a change in the introduction of Hilbert’s Grundlagen der Geometrie between the fiirst and the subsequent editions: the disappearance of the reference to the independence of the axioms. We briefly outline the theoretical relevance of402517FAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO2013/25095-42014/25342-
A note on the introduction of Hilbert's Grundlagen der Geometrie
We present and discuss a change in the introduction of Hilbert's Grundlagen der Geometrie between the first and the subsequent editions: the disappearance of the reference to the independence of the axioms. We briefly outline the theoretical relevance of the notion of independence in Hilbert's work and we suggest that a possible reason for this disappearance is the discovery that Hilbert's axioms were not, in fact, independent. In the end we show how this change gives textual evidence for the connection between the notions of independence and simplicity
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