109 research outputs found
Universe acceleration and fine structure constant variation in BSBM theory
In this work we investigate the utility of using SNe Ia observations in
constraining the cosmological parameters in BSBM theory where a scalar field is
responsible for both fine structure constant variation and late time universe
acceleration. The model is discussed in the presence of an exponential self
potential for the scalar field. Stability and phase space analysis of the
solutions are studied. The model is tested against observational data for
Hubble parameter and quasar absorption spectra. With the best fitted model
parameters, the theory predicts a good match with the experimental results and
exhibits fine structure constant variation. The analysis also shows that for
the equation of state parameter, recent universe acceleration and possible
phantom crossing in future is forecasted.Comment: 14 pages, 10 figures, final version with minor modification accepted
to be published in JCA
Do we have any hope of detecting scattering between dark energy and baryons through cosmology?
Neutron Moderation in the Oklo Natural Reactor and the Time Variation of alpha
In the analysis of the Oklo (gabon) natural reactor to test for a possible
time variation of the fine structure constant alpha, a Maxwell-Boltzmann low
energy neutron spectrum was assumed. We present here an analysis where a more
realistic spectrum is employed and show that the most recent isotopic analysis
of samples implies a non-zero change in alpha, over the last two billion years
since the reactor was operating, of \Delta\alpha/\alpha\geq 4.5\times 10^{-8}
(6\sigma confidence). Issues regarding the interpretation of the shifts of the
low energy neutron resonances are discussed.Comment: 7 pages, 4 figures; version 2 included reference to Flambaum/Shuryak
work and corrects error in abstract version three corrects a few points and
adds discussion on hydrogen and impurity concentration
Primordial Magnetic Fields via Spontaneous Breaking of Lorentz Invariance
Spontaneous breaking of Lorentz invariance compatible with observational
limits may realistically take place in the context of string theories, possibly
endowing the photon with a mass. In this process the conformal symmetry of the
electromagnetic action is broken allowing for the possibility of generating
large scale () magnetic fields within inflationary scenarios. We show
that for reheating temperatures safe from the point of view of the gravitino
and moduli problem, T_{RH} \laq 10^{9} GeV for , the
strength of the generated seed fields is, in our mechanism, consistent with
amplification by the galactic dynamo processes and can be even as large as to
explain the observed galactic magnetic fields through the collapse of
protogalactic clouds.Comment: Final version to appear in Physics Letters
Effective field theory analysis of the self-interacting chameleon
We analyse the phenomenology of a self-interacting scalar field in the
context of the chameleon scenario originally proposed by Khoury and Weltman. In
the absence of self-interactions, this type of scalar field can mediate long
range interactions and simultaneously evade constraints from violation of the
weak equivalence principle. By applying to such a scalar field the effective
field theory method proposed for Einstein gravity by Goldberger and Rothstein,
we give a thorough perturbative evaluation of the importance of non-derivative
self-interactions in determining the strength of the chameleon mediated force
in the case of orbital motion. The self-interactions are potentially dangerous
as they can change the long range behaviour of the field. Nevertheless, we show
that they do not lead to any dramatic phenomenological consequence with respect
to the linear case and solar system constraints are fulfilled.Comment: 15 pages, 2 figures. Final version accepted for publication on
General Relativity and Gravitatio
Chameleonic Generalized Brans--Dicke model and late-time acceleration
In this paper we consider Chameleonic Generalized Brans--Dicke Cosmology in
the framework of FRW universes. The bouncing solution and phantom crossing is
investigated for the model. Two independent cosmological tests: Cosmological
Redshift Drift (CRD) and distance modulus are applied to test the model with
the observation.Comment: 20 pages, 15 figures, to be published in Astrophys. Space Sci. (2011
Spherical collapse with dark energy
I discuss the work of Maor and Lahav [1], in which the inclusion of dark
energy into the spherical collapse formalism is reviewed. Adopting a
phenomenological approach, I consider the consequences of - a) allowing the
dark energy to cluster, and, b) including the dark energy in the virialization
process. Both of these issues affect the final state of the system in a
fundamental way. The results suggest a potentially differentiating signature
between a true cosmological constant and a dynamic form of dark energy. This
signature is unique in the sense that it does not depend on a measurement of
the value of the equation of state of dark energy.Comment: To appear in the proceedings of the ``Peyresq Physics 10" Workshop,
19 - 24 June 2005, Peyresq, Franc
Dynamics of the self-interacting chameleon cosmology
In this article we study the properties of the flat FRW chameleon cosmology
in which the cosmic expansion of the Universe is affected by the chameleon
field and dark energy. In particular, we perform a detailed examination of the
model in the light of numerical analysis. The results illustrate that the
interacting chameleon filed plays an important role in late time universe
acceleration and phantom crossing.Comment: 13 pages, 8 figures, to appear in Astrophysics and Space Sc
Constraining dark energy fluctuations with supernova correlations
We investigate constraints on dark energy fluctuations using type Ia
supernovae. If dark energy is not in the form of a cosmological constant, that
is if the equation of state is not equal to -1, we expect not only temporal,
but also spatial variations in the energy density. Such fluctuations would
cause local variations in the universal expansion rate and directional
dependences in the redshift-distance relation. We present a scheme for relating
a power spectrum of dark energy fluctuations to an angular covariance function
of standard candle magnitude fluctuations. The predictions for a
phenomenological model of dark energy fluctuations are compared to
observational data in the form of the measured angular covariance of Hubble
diagram magnitude residuals for type Ia supernovae in the Union2 compilation.
The observational result is consistent with zero dark energy fluctuations.
However, due to the limitations in statistics, current data still allow for
quite general dark energy fluctuations as long as they are in the linear
regime.Comment: 18 pages, 6 figures, matches the published versio
Phase transition in Schwarzschild-de Sitter spacetime
Using a static massive spherically symmetric scalar field coupled to gravity
in the Schwarzschild-de Sitter (SdS) background, first we consider some
asymptotic solutions near horizon and their local equations of state(E.O.S) on
them. We show that near cosmological and event horizons our scalar field
behaves as a dust. At the next step near two pure de-Sitter or Schwarzschild
horizons we obtain a coupling dependent pressure to energy density ratio. In
the case of a minimally couplling this ratio is -1 which springs to the mind
thermodynamical behavior of dark energy. If having a negative pressure behavior
near these horizons we concluded that the coupling constant must be
>. Therefore we derive a new constraint on the value of our coupling .
These two different behaviors of unique matter in the distinct regions of
spacetime at present era can be interpreted as a phase transition from dark
matter to dark energy in the cosmic scales and construct a unified scenario.Comment: 7 pages,no figures,RevTex, Typos corrected and references adde
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