101 research outputs found
Photon-axion conversion in intergalactic magnetic fields and cosmological consequences
Photon-axion conversion induced by intergalactic magnetic fields causes an
apparent dimming of distant sources, notably of cosmic standard candles such as
supernovae of type Ia (SNe Ia). We review the impact of this mechanism on the
luminosity-redshift relation of SNe Ia, on the dispersion of quasar spectra,
and on the spectrum of the cosmic microwave background. The original idea of
explaining the apparent dimming of distant SNe Ia without cosmic acceleration
is strongly constrained by these arguments. However, the cosmic equation of
state extracted from the SN Ia luminosity-redshift relation remains sensitive
to this mechanism. For example, it can mimic phantom energy.Comment: (14 pages, 9 eps figures) Contribution to appear in a volume of
Lecture Notes in Physics (Springer-Verlag) on Axion
Dynamical System Approach to Cosmological Models with a Varying Speed of Light
Methods of dynamical systems have been used to study homogeneous and
isotropic cosmological models with a varying speed of light (VSL). We propose
two methods of reduction of dynamics to the form of planar Hamiltonian
dynamical systems for models with a time dependent equation of state. The
solutions are analyzed on two-dimensional phase space in the variables where is a function of a scale factor . Then we show how the
horizon problem may be solved on some evolutional paths. It is shown that the
models with negative curvature overcome the horizon and flatness problems. The
presented method of reduction can be adopted to the analysis of dynamics of the
universe with the general form of the equation of state .
This is demonstrated using as an example the dynamics of VSL models filled with
a non-interacting fluid. We demonstrate a new type of evolution near the
initial singularity caused by a varying speed of light. The singularity-free
oscillating universes are also admitted for positive cosmological constant. We
consider a quantum VSL FRW closed model with radiation and show that the
highest tunnelling rate occurs for a constant velocity of light if and . It is also proved that the considered class of
models is structurally unstable for the case of .Comment: 18 pages, 5 figures, RevTeX4; final version to appear in PR
Quinstant Dark Energy Predictions for Structure Formation
We explore the predictions of a class of dark energy models, quinstant dark
energy, concerning the structure formation in the Universe, both in the linear
and non-linear regimes. Quinstant dark energy is considered to be formed by
quintessence and a negative cosmological constant. We conclude that these
models give good predictions for structure formation in the linear regime, but
fail to do so in the non-linear one, for redshifts larger than one.Comment: 9 pages, 14 figures, "Accepted for publication in Astrophysics &
Space Science
Thermodynamical Stability of Hagedorn and Radiation Regimes in Closed String Gas Cosmology
In this paper, we investigate thermal equilibrium in string gas cosmology
which is dominated by closed string.We consider two interesting regimes,
Hagedorn and radiation regimes.We find that for short strings in small radius
of Hagedorn regime very large amount of energy requested to have thermal
equilibrium but for long strings in such system a few energy is sufficient to
have thermal equilibrium. On the other hand in the large radius of Hagedorn
regime, which pressure is not negligible, we obtain a relation between the
energy and pressure in terms of cosmic time which is satisfied by thermal
equilibrium. Then we discuss about radiation regime and find that in all cases
there is thermal equilibrium.Comment: 13 pages, 12 figures. Title changed and figures improved. Journal
reference adde
Aspects of String-Gas Cosmology at Finite Temperature
We study string-gas cosmology in dilaton gravity, inspired by the fact that
it naturally arises in a string theory context. Our main interest is the
thermodynamical treatment of the string-gas and the resulting implications for
the cosmology. Within an adiabatic approximation, thermodynamical equilibrium
and a small, toroidal universe as initial conditions, we numerically solve the
corresponding equations of motions in two different regimes describing the
string-gas thermodynamics: (i) the Hagedorn regime, with a single scale factor,
and (ii) an almost-radiation dominated regime, which includes the leading
corrections due to the lightest Kaluza Klein and winding modes, with two scale
factors. The scale factor in the Hagedorn regime exhibits very slow time
evolution with nearly constant energy and negligible pressure. By contrast, in
case (ii) we find interesting cosmological solutions where the large dimensions
continue to expand and the small ones are kept undetectably small.Comment: 21 pages, 5 eps figure
Big Crunch Avoidance in k = 1 Semi-Classical Loop Quantum Cosmology
It is well known that a closed universe with a minimally coupled massive
scalar field always collapses to a singularity unless the initial conditions
are extremely fine tuned. We show that the corrections to the equations of
motion for the massive scalar field, given by loop quantum gravity in high
curvature regime, always lead to a bounce independently of the initial
conditions. In contrast to the previous works in loop quantum cosmology, we
note that the singularity can be avoided even at the semi-classical level of
effective dynamical equations with non-perturbative quantum gravity
modifications, without using a discrete quantum evolution.Comment: Minor changes, To appear in Physical Review
Non-Minimal Warm Inflation and Perturbations on the Warped DGP Brane with Modified Induced Gravity
We construct a warm inflation model with inflaton field non-minimally coupled
to induced gravity on a warped DGP brane. We incorporate possible modification
of the induced gravity on the brane in the spirit of -gravity. We study
cosmological perturbations in this setup. In the case of two field inflation
such as warm inflation, usually entropy perturbations are generated. While it
is expected that in the case of one field inflation these perturbations to be
removed, we show that even in the absence of the radiation field, entropy
perturbations are generated in our setup due to non-minimal coupling and
modification of the induced gravity.Comment: 29 pages, 7 figures, Accepted by Gen. Rel Gravi
Non-parametric Dark Energy Degeneracies
We study the degeneracies between dark energy dynamics, dark matter and
curvature using a non-parametric and non-perturbative approach. This allows us
to examine the knock-on bias induced in the reconstructed dark energy equation
of state, w(z), when there is a bias in the cosmic curvature or dark matter
content, without relying on any specific parameterisation of w. Even assuming
perfect Hubble, distance and volume measurements, we show that for z > 1, the
bias in w(z) is up to two orders of magnitude larger than the corresponding
errors in Omega_k or Omega_m. This highlights the importance of obtaining
unbiased estimators of all cosmic parameters in the hunt for dark energy
dynamics.Comment: 10 pages, 8 figures. Invited Review for special issue of General
Relativity and Gravitation issue on Dark Energy, eds. G. F.R Ellis et a
Pressure-dependence of electron-phonon coupling and the superconducting phase in hcp Fe - a linear response study
A recent experiment by Shimizu et al. has provided evidence of a
superconducting phase in hcp Fe under pressure. To study the
pressure-dependence of this superconducting phase we have calculated the phonon
frequencies and the electron-phonon coupling in hcp Fe as a function of the
lattice parameter, using the linear response (LR) scheme and the full potential
linear muffin-tin orbital (FP-LMTO) method. Calculated phonon spectra and the
Eliashberg functions indicate that conventional s-wave
electron-phonon coupling can definitely account for the appearance of the
superconducting phase in hcp Fe. However, the observed change in the transition
temperature with increasing pressure is far too rapid compared with the
calculated results. For comparison with the linear response results, we have
computed the electron-phonon coupling also by using the rigid muffin-tin (RMT)
approximation. From both the LR and the RMT results it appears that
electron-phonon interaction alone cannot explain the small range of volume over
which superconductivity is observed. It is shown that
ferromagnetic/antiferromagnetic spin fluctuations as well as scattering from
magnetic impurities (spin-ordered clusters) can account for the observed values
of the transition temperatures but cannot substantially improve the agreeemnt
between the calculated and observed presure/volume range of the superconducting
phase. A simplified treatment of p-wave pairing leads to extremely small ( K) transition temperatures. Thus our calculations seem to rule out
both - and - wave superconductivity in hcp Fe.Comment: 12 pages, submitted to PR
Multiple field inflation
Inflation offers a simple model for very early evolution of our Universe and
the origin of primordial perturbations on large scales. Over the last 25 years
we have become familiar with the predictions of single-field models, but
inflation with more than one light scalar field can alter preconceptions about
the inflationary dynamics and our predictions for the primordial perturbations.
I will discuss how future observational data could distinguish between
inflation driven by one field, or many fields. As an example, I briefly review
the curvaton as an alternative to the inflaton scenario for the origin of
structure.Comment: 27 pages, no figures. To appear in proceedings of 22nd IAP
Colloquium, Inflation +25, Paris, June 200
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