627 research outputs found
Quintessence from Shape Moduli
We show that shape moduli in sub-millimeter extra dimensional scenarios,
addressing the gauge hierarchy problem, can dominate the energy density of the
universe today. In our scenario, the volume of the extra dimensions is
stabilized at a sufficiently high scale to avoid conflicts with nucleosynthesis
and solar-system precision gravity experiments, while the shape moduli remain
light but couple extremely weakly to brane-localized matter and easily avoid
these bounds. Nonlocal effects in the bulk of the extra dimension generate a
potential for the shape moduli. The potential has the right form and order of
magnitude to account for the present day cosmic acceleration, in a way
analogous to models of quintessence as a pseudo Nambu-Goldstone boson.Comment: 8 pages, 1 figur
Spin diffusion at finite electric and magnetic fields
Spin transport properties at finite electric and magnetic fields are studied
by using the generalized semiclassical Boltzmann equation. It is found that the
spin diffusion equation for non-equilibrium spin density and spin currents
involves a number of length scales that explicitly depend on the electric and
magnetic fields. The set of macroscopic equations can be used to address a
broad range of the spin transport problems in magnetic multilayers as well as
in semiconductor heterostructure. A specific example of spin injection into
semiconductors at arbitrary electric and magnetic fields is illustrated
Intermediate inflation in Gauss-Bonnet braneworld
In this article we study an intermediate inflationary universe models using
the Gauss-Bonnet brane. General conditions required for these models to be
realizable are derived and discussed. We use recent astronomical observations
to constraint the parameters appearing in the model.Comment: 16 pages, 2 figures, accepted for publication in European Physical
Journal
A confirmation of agreement of different approaches for scalar gauge-invariant metric perturbations during inflation
We revisit an extension of the well-known formalism for gauge-invariant
scalar metric fluctuations, to study the spectrums for both, the inflaton and
gauge invariant (scalar) metric fluctuations in the framework of a single field
inflationary model where the quasi-exponential expansion is driven by an
inflation which is minimally coupled to gravity. The proposal here examined is
valid also for fluctuations with large amplitude, but for cosmological scales,
where vector and tensor perturbations can be neglected and the fluid is
irrotacional.Comment: Version accepted in EPJC with new title. 11 pages, no figure
The Detectability of Departures from the Inflationary Consistency Equation
We study the detectability, given CMB polarization maps, of departures from
the inflationary consistency equation, r \equiv T/S \simeq -5 n_T, where T and
S are the tensor and scalar contributions to the quadrupole variance,
respectively. The consistency equation holds if inflation is driven by a
slowly-rolling scalar field. Departures can be caused by: 1) higher-order terms
in the expansion in slow-roll parameters, 2) quantum loop corrections or 3)
multiple fields. Higher-order corrections in the first two slow-roll parameters
are undetectably small. Loop corrections are detectable if they are nearly
maximal and r \ga 0.1. Large departures (|\Delta n_T| \ga 0.1) can be seen if r
\ga 0.001. High angular resolution can be important for detecting non-zero
r+5n_T, even when not important for detecting non-zero r.Comment: 7 pages, 4 figures, submitted to PR
Percolation model for structural phase transitions in LiHIO mixed crystals
A percolation model is proposed to explain the structural phase transitions
found in LiHIO mixed crystals as a function of the
concentration parameter . The percolation thresholds are obtained from Monte
Carlo simulations on the specific lattices occupied by lithium atoms and
hydrogen bonds. The theoretical results strongly suggest that percolating
lithium vacancies and hydrogen bonds are indeed responsible for the solid
solution observed in the experimental range .Comment: 4 pages, 2 figure
Reionization by active sources and its effects on the cosmic microwave background
We investigate the possible effects of reionization by active sources on the
cosmic microwave background. We concentrate on the sources themselves as the
origin of reionization, rather than early object formation, introducing an
extra period of heating motivated by the active character of the perturbations.
Using reasonable parameters, this leads to four possibilities depending on the
time and duration of the energy input: delayed last scattering, double last
scattering, shifted last scattering and total reionization. We show that these
possibilities are only very weakly constrained by the limits on spectral
distortions from the COBE FIRAS measurements. We illustrate the effects of
these reionization possibilities on the angular power spectrum of temperature
anisotropies and polarization for simple passive isocurvature models and simple
coherent sources, observing the difference between passive and active models.
Finally, we comment on the implications of this work for more realistic active
sources, such as causal white noise and topological defect models. We show for
these models that non-standard ionization histories can shift the peak in the
CMB power to larger angular scales.Comment: 21 pages LaTeX with 11 eps figures; replaced with final version
accepted for publication in Phys. Rev.
Scalar field exact solutions for non-flat FLRW cosmology: A technique from non-linear Schr\"odinger-type formulation
We report a method of solving for canonical scalar field exact solution in a
non-flat FLRW universe with barotropic fluid using non-linear Schr\"{o}dinger
(NLS)-type formulation in comparison to the method in the standard Friedmann
framework. We consider phantom and non-phantom scalar field cases with
exponential and power-law accelerating expansion. Analysis on effective
equation of state to both cases of expansion is also performed. We speculate
and comment on some advantage and disadvantage of using the NLS formulation in
solving for the exact solution.Comment: 12 pages, GERG format, Reference added. accepted by Gen. Relativ. and
Gra
Scale invariant scalar metric fluctuations during inflation: non-perturbative formalism from a 5D vacuum
We extend to 5D an approach of a 4D non-perturbative formalism to study
scalar metric fluctuations of a 5D Riemann-flat de Sitter background metric. In
contrast with the results obtained in 4D, the spectrum of cosmological scalar
metric fluctuations during inflation can be scale invariant and the background
inflaton field can take sub-Planckian values.Comment: final version to be published in Eur. Phys. J.
The Primordial Perturbation Spectrum from Various Expanding and Contracting Phases
In this paper, focusing on the case of single scalar field, we discuss
various expanding and contracting phases generating primordial perturbations,
and study the relation between the primordial perturbation spectrum from these
phases and the parameter w of state equation in details. Furthermore, we offer
an interesting classification for the primordial perturbation spectrum from
various phases, which may have important implications for building an early
universe scenario embedded in possible high energy theories.Comment: 5 pages, 3 eps figure
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