5,200 research outputs found
Hunting for Isocurvature Modes in the CMB non-Gaussianities
We investigate new shapes of local primordial non-Gaussianities in the CMB.
Allowing for a primordial isocurvature mode along with the main adiabatic one,
the angular bispectrum is in general a superposition of six distinct shapes:
the usual adiabatic term, a purely isocurvature component and four additional
components that arise from correlations between the adiabatic and isocurvature
modes. We present a class of early Universe models in which various hierarchies
between these six components can be obtained, while satisfying the present
upper bound on the isocurvature fraction in the power spectrum. Remarkably,
even with this constraint, detectable non-Gaussianity could be produced by
isocurvature modes. We finally discuss the prospects of detecting these new
shapes with the Planck satellite.Comment: 9 pages, 2 figure
General treatment of isocurvature perturbations and non-Gaussianities
We present a general formalism that provides a systematic computation of the
linear and non-linear perturbations for an arbitrary number of cosmological
fluids in the early Universe going through various transitions, in particular
the decay of some species (such as a curvaton or a modulus). Using this
formalism, we revisit the question of isocurvature non-Gaussianities in the
mixed inflaton-curvaton scenario and show that one can obtain significant
non-Gaussianities dominated by the isocurvature mode while satisfying the
present constraints on the isocurvature contribution in the observed power
spectrum. We also study two-curvaton scenarios, taking into account the
production of dark matter, and investigate in which cases significant
non-Gaussianities can be produced.Comment: Substantial improvements with respect to the first version. In
particular, we added a discussion on the confrontation of the models with
future observational data. This version is accepted for publication in JCA
Relativistic stars in f(R) and scalar-tensor theories
We study relativistic stars in the context of scalar tensor theories of
gravity that try to account for the observed cosmic acceleration and satisfy
the local gravity constraints via the chameleon mechanism. More specifically,
we consider two types of models: scalar tensor theories with an inverse power
law potential and f(R) theories. Using a relaxation algorithm, we construct
numerically static relativistic stars, both for constant energy density
configurations and for a polytropic equation of state. We can reach a
gravitational potential up to at the surface of the star, even
in f(R) theories with an "unprotected" curvature singularity. However, we find
static configurations only if the pressure does not exceed one third of the
energy density, except possibly in a limited region of the star (otherwise, one
expects tachyonic instabilities to develop). This constraint is satisfied by
realistic equations of state for neutron stars.Comment: 20 pages, 10 Figs, revtex4; v2. minor corrections and changes; v3: a
few changes, matches published versio
Differential rotation of relativistic superfluid in neutron stars
It is shown how to set up a mathematically elegant and fully relativistic
superfluid model that can provide a realistic approximation (neglecting small
anisotropies due to crust solidity, magnetic fields, et cetera, but allowing
for the regions with vortex pinning) of the global structure of a rotating
neutron star, in terms of just two independently moving constituents, one of
which represents the differentially rotating neutron superfluid, while the
other part represents the combination of all the other ingredients, including
the degenerate electrons, the superfluid protons in the core, and the ions in
the crust, whose electromagnetic interactions will tend to keep them locked
together in a state of approximately rigid rotation. Order of magnitude
estimates are provided for relevant parameters such as the resistive drag
coefficient and the maximum pinning force.Comment: 35 pages, Latex, no figure, submitted to M.N.R.A.
Scalar Kaluza-Klein modes in a multiply warped braneworld
The Kaluza-Klein (KK) modes of a massive scalar field on a 3-brane embedded
in six dimensional multiply warped spacetime are determined. Due to the
presence of warping along both the extra dimensions the KK mass spectrum splits
into two closely spaced branches which is a distinct feature of this model
compared to the five dimensional Randall-Sundrum model. This new cluster of the
KK mode spectrum is expected to have interesting phenomenological implications
for the upcoming collider experiments. Such a scenario may also be extended for
even larger number of orbifolded extra dimensions.Comment: 10 pages, Revte
Cosmological CMBR dipole in open universes ?
The observed CMBR dipole is generally interpreted as a Doppler effect arising
from the motion of the Earth relative to the CMBR frame. An alternative
interpretation, proposed in the last years, is that the dipole results from
ultra-large scale isocurvature perturbations. We examine this idea in the
context of open cosmologies and show that the isocurvature interpretation is
not valid in an open universe, unless it is extremely close to a flat universe,
.Comment: 26 pages, Latex, 6 figures, to appear in Phys. Rev.
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