785 research outputs found
An Observational Test of Two-field Inflation
We study adiabatic and isocurvature perturbation spectra produced by a period
of cosmological inflation driven by two scalar fields. We show that there
exists a model-independent consistency condition for all two-field models of
slow-roll inflation, despite allowing for model-dependent linear processing of
curvature and isocurvature perturbations during and after inflation on
super-horizon scales. The scale-dependence of all spectra are determined solely
in terms of slow-roll parameters during inflation and the dimensionless
cross-correlation between curvature and isocurvature perturbations. We present
additional model-dependent consistency relations that may be derived in
specific two-field models, such as the curvaton scenario.Comment: 6 pages, latex with revtex, no figures; v2, minor changes, to appear
in Physical Review
On the Generation of a Scale-Invariant Spectrum of Adiabatic Fluctuations in Cosmological Models with a Contracting Phase
In Pre-Big-Bang and in Ekpyrotic Cosmology, perturbations on cosmological
scales today are generated from quantum vacuum fluctuations during a phase when
the Universe is contracting (viewed in the Einstein frame). The backgrounds
studied to date do not yield a scale invariant spectrum of adiabatic
fluctuations. Here, we present a new contracting background model (neither of
Pre-Big-Bang nor of the Ekpyrotic form) involving a single scalar field coupled
to gravity in which a scale-invariant spectrum of curvature fluctuations and
gravitational waves results. The equation of state of this scalar field
corresponds to cold matter. We demonstrate that if this contracting phase can
be matched via a nonsingular bounce to an expanding Friedmann cosmology, the
scale-invariance of the curvature fluctuations is maintained. We also find new
background solutions for Pre-Big-Bang and for Ekpyrotic cosmology, which
involve two scalar fields with exponential potentials with background values
which are evolving in time. We comment on the difficulty of obtaining a
scale-invariant spectrum of adiabatic fluctuations with background solutions
which have been studied in the past.Comment: 8 pages, revised version without the section on perturbations,
matching the version published on Phys. Rev. D. For cosmological
perturbations in the two field model see astro-ph/021127
Modified gravity with negative and positive powers of the curvature: unification of the inflation and of the cosmic acceleration
The modified gravity, which eliminates the need for dark energy and which
seems to be stable, is considered. The terms with positive powers of the
curvature support the inflationary epoch while the terms with negative powers
of the curvature serve as effective dark energy, supporting current cosmic
acceleration. The equivalent scalar-tensor gravity may be compatible with the
simplest solar system experiments.Comment: 23 pages, 3 figures, discussion is extended, references added,
version to appear in PR
Dark energy and dark matter from an inhomogeneous dilaton
A cosmological scenario is proposed where the dark matter (DM) and dark
energy (DE) of the universe are two simultaneous manifestations of an
inhomogenous dilaton. The equation of state of the field is scale-dependent and
pressureless at galactic and larger scales and it has negative pressure as a DE
at very large scales. The dilaton drives an inflationary phase followed by a
kinetic energy-dominated one, as in the "quintessential inflation" model
introduced by Peebles & Vilenkin, and soon after the end of inflation particle
production seeds the first inhomogeneities that lead to galaxy formation. The
dilaton is trapped near the minimum of the potential where it oscillates like a
massive field, and the excess of kinetic energy is dissipated via the mechanism
of "gravitational cooling" first introduced by Seidel & Suen. The
inhomogeneities therefore behave like solitonic oscillations around the minimum
of the potential, known as "oscillatons", that we propose account for most DM
in galaxies. Those regions where the dilaton does not transform enough kinetic
energy into reheating or carry an excess of it from regions that have cooled,
evolve to the tail of the potential as DE, driving the acceleration of the
universe.Comment: 9 pages, 8 figures, uses revtex, submitted PR
Entropic force and its cosmological implications
We investigate a possibility of realizing the entropic force into the
cosmology. A main issue is how the holographic screen is implemented in the
Newtonian cosmology. Contrary to the relativistic realization of Friedmann
equations, we do not clarify the connection between Newtonian cosmology and
entropic force because there is no way of implementing the holographic screen
in the Newtonian cosmology.Comment: 16 pages, no figures, version "Accepted for publication in
Astrophysics & Space Science
On the properties of the transition matrix in bouncing cosmologies
We elaborate further on the evolution properties of cosmological fluctuations
through a bounce. We show this evolution to be describable either by
``transmission'' and ``reflection'' coefficients or by an effective unitary
S-matrix. We also show that they behave in a time reversal invariant way.
Therefore, earlier results are now interpreted in a different perspective and
put on a firmer basis.Comment: 4 pages, 1 figure, to appear in PR
Initial-State Interactions in the Unpolarized Drell-Yan Process
We show that initial-state interactions contribute to the
distribution in unpolarized Drell-Yan lepton pair production and , without suppression. The asymmetry is expressed as a
product of chiral-odd distributions , where the quark-transversity function
is the transverse momentum dependent, light-cone
momentum distribution of transversely polarized quarks in an {\it unpolarized}
proton. We compute this (naive) -odd and chiral-odd distribution function
and the resulting asymmetry explicitly in a quark-scalar diquark
model for the proton with initial-state gluon interaction. In this model the
function equals the -odd (chiral-even) Sivers
effect function . This suggests that the
single-spin asymmetries in the SIDIS and the Drell-Yan process are closely
related to the asymmetry of the unpolarized Drell-Yan process,
since all can arise from the same underlying mechanism. This provides new
insight regarding the role of quark and gluon orbital angular momentum as well
as that of initial- and final-state gluon exchange interactions in hard QCD
processes.Comment: 22 pages, 6 figure
Tachyon warm inflationary universe model in the weak dissipative regime
Warm inflationary universe model in a tachyon field theory is studied in the
weak dissipative regime. We develop our model for an exponential potential and
the dissipation parameter =constant. We describe scalar and
tensor perturbations for this scenario.Comment: 9 pages, accepted by European Physical Journal
Modification to the power spectrum in the brane world inflation driven by the bulk inflaton
We compute the cosmological perturbations generated in the brane world
inflation driven by the bulk inflaton. Different from the model that the
inflation is a brane effect, we exhibit the modification of the power spectrum
of scalar perturbations due to the existence of the fifth dimension. With the
change of the initial vacuum, we investigate the dependence of the correction
of the power spectrum on the choice of the vacuum.Comment: replaced with the revised version, accepted for publication in PR
Cosmological Imprints of Pre-Inflationary Particles
We study some of the cosmological imprints of pre-inflationary particles. We
show that each such particle provides a seed for a spherically symmetric cosmic
defect. The profile of this cosmic defect is fixed and its magnitude is linear
in a single parameter that is determined by the mass of the pre-inflationary
particle. We study the CMB and peculiar velocity imprints of this cosmic defect
and suggest that it could explain some of the large scale cosmological
anomalies.Comment: 31 pages, 7 figure
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