3,768 research outputs found
Primordial density perturbations with running spectral index: impact on non-linear cosmic structures
(abridged) We explore the statistical properties of non-linear cosmic
structures in a flat CDM cosmology in which the index of the
primordial power spectrum for scalar perturbations is allowed to depend on the
scale. Within the inflationary paradigm, the running of the scalar spectral
index can be related to the properties of the inflaton potential, and it is
hence of critical importance to test it with all kinds of observations, which
cover the linear and non-linear regime of gravitational instability. We focus
on the amount of running allowed by an updated
combination of CMB anisotropy data and the 2dF Galaxy Redshift Survey. Our
analysis constrains
at 95% Confidence Level when (not) taking into
account primordial gravitational waves in a ratio as predicted by canonical
single field inflation, in agreement with other works. For the cosmological
models best fitting the data both with and without running we studied the
abundance of galaxy clusters and of rare objects, the halo bias, the
concentration of dark matter halos, the Baryon Acoustic Oscillation, the power
spectrum of cosmic shear, and the Integrated Sachs-Wolfe effect. We find that
counting galaxy clusters in future X-ray and Sunyaev-Zel'dovich surveys could
discriminate between the two models, more so if broad redshift information
about the cluster samples will be available. Likewise, measurements of the
power spectrum of cosmological weak lensing as performed by planned all-sky
optical surveys such as EUCLID could detect a running of the primordial
spectral index, provided the uncertainties about the source redshift
distribution and the underlying matter power spectrum are well under control.Comment: 17 pages, 14 figures, 4 tables. Accepted for publication on MNRA
Optical Potentials Derived from Nucleon-Nucleon Chiral Potentials at N4LO
Background: Elastic scattering is probably the main event in the interactions
of nucleons with nuclei. Even if this process has been extensively studied in
the last years, a consistent description, i.e., starting from microscopic two-
and many-body forces connected by the same symmetries and principles, is still
under development. Purpose: In a previous paper we derived a theoretical
optical potential from NN chiral potentials at fourth order (N3LO). In the
present work we use NN chiral potentials at fifth order (N4LO), with the
purpose to check the convergence and to assess the theoretical errors
associated with the truncation of the chiral expansion in the construction of
an optical potential. Methods: The optical potential is derived as the
first-order term within the spectator expansion of the nonrelativistic multiple
scattering theory and adopting the impulse approximation and the optimum
factorization approximation. Results: The pp and np Wolfenstein amplitudes and
the cross section, analyzing power, and spin rotation of elastic proton
scattering from 16O, 12C, and 40Ca nuclei are presented at an incident proton
energy of 200 MeV. The results obtained with different versions of chiral
potentials at N4LO are compared. Conclusions: Our results indicate that
convergence has been reached at N4LO. The agreement with the experimental data
is comparable with the agreement obtained in our previous work. We confirm that
building an optical potential within chiral perturbation theory is a promising
approach for describing elastic proton-nucleus scattering.Comment: Physical Review C, in prin
Isocurvature fluctuations in the effective Newton's constant
We present a new isocurvature mode present in scalar-tensor theories of
gravity that corresponds to a regular growing solution in which the energy of
the relativistic degrees of freedom and the scalar field that regulates the
gravitational strength compensate during the radiation dominated epoch on
scales much larger than the Hubble radius. We study this isocurvature mode and
its impact on anisotropies of the cosmic microwave background for the simplest
scalar-tensor theory, i.e. the extended Jordan-Brans-Dicke gravity, in which
the scalar field also drives the acceleration of the Universe. We use Planck
data to constrain the amplitude of this isocurvature mode in the case of fixed
correlation with the adiabatic mode and we show how this mode could be
generated in a simple two field inflation model.Comment: Version updated to match published version. No changes in the result
Study of a Class of Four Dimensional Nonsingular Cosmological Bounces
We study a novel class of nonsingular time-symmetric cosmological bounces. In
this class of four dimensional models the bounce is induced by a perfect fluid
with a negative energy density. Metric perturbations are solved in an analytic
way all through the bounce. The conditions for generating a scale invariant
spectrum of tensor and scalar metric perturbations are discussed.Comment: 16 pages, 10 figure
Microscopic Optical Potentials: recent achievements and future perspectives
Few years ago we started the investigation of microscopic Optical Potentials
(OP) in the framework of chiral effective field theories and published our
results in a series of manuscripts. Starting from the very first work, where a
microscopic OP was introduced following the multiple scattering procedure of
Watson, and then followed by more recent works, where the agreement with
experimental data and phenomenological approaches was successfully tested, we
finally arrived at a description of elastic scattering processes off non-zero
spin nuclei. Among our achievements, it is worth mentioning the partial
inclusion of three-nucleon forces, and the extension of our OP to
antiproton-nucleus elastic scattering. Despite the overall good agreement with
empirical data obtained so far, we do believe that several improvements and
upgrades of the present approach are still to be achieved. In this short essay
we would like to address some of the most relevant achievements and discuss an
interesting development that, in our opinion, is needed to further improve
microscopic OPs in order to reach in a near future the same level of accuracy
of the phenomenological ones.Comment: 8 pages, 4 figures, Conference proceedings of the 13th International
Spring Seminar on Nuclear Physics, Sant'Angelo d'Ischia, May 15-20, 202
Metric perturbations at reheating: the use of spherical symmetry
We consider decay of the inflaton with a quartic potential coupled to other
fields, including gravity, but restricted to spherical symmetry. We describe
analytically an early, quasilinear regime, during which inflaton fluctuations
and the metric functions are driven by nonlinear effects of the decay products.
We present a detailed study of the leading nonlinear effects in this regime.
Results of the quasilinear approximation, in its domain of applicability, are
found to be consistent with those of fully nonlinear lattice studies. We
discuss how these results may be promoted to the full three dimensions.Comment: 18 pages, revtex, 2 figure
Adiabatic and Isocurvature Perturbations for Multifield Generalized Einstein Models
Low energy effective field theories motivated by string theory will likely
contain several scalar moduli fields which will be relevant to early Universe
cosmology. Some of these fields are expected to couple with non-standard
kinetic terms to gravity. In this paper, we study the splitting into adiabatic
and isocurvature perturbations for a model with two scalar fields, one of which
has a non-standard kinetic term in the Einstein-frame action. Such actions can
arise, e.g., in the Pre-Big-Bang and Ekpyrotic scenarios. The presence of a
non-standard kinetic term induces a new coupling between adiabatic and
isocurvature perturbations which is non-vanishing when the potential for the
matter fields is nonzero. This coupling is un-suppressed in the long wavelength
limit and thus can lead to an important transfer of power from the entropy to
the adiabatic mode on super-Hubble scales. We apply the formalism to the case
of a previously found exact solution with an exponential potential and study
the resulting mixing of adiabatic and isocurvature fluctuations in this
example. We also discuss the possible relevance of the extra coupling in the
perturbation equations for the process of generating an adiabatic component of
the fluctuations spectrum from isocurvature perturbations without considering a
later decay of the isocurvature component.Comment: 11 pages, 3 figures, one equation corrected, typos fixed, conclusions
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