27 research outputs found
Theoretical and Observational Constraints on Brane Inflation and Study of Scalar Perturbations through the Effective Field Theory Formalism
In this thesis, consisting of two main parts, we study observational
signatures of cosmic (super)strings in the context of D-brane inflation and
properties of scalar perturbations on generic homogeneous inflating
backgrounds. In the first part we study the production, nature and decay
processes of cosmic superstrings in two widely used effective models of D-brane
inflation, namely the and models. Specifically, we show
that the strings produced in are of local axionic type and we place
constraints on the tension while arguing that the supersymmetry breaking
mechanism of the model needs to be altered according to supergravity
constraints on constant Fayet-Iliopoulos terms. Moreover, we study radiative
processes of cosmic superstrings on warped backgrounds. We argue that placing
the string formation in a natural context such as inflation,
restricts the forms of possible radiation from these objects. Motivated by
these string models, which inevitably result in the presence of heavy moduli
fields during inflation, in the second part, using the Effective Field Theory
(EFT) of inflation, we construct operators that capture the effects of massive
scalars on the low energy dynamics of inflaton perturbations. We compute the
energy scales that define the validity window of the EFT such as the scale
where ultra violet (UV) degrees of freedom become operational and the scale
where the EFT becomes strongly coupled. We show that the low energy operators
related to heavy fields induce a dispersion relation for the light modes
admitting two regimes: a linear and a non linear/dispersive one. Assuming that
these modes cross the Hubble scale within the dispersive regime, we compute
observables related to two- and three-point correlators and show how they are
directly connected with the scale of UV physics.Comment: PhD thesis, 145 p. Defended OCT 201
Constraints on inflation with LSS surveys: features in the primordial power spectrum
We analyse the efficiency of future large scale structure surveys to unveil
the presence of scale dependent features in the primordial spectrum --resulting
from cosmic inflation-- imprinted in the distribution of galaxies. Features may
appear as a consequence of non-trivial dynamics during cosmic inflation, in
which one or more background quantities experienced small but rapid deviations
from their characteristic slow-roll evolution. We consider two families of
features: localized features and oscillatory extended features. To characterise
them we employ various possible templates parametrising their scale dependence
and provide forecasts on the constraints on these parametrisations for LSST
like surveys. We perform a Fisher matrix analysis for three observables: cosmic
microwave background (CMB), galaxy clustering and weak lensing. We find that
the combined data set of these observables will be able to limit the presence
of features down to levels that are more restrictive than current constraints
coming from CMB observations only. In particular, we address the possibility of
gaining information on currently known deviations from scale invariance
inferred from CMB data, such as the feature appearing at the
multipole (which is the main contribution to the low- deficit) and a
potential feature appearing at .Comment: 37 pp., 5 Tabs., 10 Figs, v3: changed discussion around templates II,
III, added clarifications, comments and references. Matches JCAP versio
Non-Gaussian statistics of de Sitter spectators: A perturbative derivation of stochastic dynamics
Scalar fields interacting with the primordial curvature perturbation during
inflation may communicate their statistics to the latter. This situation
motivates the study of how the probability density function (PDF) of a light
spectator field in a pure de Sitter space-time, becomes non-Gaussian
under the influence of a scalar potential . One approach
to this problem is offered by the stochastic formalism introduced by
Starobinsky and Yokoyama. It results in a Fokker-Planck equation for the
time-dependent PDF describing the statistics of
which, in the limit of equilibrium gives one back the solution . We
study the derivation of using quantum field theory tools.
Our approach yields an almost Gaussian distribution function, distorted by
minor corrections comprised of terms proportional to powers of , where is the
number of -folds succeeding the Hubble-horizon crossing of 's
wavelengths, and stands for a derivative
operator acting on . This general form is obtained
perturbatively and remains valid even with loop corrections. Our solution
satisfies a Fokker-Planck equation that receives corrections with respect to
the one found within the stochastic approach, allowing us to comment on the
validity of the standard equilibrium solution for generic potentials. We posit
that higher order corrections to the Fokker-Planck equation may become
important towards the equilibrium.Comment: 53 pp. plus reference
Shapes and features of the primordial bispectrum
If time-dependent disruptions from slow-roll occur during inflation, the
correlation functions of the primordial curvature perturbation should have
scale-dependent features, a case which is marginally supported from the cosmic
microwave background (CMB) data. We offer a new approach to analyze the
appearance of such features in the primordial bispectrum that yields new
consistency relations and justifies the search of oscillating patterns
modulated by orthogonal and local templates. Under the assumption of sharp
features, we find that the cubic couplings of the curvature perturbation can be
expressed in terms of the bispectrum in two specific momentum configurations,
for example local and equilateral. This allows us to derive consistency
relations among different bispectrum shapes, which in principle could be tested
in future CMB surveys. Furthermore, based on the form of the consistency
relations, we construct new two-parameter templates for features that include
all the known shapes.Comment: (v1) 16 pages, 3 figures, 1 table; (v2) minor clarifications
including updated abstract, to appear in Journal of Cosmology and
Astroparticle Physic
On degenerate models of cosmic inflation
In this article we discuss the role of current and future CMB measurements in
pinning down the model of inflation responsible for the generation of
primordial curvature perturbations. By considering a parameterization of the
effective field theory of inflation with a modified dispersion relation arising
from heavy fields, we derive the dependence of cosmological observables on the
scale of heavy physics . Specifically, we show how the
non-linearity parameters are related to the phase velocity of
curvature perturbations at horizon exit, which is parameterized by
. Bicep2 and Planck findings are shown to be consistent with
a value . However, we find a
degeneracy in the parameter space of inflationary models that can only be
resolved with a detailed knowledge of the shape of the non-Gaussian bispectrum.Comment: 22pp., 1 fig; v2: added some clarifications and references, corrected
typos, matches published versio
Scale invariance of the primordial tensor power spectrum
Future cosmic microwave background polarization experiments will search for
evidence of primordial tensor modes at large angular scales, in the multipole
range Because in that range there is some mild evidence
of departures from scale invariance in the power spectrum of primordial
curvature perturbations, one may wonder about the possibility of similar
deviations appearing in the primordial power spectrum of tensor modes. Here we
address this issue and analyze the possible presence of features in the tensor
spectrum resulting from the dynamics of primordial fluctuations during
inflation. We derive a general, model independent, relation linking features in
the spectra of curvature and tensor perturbations. We conclude that even with
large deviations from scale invariance in the curvature power spectrum, the
tensor spectrum remains scale invariant for all observational purposes.Comment: 22 pages, 4 figures; v2: added references and clarifying comments;
v3: added reference and few more comments. Matches published versio
Effective field theory of weakly coupled inflationary models
The application of Effective Field Theory (EFT) methods to inflation has
taken a central role in our current understanding of the very early universe.
The EFT perspective has been particularly useful in analyzing the
self-interactions determining the evolution of co-moving curvature
perturbations (Goldstone boson modes) and their influence on low-energy
observables. However, the standard EFT formalism, to lowest order in spacetime
differential operators, does not provide the most general parametrization of a
theory that remains weakly coupled throughout the entire low-energy regime.
Here we study the EFT formulation by including spacetime differential operators
implying a scale dependence of the Goldstone boson self-interactions and its
dispersion relation. These operators are shown to arise naturally from the
low-energy interaction of the Goldstone boson with heavy fields that have been
integrated out. We find that the EFT then stays weakly coupled all the way up
to the cutoff scale at which ultraviolet degrees of freedom become operative.
This opens up a regime of new physics where the dispersion relation is
dominated by a quadratic dependence on the momentum \omega ~ p^2. In addition,
provided that modes crossed the horizon within this energy range, the
prediction of inflationary observables - including non-Gaussian signatures -
are significantly affected by the new scales characterizing it.Comment: 36 pages, v2: references added, minor changes to match published
versio
De Sitter bubbles from anti-de Sitter fluctuations
Cosmological acceleration is difficult to accommodate in theories of
fundamental interactions involving supergravity and superstrings. An
alternative is that the acceleration is not universal but happens in a large
localized region, which is possible in theories admitting regular black holes
with de Sitter-like interiors. We point out that, given a global anti-de Sitter
background, the formation of such 'de Sitter bubbles' will be enhanced by
mechanisms analogous to the Bizon-Rostworowski instability in general
relativity. This opens an arena for discussing the production of multiple
accelerating universes from anti-de Sitter fluctuations. We demonstrate such
collapse enhancement by explicit numerical work in the context of a simple
two-dimensional dilaton-gravity model that mimics the spherically symmetric
sector of higher-dimensional gravities.Comment: v2: small change