97 research outputs found
Non-gaussianity at tree and one-loop levels from vector field perturbations
We study the spectrum P_\zeta and bispectrum B_\zeta of the primordial
curvature perturbation \zeta when the latter is generated by scalar and vector
field perturbations. The tree-level and one-loop contributions from vector
field perturbations are worked out considering the possibility that the
one-loop contributions may be dominant over the tree level terms (both (either)
in P_\zeta and (or) in B_\zeta) and viceversa. The level of non-gaussianity in
the bispectrum, f_{NL}, is calculated and related to the level of statistical
anisotropy in the power spectrum, g_\zeta. For very small amounts of
statistical anisotropy in the power spectrum, the level of non-gaussianity may
be very high, in some cases exceeding the current observational limit.Comment: LaTeX file, 11 pages, Main body: 8 pages, References: 3 pages. v2:
Minor corrections. References added. Conclusions unchanged. v3: Minor
corrections. Some references added and others updated. Version accepted for
publication in Physical Review
Anisotropic Dark Energy from String Compactifications
We explore the cosmological dynamics of a minimalistic yet generic
string-inspired model for multifield dark energy. Adopting a supergravity
four-dimensional viewpoint, we motivate the model's structure arising from
superstring compactifications involving a chiral superfield and a pure
gauge sector. The chiral sector gives rise to a pair of scalar fields, such as
the axio-dilaton, which are kinetically coupled. However, the scalar potential
depends on only one of them, further entwined with the vector field through the
gauge kinetic function. The model has two anisotropic attractor solutions that,
despite a steep potential and thanks to multifield dynamics, could explain the
current accelerated expansion of the Universe while satisfying observational
constraints on the late-times cosmological anisotropy. Nevertheless, justifying
the parameter space allowing for slow roll dynamics together with the correct
cosmological parameters, would be challenging within the landscape of string
theory. Intriguingly, we find that the vector field, particularly at one of the
studied fixed points, plays a crucial role in enabling geodesic trajectories in
the scalar field space while realizing slow-roll dynamics with a steep
potential. This observation opens a new avenue for exploring multifield dark
energy models within the superstring landscape.Comment: Submitted to JHEP. Comments are welcome
Scrutinizing coupled vector dark energy in light of data
Since current challenges faced by CDM might be hinting at new
unravelled physics, here we investigate a plausible cosmological model where a
vector field acts as source of dark energy. In particular, we examine whether
an energy-momentum exchange between dark energy and dark matter could provide
an explanation for current discrepancies in cosmological parameters. We
carefully work out equations governing both background and linear order
perturbations and implement them in a Boltzmann code. We found that a negative
coupling makes the dark energy equation of state less negative and closer to a
cosmological constant during the matter dominated epoch than an uncoupled
vector dark energy model. While the effect of the coupling is hardly noticeable
on the growth of matter density perturbations, matter velocity perturbations
are enhanced at late-times when dark energy dominates. Therefore, data of
redshift space distortions help to narrow down these kinds of couplings in the
dark sector. We computed cosmological constraints and found common parameters
also present in CDM are in good agreement with the Planck
Collaboration baseline result. However, our best fit predicts a much higher
growth rate of matter perturbations at low redshift, thus exacerbating the
disagreement with redshift space distortions data. We conclude that our coupled
vector dark energy model does not solve current tensions (i.e., and
). Moreover, having three additional parameters with respect to
CDM, the coupled vector dark energy model is heavily disfavoured by
Bayesian evidence.Comment: 30 pages, 7 figures, 2 tables. A few references were adde
Reconstructing the parameter space of non-analytical cosmological fixed points
Dynamical system theory is a widely used technique in the analysis of
cosmological models. Within this framework, the equations describing the
dynamics of a model are recast in terms of dimensionless variables, which
evolve according to a set of autonomous first-order differential equations. The
fixed points of this autonomous set encode the asymptotic evolution of the
model. Usually, these points can be written as analytical expressions for the
variables in terms of the parameters of the model, which allows a complete
characterization of the corresponding parameter space. However, a thoroughly
analytical treatment is impossible in some cases. In this work, we give an
example of a dark energy model, a scalar field coupled to a vector field in an
anisotropic background, where not all the fixed points can be analytically
found. Then, we put forward a general scheme that provides a numerical
description of the parameter space. This allows us to find interesting
accelerated attractors of the system with no analytical representation. This
work may serve as a template for the numerical analysis of highly complicated
dynamical systems.Comment: 13 pages, 13 figures, 1 table. Changes match the published versio
- …