1,742 research outputs found
Cosmology of modified (but second order) gravity
This is a brief review of modified gravity cosmologies. Generically
extensions of gravity action involve higher derivative terms, which can result
in ghosts and instabilities. There are three ways to circumvent this:
Chern-Simons terms, first order variational principle and nonlocality. We
consider recent cosmological applications of these three classes of modified
gravity models, in particular to the dark energy problem. The viable parameter
spaces can be very efficiently constrained by taking into account cosmological
data from all epochs in addition to Solar system tests and stability
considerations. We make some new remarks concerning so called algebraic
scalar-tensor theories, biscalar reformulation of nonlocal actions involving
the inverse d'Alembertian, and a possible covariant formulation holographic
cosmology with nonperturbative gravity.Comment: 18 pages, 2 figures. Based on a talk given at the 4th Gamow
conference "Astrophysics and Cosmology after Gamow: recent progress and new
horizons" at Odessa, Ukraine, 17-23 August 200
CMB statistics in noncommutative inflation
Noncommutative geometry can provide effective description of physics at very
short distances taking into account generic effects of quantum gravity.
Inflation amplifies tiny quantum fluctuations in the early universe to
macroscopic scales and may thus imprint high energy physics signatures in the
cosmological perturbations that could be detected in the CMB. It is shown here
that this can give rise to parity-violating modulations of the primordial
spectrum and odd non-Gaussian signatures. The breaking of rotational invariance
of the CMB provides constraints on the scale of noncommutativity that are
competitive with the existing noncosmological bounds, and could explain the
curious hemispherical asymmetry that has been claimed to be observed in the
sky. This introduces also non-Gaussianity with peculiar shape- and
scale-dependence, which in principle allows an independent cross-check of the
presence of noncommutativity at inflation.Comment: 9 pages, no figure
Three-magnetic fields
A completely new mechanism to generate the observed amount of large-scale
cosmological magnetic fields is introduced in the context of three-form
inflation. The amplification of the fields occurs via fourth order dynamics of
the vector perturbations and avoids the backreaction problem that plagues most
previously introduced mechanisms.Comment: 4 pages, 2 figures -- v2 as published (title changed in the published
version to "Cosmic magnetization in three-form inflation"
Spacetimes with vector distortion: Inflation from generalised Weyl geometry
Spacetime with general linear vector distortion is introduced. Thus, the
torsion and the nonmetricity of the affine connection are assumed to be
proportional to a vector field (and not its derivatives). The resulting
two-parameter family of non-Riemannian geometries generalises the conformal
Weyl geometry and some other interesting special cases. Taking into account the
leading order quadratic curvature correction to the Einstein-Hilbert action
results uniquely in the one-parameter extension of the Starobinsky inflation
known as the alpha-attractor. The most general quadratic curvature action
introduces, in addition to the canonical vector kinetic term, novel ghost-free
vector-tensor interactions.Comment: 5 pages, 1 table, 0 figures. V2: Added the general quadratic theor
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