11 research outputs found
Cosmic Acceleration and Modified Gravity
I briefly discuss some attempts to construct a consistent modification to
General Relativity (GR) that might explain the observed late-time acceleration
of the universe and provide an alternative to dark energy. I mention the issues
facing extensions to GR, illustrate these with two specific examples, and
discuss the resulting observational and theoretical obstacles. This article
comprises an invited talk at the NASA workshop {\it From Quantum to Cosmos:
Fundamental Physics Research in Space}Comment: 12 pages, 1 figure. Invited talk at the NASA workshop - From Quantum
to Cosmos: Fundamental Physics Research in Space, May 21-24 200
The accelerating universe and a limiting curvature proposal
We consider the hypothesis of a limiting minimal curvature in gravity as a
way to construct a class of theories exhibiting late-time cosmic acceleration.
Guided by the minimal curvature conjecture (MCC) we are naturally lead to a set
of scalar tensor theories in which the scalar is non-minimally coupled both to
gravity and to the matter Lagrangian. The model is compared to the Lambda Cold
Dark Matter concordance model and to the observational data using the gold
SNeIa sample of Riess et. al. (2004). An excellent fit to the data is achieved.
We present a toy model designed to demonstrate that such a new, possibly
fundamental, principle may be responsible for the recent period of cosmological
acceleration. Observational constraints remain to be imposed on these models.Comment: 22 pages, 7 figures; revised version to appear in JCAP; references
adde
Generalized Gravity and a Ghost
We show that generalized gravity theories involving the curvature invariants
of the Ricci tensor and the Riemann tensor as well as the Ricci scalar are
equivalent to multi- scalar-tensor gravities with four derivatives terms. By
expanding the action around a vacuum spacetime, the action is reduced to that
of the Einstein gravity with four derivative terms, and consequently there
appears a massive spin-2 ghost in such generalized gravity theories in addition
to a massive spin-0 field.Comment: 8 pages, a reference adde
Moduli Stabilization in Brane Gas Cosmology with Superpotentials
In the context of brane gas cosmology in superstring theory, we show why it
is impossible to simultaneously stabilize the dilaton and the radion with a
general gas of strings (including massless modes) and D-branes. Although this
requires invoking a different mechanism to stabilize these moduli fields, we
find that the brane gas can still play a crucial role in the early universe in
assisting moduli stabilization. We show that a modest energy density of
specific types of brane gas can solve the overshoot problem that typically
afflicts potentials arising from gaugino condensation.Comment: minor changes to match the journal versio
One-loop f(R) gravity in de Sitter universe
Motivated by the dark energy issue, the one-loop quantization approach for a
family of relativistic cosmological theories is discussed in some detail.
Specifically, general gravity at the one-loop level in a de Sitter
universe is investigated, extending a similar program developed for the case of
pure Einstein gravity. Using generalized zeta regularization, the one-loop
effective action is explicitly obtained off-shell, what allows to study in
detail the possibility of (de)stabilization of the de Sitter background by
quantum effects. The one-loop effective action maybe useful also for the study
of constant curvature black hole nucleation rate and it provides the plausible
way of resolving the cosmological constant problem.Comment: 25 pages, Latex file. Discussion enlarged, new references added.
Version accepted in JCA
Primordial fluctuations and non-Gaussianities from multifield DBI Galileon inflation
We study a cosmological scenario in which the DBI action governing the motion
of a D3-brane in a higher-dimensional spacetime is supplemented with an induced
gravity term. The latter reduces to the quartic Galileon Lagrangian when the
motion of the brane is non-relativistic and we show that it tends to violate
the null energy condition and to render cosmological fluctuations ghosts. There
nonetheless exists an interesting parameter space in which a stable phase of
quasi-exponential expansion can be achieved while the induced gravity leaves
non trivial imprints. We derive the exact second-order action governing the
dynamics of linear perturbations and we show that it can be simply understood
through a bimetric perspective. In the relativistic regime, we also calculate
the dominant contribution to the primordial bispectrum and demonstrate that
large non-Gaussianities of orthogonal shape can be generated, for the first
time in a concrete model. More generally, we find that the sign and the shape
of the bispectrum offer powerful diagnostics of the precise strength of the
induced gravity.Comment: 34 pages including 9 figures, plus appendices and bibliography.
Wordings changed and references added; matches version published in JCA