3,794 research outputs found
Mimetic gravity: a review of recent developments and applications to cosmology and astrophysics
Mimetic gravity is a Weyl-symmetric extension of General Relativity, related
to the latter by a singular disformal transformation, wherein the appearance of
a dust-like perfect fluid can mimic cold dark matter at a cosmological level.
Within this framework, it is possible to provide an unified geometrical
explanation for dark matter, the late-time acceleration, and inflation, making
it a very attractive theory. In this review, we summarize the main aspects of
mimetic gravity, as well as extensions of the minimal formulation of the model.
We devote particular focus to the reconstruction technique, which allows the
realization of any desired expansionary history of the Universe by an accurate
choice of potential, or other functions defined within the theory (as in the
case of mimetic gravity). We briefly discuss cosmological perturbation
theory within mimetic gravity. As a case study within which we apply the
concepts previously discussed, we study a mimetic Ho\v{r}ava-like theory, of
which we explore solutions and cosmological perturbations in detail. Finally,
we conclude the review by discussing static spherically symmetric solutions
within mimetic gravity, and apply our findings to the problem of galactic
rotation curves. Our review provides an introduction to mimetic gravity, as
well as a concise but self-contained summary of recent findings, progresses,
open questions, and outlooks on future research directions.Comment: 68 pages, invited review to appear in Advances in High Energy Physic
On Optimization Modulo Theories, MaxSMT and Sorting Networks
Optimization Modulo Theories (OMT) is an extension of SMT which allows for
finding models that optimize given objectives. (Partial weighted) MaxSMT --or
equivalently OMT with Pseudo-Boolean objective functions, OMT+PB-- is a
very-relevant strict subcase of OMT. We classify existing approaches for MaxSMT
or OMT+PB in two groups: MaxSAT-based approaches exploit the efficiency of
state-of-the-art MAXSAT solvers, but they are specific-purpose and not always
applicable; OMT-based approaches are general-purpose, but they suffer from
intrinsic inefficiencies on MaxSMT/OMT+PB problems.
We identify a major source of such inefficiencies, and we address it by
enhancing OMT by means of bidirectional sorting networks. We implemented this
idea on top of the OptiMathSAT OMT solver. We run an extensive empirical
evaluation on a variety of problems, comparing MaxSAT-based and OMT-based
techniques, with and without sorting networks, implemented on top of
OptiMathSAT and {\nu}Z. The results support the effectiveness of this idea, and
provide interesting insights about the different approaches.Comment: 17 pages, submitted at Tacas 1
Unification of Constant-roll Inflation and Dark Energy with Logarithmic -corrected and Exponential Gravity
In this paper we investigate how to describe in a unified way a constant-roll
inflationary era with a dark energy era, by using the theoretical framework of
gravity. To this end, we introduce some classes of appropriately chosen
gravity models, and we examine in detail how the unification of early
and late-time acceleration eras can be achieved. We study in detail the
inflationary era, and as we demonstrate it is possible to achieve a viable
inflationary era, for which the spectral index of primordial curvature
perturbations and the scalar-to-tensor ratio can be compatible with the latest
observational data. In addition, the graceful exit issue is briefly discussed
for a class of models. Finally, we discuss the dark energy oscillations issue,
and we investigate which model from one of the classes we introduced, can
produce oscillations with the smallest amplitude.Comment: Nuclear Physics B accepte
Beyond-one-loop quantum gravity action yielding both inflation and late-time acceleration
A unified description of early-time inflation with the current cosmic
acceleration is achieved by means of a new theory that uses a quadratic model
of gravity, with the inclusion of an exponential -gravity contribution
for dark energy. High-curvature corrections of the theory come from
higher-derivative quantum gravity and yield an effective action that goes
beyond the one-loop approximation. It is shown that, in this theory, viable
inflation emerges in a natural way, leading to a spectral index and
tensor-to-scalar ratio that are in perfect agreement with the most reliable
Planck results. At low energy, late-time accelerated expansion takes place. As
exponential gravity, for dark energy, must be stabilized during the matter and
radiation eras, we introduce a curing term in order to avoid nonphysical
singularities in the effective equation of state parameter. The results of our
analysis are confirmed by accurate numerical simulations, which show that our
model does fit the most recent cosmological data for dark energy very
precisely.Comment: 20 pages, to appear in NP
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