3,845 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
Classifying document types to enhance search and recommendations in digital libraries
In this paper, we address the problem of classifying documents available from
the global network of (open access) repositories according to their type. We
show that the metadata provided by repositories enabling us to distinguish
research papers, thesis and slides are missing in over 60% of cases. While
these metadata describing document types are useful in a variety of scenarios
ranging from research analytics to improving search and recommender (SR)
systems, this problem has not yet been sufficiently addressed in the context of
the repositories infrastructure. We have developed a new approach for
classifying document types using supervised machine learning based exclusively
on text specific features. We achieve 0.96 F1-score using the random forest and
Adaboost classifiers, which are the best performing models on our data. By
analysing the SR system logs of the CORE [1] digital library aggregator, we
show that users are an order of magnitude more likely to click on research
papers and thesis than on slides. This suggests that using document types as a
feature for ranking/filtering SR results in digital libraries has the potential
to improve user experience.Comment: 12 pages, 21st International Conference on Theory and Practise of
Digital Libraries (TPDL), 2017, Thessaloniki, Greec
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
Black hole and de Sitter solutions in a covariant renormalizable field theory of gravity
It is shown that Schwarzschild black hole and de Sitter solutions exist as
exact solutions of a recently proposed relativistic covariant formulation of
(power-counting) renormalizable gravity with a fluid. The formulation without a
fluid is also presented here. The stability of the solutions is studied and
their corresponding entropies are computed, by using the covariant Wald method.
The area law is shown to hold both for the Schwarzschild and for the de Sitter
solutions found, confirming that, for the case, one is dealing with a
minimal modification of GR.Comment: 7 paages, latex fil
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