474 research outputs found
A dynamical system analysis of hybrid metric-Palatini cosmologies
The so called hybrid metric-Palatini gravity presents a unique viable
generalisation of the theories within the metric-affine formalism. Here
the cosmology of the theories is studied using the dynamical system
approach. The method consists of formulating the propagation equation in terms
of suitable (expansion-normalised) variables as an autonomous system. The fixed
points of the system then represent exact cosmological solutions described by
power-law or de Sitter expansion. The formalism is applied to two classes of
models, revealing both standard cosmological fixed points and new
accelerating solutions that can be attractors in the phase space. In addition,
the fixed point with vanishing expansion rate are considered with special care
in order to characterise the stability of Einstein static spaces and bouncing
solutions.Comment: 13 page
Quantum backreaction in evolving FLRW spacetimes
Quantum fluctuations of a nonminimally coupled scalar field in D-dimensional
homogeneous and isotropic background are calculated within the operator
formalism in curved models with time evolutions of the scale factor that allow
smooth transitions between contracting and expanding and between decelerating
and accelerating regimes. The coincident propagator is derived and used to
compute the one-loop backreaction from the scalar field. The inflationary
infrared divergences are absent in Bunch-Davies vacuum when taking into account
a preceding cosmological era or spatial curvature which can be either positive
or negative. It is found that asymptotically, the backreaction energy density
in the minimally coupled case grows logarithmically with the scale factor in
quasi-de Sitter space, and in a class of models decays in slow-roll inflation
and grows as a power-law during super-inflation. The backreaction increases
generically in a contracting phase or in the presence of a negative nonminimal
coupling. The effects of the coupling and renormalization scale upon the
quantum fluctuations together with the novel features due to nontrivial time
evolution and spatial curvature are clarified with exact solutions and
numerical examples.Comment: 23 pages, 6 figure
Cosmology of hybrid metric-Palatini f(X)-gravity
A new class of modified theories of gravity, consisting of the superposition
of the metric Einstein-Hilbert Lagrangian with an term constructed
\`{a} la Palatini was proposed recently. The dynamically equivalent
scalar-tensor representation of the model was also formulated, and it was shown
that even if the scalar field is very light, the theory passes the Solar System
observational constraints. Therefore the model predicts the existence of a
long-range scalar field, modifying the cosmological and galactic dynamics. An
explicit model that passes the local tests and leads to cosmic acceleration was
also obtained. In the present work, it is shown that the theory can be also
formulated in terms of the quantity , where T and R are
the traces of the stress-energy and Ricci tensors, respectively. The variable X
represents the deviation with respect to the field equation trace of general
relativity. The cosmological applications of this hybrid metric-Palatini
gravitational theory are also explored, and cosmological solutions coming from
the scalar-tensor representation of f(X)-gravity are presented. Criteria to
obtain cosmic acceleration are discussed and the field equations are analyzed
as a dynamical system. Several classes of dynamical cosmological solutions,
depending on the functional form of the effective scalar field potential,
describing both accelerating and decelerating Universes are explicitly
obtained. Furthermore, the cosmological perturbation equations are derived and
applied to uncover the nature of the propagating scalar degree of freedom and
the signatures these models predict in the large-scale structure.Comment: 17 pages. V2: 18 pages; minor revision and references added; to
appear in JCA
Wormholes supported by hybrid metric-Palatini gravity
Recently, a modified theory of gravity was presented, which consists of the
superposition of the metric Einstein-Hilbert Lagrangian with an
term constructed \`{a} la Palatini. The theory possesses extremely interesting
features such as predicting the existence of a long-range scalar field, that
explains the late-time cosmic acceleration and passes the local tests, even in
the presence of a light scalar field. In this brief report, we consider the
possibility that wormholes are supported by this hybrid metric-Palatini
gravitational theory. We present here the general conditions for wormhole
solutions according to the null energy conditions at the throat and find
specific examples. In the first solution, we specify the redshift function, the
scalar field and choose the potential that simplifies the modified Klein-Gordon
equation. This solution is not asymptotically flat and needs to be matched to a
vacuum solution. In the second example, by adequately specifying the metric
functions and choosing the scalar field, we find an asymptotically flat
spacetime.Comment: 4 pages. V2: 5 pages, discussion added; matches published versio
The virial theorem and the dark matter problem in hybrid metric-Palatini gravity
Hybrid metric-Palatini gravity is a recently proposed theory, consisting of
the superposition of the metric Einstein-Hilbert Lagrangian with an
term constructed \`{a} la Palatini. The theory predicts the existence of a
long-range scalar field, which passes the Solar System observational
constraints, even if the scalar field is very light, and modifies the
cosmological and galactic dynamics. Thus, the theory opens new possibilities to
approach, in the same theoretical framework, the problems of both dark energy
and dark matter. In this work, we consider the generalized virial theorem in
the scalar-tensor representation of the hybrid metric-Palatini gravity. More
specifically, taking into account the relativistic collisionless Boltzmann
equation, we show that the supplementary geometric terms in the gravitational
field equations provide an effective contribution to the gravitational
potential energy. We show that the total virial mass is proportional to the
effective mass associated with the new terms generated by the effective scalar
field, and the baryonic mass. This shows that the geometric origin in the
generalized virial theorem may account for the well-known virial theorem mass
discrepancy in clusters of galaxies. In addition to this, we also consider
astrophysical applications of the model and show that the model predicts that
the mass associated to the scalar field and its effects extend beyond the
virial radius of the clusters of galaxies. In the context of the galaxy cluster
velocity dispersion profiles predicted by the hybrid metric-Palatini model, the
generalized virial theorem can be an efficient tool in observationally testing
the viability of this class of generalized gravity models.Comment: 11 pages, no figures; minor modifications, accepted for publication
In JCA
Paleolimnological evidence for increased sexual reproduction in chydorids (Chydoridae, Cladocera) under environmental stress
To investigate the extent to which anthropogenic perturbations such as eutrophication and trace metal pollution (i.e., environmental stress sensu Odum 1985) influence the reproductive modes of cladoceran populations, we analyzed the abundance of subfossils of the chydorids Alonella nana (Baird, 1850) and Alona affinis (Leydig, 1860) in sediment cores from three Finnish lakes. Reconstruction of lakes\u27 pollutant history and the biological response of chydorids indicate that in two of the lakes the proportion of individuals reproducing sexually increased with environmental stressors. More specifically, A. nana responded to eutrophication in Lake Hamptr?sk with greater production of ephippia, while A. affinis responded to aluminum pollution or acidification in Lake Pieni Majaslampi. In contrast, the reference lake, Lake Iso Lehm?lampi, showed no radical changes in sexual reproduction over the twomillennium long sediment record. We conclude that chydorids may use sexual reproduction as a strategy for overcoming unexpected environmental stresses
Correlating Pedestrian Flows and Search Engine Queries
An important challenge for ubiquitous computing is the development of
techniques that can characterize a location vis-a-vis the richness and
diversity of urban settings. In this paper we report our work on correlating
urban pedestrian flows with Google search queries. Using longitudinal data we
show pedestrian flows at particular locations can be correlated with the
frequency of Google search terms that are semantically relevant to those
locations. Our approach can identify relevant content, media, and
advertisements for particular locations.Comment: 4 pages, 1 figure, 1 tabl
Palatini formulation of modified gravity with a nonminimal curvature-matter coupling
We derive the field equations and the equations of motion for massive test
particles in modified theories of gravity with an arbitrary coupling between
geometry and matter by using the Palatini formalism. We show that the
independent connection can be expressed as the Levi-Civita connection of an
auxiliary, matter Lagrangian dependent metric, which is related with the
physical metric by means of a conformal transformation. Similarly to the metric
case, the field equations impose the non-conservation of the energy-momentum
tensor. We derive the explicit form of the equations of motion for massive test
particles in the case of a perfect fluid, and the expression of the extra-force
is obtained in terms of the matter-geometry coupling functions and of their
derivatives. Generally, the motion is non-geodesic, and the extra force is
orthogonal to the four-velocity.Comment: 7 pages, no figures; v2, revised and corrected version; new Section
adde
- …