104 research outputs found
The role of nonmetricity in metric-affine theories of gravity
The intriguing choice to treat alternative theories of gravity by means of
the Palatini approach, namely elevating the affine connection to the role of
independent variable, contains the seed of some interesting (usually
under-explored) generalizations of General Relativity, the metric-affine
theories of gravity. The peculiar aspect of these theories is to provide a
natural way for matter fields to be coupled to the independent connection
through the covariant derivative built from the connection itself. Adopting a
procedure borrowed from the effective field theory prescriptions, we study the
dynamics of metric-affine theories of increasing order, that in the complete
version include invariants built from curvature, nonmetricity and torsion. We
show that even including terms obtained from nonmetricity and torsion to the
second order density Lagrangian, the connection lacks dynamics and acts as an
auxiliary field that can be algebraically eliminated, resulting in some extra
interactions between metric and matter fields.Comment: 15 pages. v2: some misconceptions clarified, references added.
Accepted for publication on Classical and Quantum Gravit
Horizon thermodynamics and spacetime mappings
When black holes are dynamical, event horizons are replaced by apparent and
trapping horizons. Conformal and Kerr-Schild transformations are widely used in
relation with dynamical black holes and we study the behaviour under such
transformations of quantities related to the thermodynamics of these horizons,
such as the Misner-Sharp-Hernandez mass (internal energy), the Kodama vector,
surface gravity, and temperature. The transformation properties are not those
expected on the basis of naive arguments.Comment: 12 page
Geometrically induced magnetic catalysis and critical dimensions
We discuss the combined effect of magnetic fields and geometry in interacting
fermionic systems. At leading order in the heat-kernel expansion, the infrared
singularity (that in flat space leads to the magnetic catalysis) is regulated
by the chiral gap effect, and the catalysis is deactivated by the effect of the
scalar curvature. We discover that an infrared singularity is found in
higher-order terms that mix the magnetic field with curvature, and these lead
to a novel form of geometrically induced magnetic catalysis. The dynamical mass
squared is then modified not only due to the chiral gap effect by an amount
proportional to the curvature, but also by a magnetic shift ,
where represents the number of space-time dimensions. We argue that
is a critical dimension across which the behavior of the magnetic shift changes
qualitatively.Comment: 5 pages; minor change
Gravity beyond general relativity : theory and phenomenology
Despite the notorious achievements of General Relativity, Einstein's theory is under scrutiny due to the lack of a suitable scheme to quantize gravity as well as for the puzzling features it shows both at strong (early universe, black holes) and weak (Dark Energy problem) regime. The proposal to extend the classical theory of gravity harbours the intriguing goals to cure some of these inconsistencies. A large class of modi cations of General Relativity (GR) has been widely explored in the past; in principle, the main motivation for such early e orts was to solve the problem of non-renormalizability by providing a new framework in which (thanks to higher order corrections in the gravitational action) gravity could be quantized. The analysis of the cosmological implications of such models also showed a number of peculiar features that justi ed further developments. The ultraviolet modi cations that naturally arise at high energy in the context of quantum gravity have been taken into account for their impact on the phenomenology of the very early universe. Furthermore, it was recently argued that alternative infrared extensions of the Einstein-Hilbert (EH) action could be invoked to presumably alleviate the Dark Sector problem
The dynamics of generalized Palatini Theories of Gravity
It is known that in f(R) theories of gravity with an independent connection
which can be both non-metric and non symmetric, this connection can always be
algebraically eliminated in favour of the metric and the matter fields, so long
as it is not coupled to the matter explicitly. We show here that this is a
special characteristic of f(R) actions, and it is not true for actions that
include other curvature invariants. This contradicts some recent claims in the
literature. We clarify the reasons of this contradiction.Comment: v1: 6 pages; v2: minor changes to match published versio
The dynamics of metric-affine gravity
Metric-affine theories of gravity provide an interesting alternative to
General Relativity: in such an approach, the metric and the affine (not
necessarily symmetric) connection are independent quantities. Furthermore, the
action should include covariant derivatives of the matter fields, with the
covariant derivative naturally defined using the independent connection. As a
result, in metric-affine theories a direct coupling involving matter and
connection is also present. The role and the dynamics of the connection in such
theories is explored. We employ power counting in order to construct the action
and search for the minimal requirements it should satisfy for the connection to
be dynamical. We find that for the most general action containing lower order
invariants of the curvature and the torsion the independent connection does not
carry any dynamics. It actually reduces to the role of an auxiliary field and
can be completely eliminated algebraically in favour of the metric and the
matter field, introducing extra interactions with respect to general
relativity. However, we also show that including higher order terms in the
action radically changes this picture and excites new degrees of freedom in the
connection, making it (or parts of it) dynamical. Constructing actions that
constitute exceptions to this rule requires significant fine tuned and/or extra
a priori constraints on the connection. We also consider f(R) actions as a
particular example in order to show that they constitute a distinct class of
metric-affine theories with special properties, and as such they cannot be used
as representative toy theories to study the properties of metric-affine
gravity.Comment: 26 pages. v2: some footnotes, references and minor changes added to
match the published version. v3: some equations corrected to account for a
term that had been missed, results unaffecte
Cosmography beyond standard candles and rulers
We perform a cosmographic analysis using several cosmological observables
such as the luminosity distance moduli, the volume distance, the angular
diameter distance and the Hubble parameter. These quantities are determined
using different data sets: Supernovae type Ia and Gamma Ray Bursts, the
Baryonic Acoustic Oscillations, the cosmic microwave background power spectrum
and the Hubble parameter as measured from surveys of galaxies. This data set
allows to put constraints on the cosmographic expansion with unprecedented
precision. We also present forecasts for the coefficients of the kinematic
expansion using future but realistic data sets: constraints on the coefficients
of the expansions are likely to improve by a factor ten with the upcoming large
scale structure probes. Finally, we derive the set of the cosmographic
parameters for several cosmological models (including CDM) and compare
them with our best fit set. While distance measurements are unable to
discriminate among these models, we show that the inclusion of the Hubble data
set leads to strong constraints on the lowest order coefficients and in
particular it is incompatible with CDM at 3- confidence level.
We discuss the reliability of this determination and suggest further
observations which might be of crucial importance for the viability of
cosmographic tests in the next future.Comment: 15 pages, 2 figures, 2 tables, Accepted for publication in PR
Raychaudhuri equation in spacetimes with torsion
Given a spacetime with nonvanishing torsion, we discuss the equation for the
evolution of the separation vector between infinitesimally close curves in a
congruence. We show that the presence of a torsion field leads, in general, to
tangent and orthogonal effects on the congruence; in particular, the presence
of a completely generic torsion field contributes to a relative acceleration
between test particles. We derive, for the first time in the literature, the
Raychaudhuri equation for a congruence of timelike and null curves in a
spacetime with the most generic torsion field.The Authors wish to thank José P. S. Lemos for early discussions on a first version of the paper. We thank FCT-Portugal for financial support through Project No. PEst-OE/FIS/UI0099/2015. PL thanks IDPASC and FCT-Portugal for financial support through Grant No. PD/BD/114074/2015. VV is supported by the FCTPortugal grant SFRH/BPD/77678/2011.info:eu-repo/semantics/publishedVersio
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