2,610 research outputs found
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
Scale hierarchy in Horava-Lifshitz gravity: a strong constraint from synchrotron radiation in the Crab nebula
Horava-Lifshitz gravity models contain higher order operators suppressed by a
characteristic scale, which is required to be parametrically smaller than the
Planck scale. We show that recomputed synchrotron radiation constraints from
the Crab nebula suffice to exclude the possibility that this scale is of the
same order of magnitude as the Lorentz breaking scale in the matter sector.
This highlights the need for a mechanism that suppresses the percolation of
Lorentz violation in the matter sector and is effective for higher order
operators as well.Comment: 4 page, 2 figures; v2: minor changes to match published versio
Gedanken experiments on nearly extremal black holes and the Third Law
A gedanken experiment in which a black hole is pushed to spin at its maximal
rate by tossing into it a test body is considered. After demonstrating that
this is kinematically possible for a test body made of reasonable matter, we
focus on its implications for black hole thermodynamics and the apparent
violation of the third law (unattainability of the extremal black hole). We
argue that this is not an actual violation, due to subtleties in the absorption
process of the test body by the black hole, which are not captured by the
purely kinematic considerations.Comment: v2: minor edits, references added; v3: minor edits to match published
versio
Reply to "Can gravitational dynamics be obtained by diffeomorphism invariance of action?"
In a previous work we showed that, in a suitable setting, one can use
diffeomorphism invariance in order to derive gravitational field equations from
boundary terms of the gravitational action. Standing by our results we reply
here to a recent comment questioning their validity.Comment: Accepted for publication in PR
Analogue Cosmological Particle Creation: Quantum Correlations in Expanding Bose Einstein Condensates
We investigate the structure of quantum correlations in an expanding Bose
Einstein Condensate (BEC) through the analogue gravity framework. We consider
both a 3+1 isotropically expanding BEC as well as the experimentally relevant
case of an elongated, effectively 1+1 dimensional, expanding condensate. In
this case we include the effects of inhomogeneities in the condensate, a
feature rarely included in the analogue gravity literature. In both cases we
link the BEC expansion to a simple model for an expanding spacetime and then
study the correlation structure numerically and analytically (in suitable
approximations). We also discuss the expected strength of such correlation
patterns and experimentally feasible BEC systems in which these effects might
be detected in the near future.Comment: Reference adde
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