7,974 research outputs found
Surface terms, Asymptotics and Thermodynamics of the Holst Action
We consider a first order formalism for general relativity derived from the
Holst action. This action is obtained from the standard Palatini-Hilbert form
by adding a topological-like term and can be taken as the starting point for
loop quantum gravity and spin foam models. The equations of motion derived from
the Holst action are, nevertheless, the same as in the Palatini formulation.
Here we study the form of the surface terms of the action for general
boundaries as well as the symplectic current in the covariant formulation of
the theory. Furthermore, we analyze the behavior of the surface terms in
asymptotically flat space-times. We show that the contribution to the
symplectic structure from the Holst term vanishes and one obtains the same
asymptotic expressions as in the Palatini action. It then follows that the
asymptotic Poincare symmetries and conserved quantities such as energy, linear
momentum and relativistic angular momentum found here are equivalent to those
obtained from the standard Arnowitt, Deser and Misner formalism. Finally, we
consider the Euclidean approach to black hole thermodynamics and show that the
on-shell Holst action, when evaluated on some static solutions containing
horizons, yields the standard thermodynamical relations.Comment: 16 page
The Big Bang as a Phase Transition
We study a five-dimensional cosmological model, which suggests that the
universe bagan as a discontinuity in a (Higgs-type) scalar field, or
alternatively as a conventional four-dimensional phase transition.Comment: 10 pages, 2 figures; typo corrected in equation (18); 1 reference
added; version to appear in International Journal of Modern Physics
Isolated horizons in higher-dimensional Einstein-Gauss-Bonnet gravity
The isolated horizon framework was introduced in order to provide a local
description of black holes that are in equilibrium with their (possibly
dynamic) environment. Over the past several years, the framework has been
extended to include matter fields (dilaton, Yang-Mills etc) in D=4 dimensions
and cosmological constant in dimensions. In this article we present a
further extension of the framework that includes black holes in
higher-dimensional Einstein-Gauss-Bonnet (EGB) gravity. In particular, we
construct a covariant phase space for EGB gravity in arbitrary dimensions which
allows us to derive the first law. We find that the entropy of a weakly
isolated and non-rotating horizon is given by
.
In this expression is the -dimensional cross section of the
horizon with area form and Ricci scalar ,
is the -dimensional Newton constant and is the Gauss-Bonnet
parameter. This expression for the horizon entropy is in agreement with those
predicted by the Euclidean and Noether charge methods. Thus we extend the
isolated horizon framework beyond Einstein gravity.Comment: 18 pages; 1 figure; v2: 19 pages; 2 references added; v3: 19 pages;
minor corrections; 1 reference added; to appear in Classical and Quantum
Gravit
Fundamental properties and applications of quasi-local black hole horizons
The traditional description of black holes in terms of event horizons is
inadequate for many physical applications, especially when studying black holes
in non-stationary spacetimes. In these cases, it is often more useful to use
the quasi-local notions of trapped and marginally trapped surfaces, which lead
naturally to the framework of trapping, isolated, and dynamical horizons. This
framework allows us to analyze diverse facets of black holes in a unified
manner and to significantly generalize several results in black hole physics.
It also leads to a number of applications in mathematical general relativity,
numerical relativity, astrophysics, and quantum gravity. In this review, I will
discuss the basic ideas and recent developments in this framework, and
summarize some of its applications with an emphasis on numerical relativity.Comment: 14 pages, 2 figures. Based on a talk presented at the 18th
International Conference on General Relativity and Gravitation, 8-13 July
2007, Sydney, Australi
Barbero-Immirzi parameter, manifold invariants and Euclidean path integrals
The Barbero-Immirzi parameter appears in the \emph{real} connection
formulation of gravity in terms of the Ashtekar variables, and gives rise to a
one-parameter quantization ambiguity in Loop Quantum Gravity. In this paper we
investigate the conditions under which will have physical effects in
Euclidean Quantum Gravity. This is done by constructing a well-defined
Euclidean path integral for the Holst action with non-zero cosmological
constant on a manifold with boundary. We find that two general conditions must
be satisfied by the spacetime manifold in order for the Holst action and its
surface integral to be non-zero: (i) the metric has to be non-diagonalizable;
(ii) the Pontryagin number of the manifold has to be non-zero. The latter is a
strong topological condition, and rules out many of the known solutions to the
Einstein field equations. This result leads us to evaluate the on-shell
first-order Holst action and corresponding Euclidean partition function on the
Taub-NUT-ADS solution. We find that shows up as a finite rotation of
the on-shell partition function which corresponds to shifts in the energy and
entropy of the NUT charge. In an appendix we also evaluate the Holst action on
the Taub-NUT and Taub-bolt solutions in flat spacetime and find that in that
case as well shows up in the energy and entropy of the NUT and bolt
charges. We also present an example whereby the Euler characteristic of the
manifold has a non-trivial effect on black-hole mergers.Comment: 18 pages; v2: references added; to appear in Classical and Quantum
Gravity; v3: typos corrected; minor revisions to match published versio
On Bouncing Brane-Worlds, S-branes and Branonium Cosmology
We present several higher-dimensional spacetimes for which observers living
on 3-branes experience an induced metric which bounces. The classes of examples
include boundary branes on generalised S-brane backgrounds and probe branes in
D-brane/anti D-brane systems. The bounces we consider normally would be
expected to require an energy density which violates the weak energy condition,
and for our co-dimension one examples this is attributable to bulk curvature
terms in the effective Friedmann equation. We examine the features of the
acceleration which provides the bounce, including in some cases the existence
of positive acceleration without event horizons, and we give a geometrical
interpretation for it. We discuss the stability of the solutions from the point
of view of both the brane and the bulk. Some of our examples appear to be
stable from the bulk point of view, suggesting the possible existence of stable
bouncing cosmologies within the brane-world framework.Comment: 35 pages, 7 figures, JHEP style. Title changed and references adde
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Measurement of masses in the [Formula: see text] system by kinematic endpoints in pp collisions at [Formula: see text].
A simultaneous measurement of the top-quark, W-boson, and neutrino masses is reported for [Formula: see text] events selected in the dilepton final state from a data sample corresponding to an integrated luminosity of 5.0 fb-1 collected by the CMS experiment in pp collisions at [Formula: see text]. The analysis is based on endpoint determinations in kinematic distributions. When the neutrino and W-boson masses are constrained to their world-average values, a top-quark mass value of [Formula: see text] is obtained. When such constraints are not used, the three particle masses are obtained in a simultaneous fit. In this unconstrained mode the study serves as a test of mass determination methods that may be used in beyond standard model physics scenarios where several masses in a decay chain may be unknown and undetected particles lead to underconstrained kinematics
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