21 research outputs found
Transgression forms and extensions of Chern-Simons gauge theories
A gauge invariant action principle, based on the idea of transgression forms,
is proposed. The action extends the Chern-Simons form by the addition of a
boundary term that makes the action gauge invariant (and not just
quasi-invariant). Interpreting the spacetime manifold as cobordant to another
one, the duplication of gauge fields in spacetime is avoided. The advantages of
this approach are particularly noticeable for the gravitation theory described
by a Chern-Simons lagrangian for the AdS group, in which case the action is
regularized and finite for black hole geometries in diverse situations. Black
hole thermodynamics is correctly reproduced using either a background field
approach or a background-independent setting, even in cases with asymptotically
nontrivial topologies. It is shown that the energy found from the thermodynamic
analysis agrees with the surface integral obtained by direct application of
Noether's theorem.Comment: 28 pages, no figures. Minor changes in the introduction, final
comments and reference
Higher codimension braneworlds from intersecting branes
We study the matching conditions of intersecting brane worlds in Lovelock
gravity in arbitrary dimension. We show that intersecting various codimension 1
and/or codimension 2 branes one can find solutions that represent
energy-momentum densities localized in the intersection, providing thus the
first examples of infinitesimally thin higher codimension braneworlds that are
free of singularities and where the backreaction of the brane in the background
is fully taken into account.Comment: 20 pages; v2. references and comments added to match the published
versio
Supersymmetry of gravitational ground states
A class of black objects which are solutions of pure gravity with negative
cosmological constant are classified through the mapping between the Killing
spinors of the ground state and those of the transverse section. It is shown
that these geometries must have transverse sections of constant curvature for
spacetime dimensions d below seven. For d > 6, the transverse sections can also
be Euclidean Einstein manifolds. In even dimensions, spacetimes with transverse
section of nonconstant curvature exist only in d = 8 and 10. This
classification goes beyond standard supergravity and the eleven dimensional
case is analyzed. It is shown that if the transverse section has negative
scalar curvature, only extended objects can have a supersymmetric ground state.
In that case, some solutions are explicitly found whose ground state resembles
a wormhole.Comment: 16 pages, CECS style, minor correction
On the underlying gauge group structure of D=11 supergravity
The underlying gauge group structure of D=11 supergravity is revisited (see
paper for detailed abstract).Comment: 16 pages, no figure
Brane cosmology with curvature corrections
We study the cosmology of the Randall-Sundrum brane-world where the
Einstein-Hilbert action is modified by curvature correction terms: a
four-dimensional scalar curvature from induced gravity on the brane, and a
five-dimensional Gauss-Bonnet curvature term. The combined effect of these
curvature corrections to the action removes the infinite-density big bang
singularity, although the curvature can still diverge for some parameter
values. A radiation brane undergoes accelerated expansion near the minimal
scale factor, for a range of parameters. This acceleration is driven by the
geometric effects, without an inflaton field or negative pressures. At late
times, conventional cosmology is recovered.Comment: RevTex4, 8 pages, no figures, minor change
(p,q) D=3 Poincare supergravities from Lie algebra expansions
We use the expansion of superalgebras procedure (summarized in the text) to
derive Chern-Simons (CS) actions for the (p,q)-Poincare supergravities in
three-dimensional spacetime. After deriving the action for the (p,0)-Poincare
supergravity as a CS theory for the expansion osp(p|2;R)(2,1) of osp(p|2;R), we
find the general (p,q)-Poincare superalgebras and their associated D=3
supergravity actions as CS gauge theories from an expansion of the simple
osp(p+q|2,R) superalgebras, namely osp(p+q|2,R)(2,1,2).Comment: v2: two references added, 17 pages. To appear in NP
Chemical and mineralogical changes in a Brazilian Rhodic Paleudult under different land use and managements
Bilateral hippocampal increase following first-episode psychosis is associated with good clinical, functional and cognitive outcomes
Background - Hippocampal pathology has been proposed to underlie clinical, functional and cognitive impairments in schizophrenia. The hippocampus is a highly plastic brain region; examining change in volume, or change bilaterally, over time, can advance understanding of the substrate of recovery in psychosis.
Method - Magnetic resonance imaging and outcome data were collected at baseline and 6-year follow-up in 42 first-episode psychosis subjects and 32 matched controls, to investigate whether poorer outcomes are associated with loss of global matter and hippocampal volumes. Bilateral hippocampal increase (BHI) over time, as a marker of hippocampal plasticity was hypothesized to be associated with better outcomes. Regression analyses were performed on: (i) clinical and functional outcomes with grey matter volume change and BHI as predictor variables; and (ii) cognitive outcome with BHI as predictor.
Results - BHI was present in 29% of psychosis participants. There was no significant grey matter loss over time in either patient or control groups. Less severe illness course and lesser symptom severity were associated with BHI, but not with grey matter change. Employment and global function were associated with BHI and with less grey matter loss. Superior delayed verbal recall was also associated with BHI.
Conclusions - BHI occurs in a minority of patients following their first psychotic episode and is associated with good outcome across clinical, functional and cognitive domains
Neuropsychological functioning in first-episode schizophrenia
Background - Identifying neurocognitive subtypes in schizophrenia may help establish neurobiologically meaningful subtypes of the disorder, but is frequently confounded by differences in intellectual function between individuals with schizophrenia and controls.
Aims - To examine neuropsychological performance in individuals with epidemiologically based, first-onset schizophrenia and intellectually matched controls.
Method - Using standard IQ and reading tests, we examined the proportions of 101 people with epidemiologically derived, first-onset schizophrenia/schizoaffective disorder and 317 community controls, falling into three a priori defined intellectual categories: ‘stable good’, ‘deteriorated poor’ and ‘stable poor’. Neuropsychological function was compared between intellectually matched participants with schizophrenia and control subgroups.
Results - Multiple deficits in executive function, processing speed and verbal memory, but not visual/spatial perception/memory, were detected in all participant groups with schizophrenia compared with controls. The average effect size across the affected domains ranged from small to medium to large in the stable good, deteriorated poor and stable poor subgroups of participants with schizophrenia, respectively.
Conclusions - Compared with intellectually matched controls, people with epidemiologically derived, first-onset schizophrenia/schizoaffective disorder show multiple deficits in executive function, processing speed and verbal memory