1,764 research outputs found
`Stringy' Newton-Cartan Gravity
We construct a "stringy" version of Newton-Cartan gravity in which the
concept of a Galilean observer plays a central role. We present both the
geodesic equations of motion for a fundamental string and the bulk equations of
motion in terms of a gravitational potential which is a symmetric tensor with
respect to the longitudinal directions of the string. The extension to include
a non-zero cosmological constant is given. We stress the symmetries and
(partial) gaugings underlying our construction. Our results provide a
convenient starting point to investigate applications of the AdS/CFT
correspondence based on the non-relativistic "stringy" Galilei algebra.Comment: 44 page
On massive gravitons in 2+1 dimensions
The Fierz-Pauli (FP) free field theory for massive spin 2 particles can be
extended, in a spacetime of (1+2) dimensions (3D), to a generally covariant
parity-preserving interacting field theory, in at least two ways. One is "new
massive gravity" (NMG), with an action that involves curvature-squared terms.
Another is 3D "bigravity", which involves non-linear couplings of the FP tensor
field to 3D Einstein-Hilbert gravity. We review the proof of the linearized
equivalence of both "massive 3D gravity" theories to FP theory, and we comment
on their similarities and differences.Comment: 6 pages, to appear in the proceedings of the Spanish Relativity
Meeting ERE2009, Bilbao; minor changes, reference adde
Visual and auditory temporal integration in healthy younger and older adults
As people age, they tend to integrate successive visual stimuli over longer intervals than younger adults. It may be expected that temporal integration is affected similarly in other modalities, possibly due to general, age-related cognitive slowing of the brain. However, the previous literature does not provide convincing evidence that this is the case in audition. One hypothesis is that the primacy of time in audition attenuates the degree to which temporal integration in that modality extends over time as a function of age. We sought to settle this issue by comparing visual and auditory temporal integration in younger and older adults directly, achieved by minimizing task differences between modalities. Participants were presented with a visual or an auditory rapid serial presentation task, at 40-100 ms/item. In both tasks, two subsequent targets were to be identified. Critically, these could be perceptually integrated and reported by the participants as such, providing a direct measure of temporal integration. In both tasks, older participants integrated more than younger adults, especially when stimuli were presented across longer time intervals. This difference was more pronounced in vision and only marginally significant in audition. We conclude that temporal integration increases with age in both modalities, but that this change might be slightly less pronounced in audition
Newtonian Gravity and the Bargmann Algebra
We show how the Newton-Cartan formulation of Newtonian gravity can be
obtained from gauging the Bargmann algebra, i.e., the centrally extended
Galilean algebra. In this gauging procedure several curvature constraints are
imposed. These convert the spatial (time) translational symmetries of the
algebra into spatial (time) general coordinate transformations, and make the
spin connection gauge fields dependent. In addition we require two independent
Vielbein postulates for the temporal and spatial directions. In the final step
we impose an additional curvature constraint to establish the connection with
(on-shell) Newton-Cartan theory. We discuss a few extensions of our work that
are relevant in the context of the AdS-CFT correspondence.Comment: Latex, 20 pages, typos corrected, published versio
Hamiltonian analysis of BHT massive gravity
We study the Hamiltonian structure of the Bergshoeff-Hohm-Townsend (BHT)
massive gravity with a cosmological constant. In the space of coupling
constants , our canonical analysis reveals the special role of
the condition . In this sector, the dimension of the
physical phase space is found to be , which corresponds to two
Lagrangian degree of freedom. When applied to the AdS asymptotic region, the
canonical approach yields the conserved charges of the BTZ black hole, and
central charges of the asymptotic symmetry algebra.Comment: LATEX, 21 pages; v2: minor correction
Some interesting features of new massive gravity
A proof that new massive gravity - the massive 3D gravity model proposed by
Bergshoeff, Hohm and Townsend (BHT) - is the only unitary system at the tree
level that can be constructed by augmenting planar gravity through the
curvature-squared terms, is presented. Two interesting gravitational properties
of the BHT model, namely, time dilation and time delay, which have no
counterpart in the usual Einstein 3D gravity, are analyzed as well.Comment: Submitted to Classical and Quantum Gravit
A Lifshitz Black Hole in Four Dimensional R^2 Gravity
We consider a higher derivative gravity theory in four dimensions with a
negative cosmological constant and show that vacuum solutions of both Lifshitz
type and Schr\"{o}dinger type with arbitrary dynamical exponent z exist in this
system. Then we find an analytic black hole solution which asymptotes to the
vacuum Lifshitz solution with z=3/2 at a specific value of the coupling
constant. We analyze the thermodynamic behavior of this black hole and find
that the black hole has zero entropy while non-zero temperature, which is very
similar to the case of BTZ black holes in new massive gravity at a specific
coupling. In addition, we find that the three dimensional Lifshitz black hole
recently found by E. Ayon-Beato et al. has a negative entropy and mass when the
Newton constant is taken to be positive.Comment: 11 pages, no figure; v2, a minor error correcte
On Critical Massive (Super)Gravity in adS3
We review the status of three-dimensional "general massive gravity" (GMG) in
its linearization about an anti-de Sitter (adS) vacuum, focusing on critical
points in parameter space that yield generalizations of "chiral gravity". We
then show how these results extend to N=1 super-GMG, expanded about a
supersymmetric adS vacuum, and also to the most general `curvature-squared' N=1
supergravity model.Comment: 10 pages, Proceedings of ERE 2010, Granada, 6-10 september 2010;
reference adde
On the new massive gravity and AdS/CFT
Demanding the existence of a simple holographic -theorem, it is shown that
a general (parity preserving) theory of gravity in 2+1 dimensions involving
upto four derivative curvature invariants reduces to the new massive gravity
theory. We consider extending the theory including upto six derivative
curvature invariants. Black hole solutions are presented and consistency with
1+1 CFTs is checked. We present evidence that bulk unitarity is still in
conflict with a positive CFT central charge for generic choice of parameters.
However, for a special choice of parameters appearing in the four and six
derivative terms reduces the linearized equations to be two derivative, thereby
ameliorating the unitarity problem.Comment: 16 pages, 2 figures. v4: typo correcte
Mars Surface Mobility: Comparison of Past, Present, and Future Rover Systems
The future robotic and human exploration of Mars will rely heavily on mobile system to meet exploration objectives. In particular, the next decade of exploration (2009-2020) will utilize rovers and other mobile surface platforms to conduct a wide variety of tasks, including in the search for water and life, characterization of terrain and its geology, and conduct precursor measurements prepare for future human exploration
- …