589 research outputs found
Static and symmetric wormholes respecting energy conditions in Einstein-Gauss-Bonnet gravity
Properties of -dimensional static wormhole solutions are
investigated in Einstein-Gauss-Bonnet gravity with or without a cosmological
constant . We assume that the spacetime has symmetries corresponding
to the isometries of an -dimensional maximally symmetric space with the
sectional curvature . It is also assumed that the metric is at
least and the -dimensional maximally symmetric subspace is
compact. Depending on the existence or absence of the general relativistic
limit , solutions are classified into general relativistic (GR)
and non-GR branches, respectively, where is the Gauss-Bonnet coupling
constant. We show that a wormhole throat respecting the dominant energy
condition coincides with a branch surface in the GR branch, otherwise the null
energy condition is violated there. In the non-GR branch, it is shown that
there is no wormhole solution for . For the matter field with
zero tangential pressure, it is also shown in the non-GR branch with
and that the dominant energy condition holds at the
wormhole throat if the radius of the throat satisfies some inequality. In the
vacuum case, a fine-tuning of the coupling constants is shown to be necessary
and the radius of a wormhole throat is fixed. Explicit wormhole solutions
respecting the energy conditions in the whole spacetime are obtained in the
vacuum and dust cases with and .Comment: 10 pages, 2 tables; v2, typos corrected, references added; v3,
interpretation of the solution for n=5 in section IV corrected; v4, a very
final version to appear in Physical Review
SPRINT RUNNERS' INTENTIONS DURING ACCELERATION AND CHANGES IN THEIR RUNNING SPEED
This study investigated the relationship between intention during acceleration and changes in running speed during a sprint. Changes in running speed over each entire sprint were measured using a laser distance meter (100 Hz). Seven male sprinters performed two sprints with different intentions during acceleration: in one sprint, sprinters were instructed to immediately reach their maximum speed (ACinst), and in the other, sprinters were instructed to sprint 100 m as in a typical sprint race (ACloo). AClm showed significantly higher values for the upper limit of the sprinter's top speed compared to the ACimt. The ACinst showed significantly higher values for initial acceleration compared to the ACiw. These results suggest that changes in running speed are affected by the intention in the acceleration
Dynamical p-branes with a cosmological constant
We present a class of dynamical solutions in a D-dimensional gravitational
theory coupled to a dilaton, a form field strength, and a cosmological
constant. We find that for any D due to the presence of a cosmological
constant, the metric of solutions depends on a quadratic function of the brane
world volume coordinates, and the transverse space cannot be Ricci flat except
for the case of 1-branes. We then discuss the dynamics of 1-branes in a
D-dimensional spacetime. For a positive cosmological constant, 1-brane
solutions with D>4 approach the Milne universe in the far-brane region. On the
other hand, for a negative cosmological constant, each 1-brane approaches the
others as the time evolves from a positive value, but no brane collision occurs
for D>4, since the spacetime close to the 1-branes eventually splits into the
separate domains. In contrast, the D=3 case provides an example of colliding
1-branes. Finally, we discuss the dynamics of 0-branes and show that for D>2,
they behave like the Milne universe after the infinite cosmic time has passed.Comment: 21 pages, 7 figures; v2: minor correction
Cosmological rotating black holes in five-dimensional fake supergravity
In recent series of papers, we found an arbitrary dimensional, time-evolving
and spatially-inhomogeneous solutions in Einstein-Maxwell-dilaton gravity with
particular couplings. Similar to the supersymmetric case the solution can be
arbitrarily superposed in spite of non-trivial time-dependence, since the
metric is specified by a set of harmonic functions. When each harmonic has a
single point source at the center, the solution describes a spherically
symmetric black hole with regular Killing horizons and the spacetime approaches
asymptotically to the Friedmann-Lema\^itre-Robertson-Walker (FLRW) cosmology.
We discuss in this paper that in 5-dimensions this equilibrium condition traces
back to the 1st-order "Killing spinor" equation in "fake supergravity" coupled
to arbitrary U(1) gauge fields and scalars. We present a 5-dimensional,
asymptotically FLRW, rotating black-hole solution admitting a nontrivial
"Killing spinor," which is a spinning generalization of our previous solution.
We argue that the solution admits nondegenerate and rotating Killing horizons
in contrast with the supersymmetric solutions. It is shown that the present
pseudo-supersymmetric solution admits closed timelike curves around the central
singularities. When only one harmonic is time-dependent, the solution oxidizes
to 11-dimensions and realizes the dynamically intersecting M2/M2/M2-branes in a
rotating Kasner universe. The Kaluza-Klein type black holes are also discussed.Comment: 24 pages, 2 figures; v2: references added, to appear in PR
Black hole thermodynamics in Horndeski theories
We investigate thermodynamics of static and spherically symmetric black holes
(BHs) in the Horndeski theories. Because of the presence of the
higher-derivative interactions and the nonminimal derivative couplings of the
scalar field, the standard Wald entropy formula may not be directly applicable.
Hence, following the original formulation by Iyer and Wald, we obtain the
differentials of the BH entropy and the total mass of the system in the
Horndeski theories, which lead to the first-law of thermodynamics via the
conservation of the Hamiltonian. Our formulation covers the case of the static
and spherically symmetric BH solutions with the static scalar field and those
with the linearly time-dependent scalar field in the shift-symmetric Horndeski
theories. We then apply our results to explicit BH solutions in the Horndeski
theories. In the case of the conventional scalar-tensor theories and the
Einstein-scalar-Gauss-Bonnet theories, we recover the BH entropy obtained by
the Wald entropy formula. In the shift-symmetric theories, in the case of the
BH solutions with the the static scalar field we show that the BH entropy
follows the ordinary area law even in the presence of the nontrivial profile of
the scalar field. On the other hand, in the case of the BH solutions where the
scalar field linearly depends on time, i.e., the stealth Schwarzschild and
Schwarzschild-(anti-) de Sitter solutions, the BH entropy also depends on the
profile of the scalar field. By use of the entropy, we find that there exists
some range of the parameters in which Schwarzschild(AdS) BH with non-trivial
scalar field is thermodynamically stable than Schwarzschild(AdS) BH without
scalar field in general relativity.Comment: 21 pages, 2 figure
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