1,145 research outputs found
Godel Metrics with Chronology Protection in Horndeski Gravities
G\"odel universe, one of the most interesting exact solutions predicted by
General Relativity, describes a homogeneous rotating universe containing naked
closed time-like curves (CTCs). It was shown that such CTCs are the consequence
of the null energy condition in General Relativity. In this paper, we show that
the G\"odel-type metrics with chronology protection can emerge in
Einstein-Horndeski gravity. We construct such exact solutions also in
Einstein-Horndeski-Maxwell and Einstein-Horndeski-Proca theories.Comment: Latex, 11 pages, references adde
Godel Universe from String Theory
G\"odel universe is a direct product of a line and a three-dimensional
spacetime we call G. In this paper, we show that the G\"odel metrics
can arise as exact solutions in Einstein-Maxwell-Axion, Einstein-Proca-Axion,
or Freedman-Schwarz gauged supergravity theories. The last allows us to embed
G\"odel universe in string theory. The ten-dimensional spacetime is a direct
product of a line and the nine-dimensional one of an bundle
over G, and it can be interpreted as some decoupling limit of the
rotating D1/D5/D5 intersection. For some appropriate parameter choice, the
nine-dimensional metric becomes an AdS bundle over squashed
3-sphere. We also study the properties of the G\"odel black holes that are
constructed from the double Wick rotations of the G\"odel metrics.Comment: latex, 20 pages, discussion on null-energy condition included, typos
corrected and references adde
Criticality in Gauged Supergravities
AdS black holes show richer transition behaviors in extended phase space by
assuming the cosmological constant and its conjugate quantity to behave like
thermodynamic pressure and thermodynamic volume. We study the extended
thermodynamics of charged dilatonic AdS black holes in a class of
Einstein-Maxwell-dilaton theories that can be embedded in gauged supergravities
in various dimensions. We find that the transition behaviors of higher
dimensional dilatonic AdS black holes are different from the four dimensional
counterparts, and new transition behaviors emerges in higher dimensions. First,
there exists standard Van der Waals transition only in a five dimensional
dilatonic AdS black hole with two equal charges. Second, there emerge a new
phase transition branch in negative pressure region in six and seven
dimensional dilatonic black holes with two equal charges. Third, there emerge
transition behaviors in higher dimensional black hole with single charge cases,
which are absent in four dimensions.Comment: Latex, 18 pages, 8 figures; published versio
Magnetization plateau and incommensurate spin modulation in Ca3Co2O6
The magnetic properties of a trigonal prism unit of the spin-2 frustrated
compound Ca3Co2O6 are studied by means of the density-matrix renormalization
group method. A magnetization plateau at ( is the saturation
magnetization) with ferrimagnetic structure is observed. By fitting the
experimental data of magnetic curve, an estimation of the couplings gives
J1=-26.84K, J_{2}=0.39K, and J_{3}=0.52K. The local magnetic moments are
unveiled to exhibit an incommensurate sinusoidally modulation along the three
chains of the trigonal prism, which gives a strong theoretical support to the
experimentally observed incommensurate partially disordered antiferromagnetic
state for Ca3Co2O6. The present result suggests that the modulation indeed
originates from the competition of antiferromagnetic and ferromagnetic
couplings.Comment: 4 pages, 4 figures, accepted by Appl. Phys. Lett
Scales, Couplings Revisited and Low Energy Phenomenology in M-theory on
We revisit the eleven dimension Planck scale, the physical scale of the
eleventh dimension, the physical scale of Calabi-Yau manifold and coupling in
hidden sector in M-theory on . And we discuss the reasonable bound on
them. Considering F-term of dilaton and moduli SUSY breaking and choosing two
repersentative points which correspond to scalar quasi-massless scenario and
dilaton dominant SUSY breaking scenario respectively, we analyze experimental
constraints to the parameter space. The sparticle spectrum and some
phenomenological predictions are also given.Comment: 35 pages and 8 eps files. One error on fig1b is correcte
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