6,400 research outputs found
Higher-Derivative Gravity with Non-minimally Coupled Maxwell Field
We construct higher-derivative gravities with a non-minimally coupled Maxwell
field. The Lagrangian consists of polynomial invariants built from the Riemann
tensor and the Maxwell field strength in such a way that the equations of
motion are second order for both the metric and the Maxwell potential. We also
generalize the construction to involve a generic non-minimally coupled -form
field strength. We then focus on one low-lying example in four dimensions and
construct the exact magnetically-charged black holes. We also construct exact
electrically-charged Lifshitz black holes. We obtain approximate dyonic
black holes for the small coupling constant or small charges. We find that the
thermodynamics based on the Wald formalism disagrees with that derived from the
Euclidean action procedure, suggesting this may be a general situation in
higher-derivative gravities with non-minimally coupled form fields. As an
application in the AdS/CFT correspondence, we study the entropy/viscosity ratio
for the AdS or Lifshitz planar black holes, and find that the exact ratio can
be obtained without having to know the details of the solutions, even for this
higher-derivative theory.Comment: Latex, 23 page
Intraday pattern in bid-ask spreads and its power-law relaxation for Chinese A-share stocks
We use high-frequency data of 1364 Chinese A-share stocks traded on the
Shanghai Stock Exchange and Shenzhen Stock Exchange to investigate the intraday
patterns in the bid-ask spreads. The daily periodicity in the spread time
series is confirmed by Lomb analysis and the intraday bid-ask spreads are found
to exhibit -shaped pattern with idiosyncratic fine structure. The intraday
spread of individual stocks relaxes as a power law within the first hour of the
continuous double auction from 9:30AM to 10:30AM with exponents
for the Shanghai market and
for the Shenzhen market. The power-law
relaxation exponent of individual stocks is roughly normally
distributed. There is evidence showing that the accumulation of information
widening the spread is an endogenous process.Comment: 12 Elsart pages including 7 eps figure
Non-Abelian (Hyperscaling Violating) Lifshitz Black Holes in General Dimensions
We consider Einstein gravities coupled to a cosmological constant and
multiple Yang-Mills fields in general dimensions and find that the
theories admit colored Lifshitz solutions with dynamic exponents . We also
introduce a Maxwell field and construct exact electric charged black holes that
asymptote to the colored Lifshitz spacetimes and analyse their
thermodynamical first law. Furthermore, we introduce a dilaton to the system
and construct Lifshitz spacetimes with hyperscaling violations. After turning
on the Maxwell field, we obtain a class of hyperscaling violating Lifshitz
black holes when .Comment: Latex, 12 pages, a published version in PL
Holographic Complexity Growth Rate in Horndeski Theory
Based on the context of complexity = action (CA) conjecture, we calculate the
holographic complexity of AdS black holes with planar and spherical topologies
in Horndeski theory. We find that the rate of change of holographic complexity
for neutral AdS black holes saturates the Lloyd's bound. For charged black
holes, we find that there exists only one horizon and thus the corresponding
holographic complexity can't be expressed as the difference of some
thermodynamical potential between two horizons as that of Reissner-Nordstrom
AdS black hole in Einstein-Maxwell theory. However, the Lloyd's bound is not
violated for charged AdS black hole in Horndeski theory.Comment: 20 pages, 6 figures, references added, typos correcte
New Active Asteroid (6478) Gault
Main-belt asteroid (6478) Gault was observed to show cometary features in
early 2019. To investigate the cause, we conducted {\it BVR} observations at
Xingming Observatory, China, from 2019 January to April. The two tails were
formed around 2018 October 26--November 08, and 2018 December 29--2019 January
08, respectively, and consisted of dust grains of 20 m to 3 mm in
radius ejected at a speed of m s and following a broken
power-law size distribution bending at grain radius 70 m (bulk
density 1 g cm assumed). The total mass of dust within a
km-radius aperture around Gault declined from kg since
2019 January at a rate of kg s, but temporarily surged
around 2019 March 25, because Earth thence crossed the orbital plane of Gault,
within which the ejected dust was mainly distributed. No statistically
significant colour or short-term lightcurve variation was seen. Nonetheless we
argue that Gault is currently subjected to rotational instability. Using the
available astrometry, we did not detect any nongravitational acceleration in
the orbital motion of Gault.Comment: 6 pages, 4 figures, 2 table
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