14 research outputs found
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Stability of Horava-Lifshitz Black Holes in the Context of AdS/CFT
The anti-de Sitter/conformal field theory (AdS/CFT) correspondence is a powerful tool that promises to provide new insights toward a full understanding of field theories under extreme conditions, including but not limited to quark-gluon plasma, Fermi liquid, and superconductor. In many such applications, one typically models the field theory with asymptotically AdS black holes. These black holes are subjected to stringy effects that might render them unstable. Horava-Lifshitz gravity, in which space and time undergo different transformations, has attracted attention due to its power-counting renormalizability. In terms of AdS/CFT correspondence, Horava-Lifshitz black holes might be useful to model holographic superconductors with Lifshitz scaling symmetry. It is thus interesting to study the stringy stability of Horava-Lifshitz black holes in the context of AdS/CFT. We find that uncharged topological black holes in {lambda} = 1 Horava-Lifshitz theory are nonperturbatively stable, unlike their counterparts in Einstein gravity, with the possible exceptions of negatively curved black holes with detailed balance parameter {epsilon} close to unity. Sufficiently charged flat black holes for {epsilon} close to unity, and sufficiently charged positively curved black holes with {epsilon} close to zero, are also unstable. The implication to the Horava-Lifshitz holographic superconductor is discussed
Upper Limit on the Cosmological Gamma-ray Background
We show that the current extragalactic gamma-ray background (EGB) measurement
below 100 GeV sets an upper limit on EGB itself at very high energy (VHE) above
100 GeV. The limit is conservative for the electromagnetic cascade emission
from VHE EGB interacting with the cosmic microwave-to-optical background
radiation not to exceed the current EGB measurement. The cascade component fits
the measured VHE EGB spectrum rather well. However, once we add the
contribution from known source classes, the Fermi VHE EGB observation exceeds
or even violates the limit, which is approximated as E^2dN/dE < 4.5x10^-5
(E/100 GeV)^-0.7 MeV/cm^2/s/sr. The upper limit above 100 GeV is useful in the
future to probe the EGB origin and the new physics like axion-like particles
and Lorentz-invariance violation.Comment: 8 pages, 8 figures, accepted for publication in PRD, Interested
readers may wish to consult the recent paper by Murase, Beacom and Takami
(arXiv:1205.5755
Magnetowave Induced Plasma Wakefield Acceleration for Ultra High Energy Cosmic Rays
Magnetowave induced plasma wakefield acceleration (MPWA) in a relativistic
astrophysical outflow has been proposed as a viable mechanism for the
acceleration of cosmic particles to ultra high energies. Here we present
simulation results that clearly demonstrate the viability of this mechanism for
the first time. We invoke the high frequency and high speed whistler mode for
the driving pulse. The plasma wakefield so induced validates precisely the
theoretical prediction. We show that under appropriate conditions, the plasma
wakefield maintains very high coherence and can sustain high-gradient
acceleration over a macroscopic distance. Invoking gamma ray burst (GRB) as the
source, we show that MPWA production of ultra high energy cosmic rays (UHECR)
beyond ZeV 10^21 eV is possible.Comment: 4 pages, 4 figure
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A GPU-based Calculation Method for Near Field Effects of Cherenkov Radiation Induced by Ultra High Energy Cosmic Neutrinos
Gravitomagnetism and spinor quantum mechanics
We give a systematic treatment of a spin 1/2 particle in a combined
electromagnetic field and a weak gravitational field that is produced by a
slowly moving matter source. This paper continues previous work on a spin zero
particle, but it is largely self-contained and may serve as an introduction to
spinors in a Riemann space. The analysis is based on the Dirac equation
expressed in generally covariant form and coupled minimally to the
electromagnetic field. The restriction to a slowly moving matter source, such
as the earth, allows us to describe the gravitational field by a
gravitoelectric (Newtonian) potential and a gravitomagnetic (frame-dragging)
vector potential, the existence of which has recently been experimentally
verified. Our main interest is the coupling of the orbital and spin angular
momenta of the particle to the gravitomagnetic field. Specifically we calculate
the gravitational gyromagnetic ratio as gsubg=1 ; this is to be compared with
the electromagnetic gyromagnetic ratio of gsube=2 for a Dirac electron.Comment: 12 pages, 1 figur
Observation of thundercloud-related gamma rays and neutrons in Tibet
During the 2010 rainy season in Yangbajing (4300 m above sea level) in Tibet, China, a long-duration count enhancement associated with thunderclouds was detected by a solar-neutron telescope and neutron monitors installed at the Yangbajing Comic Ray Observatory. The event, lasting for ∼40  min, was observed on July 22, 2010. The solar-neutron telescope detected significant γ-ray signals with energies >40  MeV in the event. Such a prolonged high-energy event has never been observed in association with thunderclouds, clearly suggesting that electron acceleration lasts for 40 min in thunderclouds. In addition, Monte Carlo simulations showed that >10  MeV γ rays largely contribute to the neutron monitor signals, while >1  keV neutrons produced via a photonuclear reaction contribute relatively less to the signals. This result suggests that enhancements of neutron monitors during thunderstorms are not necessarily clear evidence for neutron production, as previously thought
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Comments on 'Modeling Galaxy Halos Using Dark Matter with Pressure'
We comment on the calculational mistake in the paper 'Modeling galaxy halos using dark matter with pressure' by Somnath Bharadwaj and Sayan Kar. The authors made a mistake while calculating the metric, which led to an overestimate of the deflection angle of light passing through the halos for -1 < w{sub r} < -0.5 and an underestimate of the deflection angle for -0.5 < w{sub r} < 0. In addition, the solution for w{sub r} > 0 should not exist. Although the Bharadwaj-Kar solution should be corrected, it appears that the characteristics of the deflection angle under the supposed nonconventional, nonideal fluid equation of state for the dark matter halo remain sensitive to the impact parameter and may be verifiable through observations
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