3,202 research outputs found
Spontaneous violation of the energy conditions
A decade ago, it was shown that a wide class of scalar-tensor theories can
pass very restrictive weak field tests of gravity and yet exhibit
non-perturbative strong field deviations away from General Relativity. This
phenomenon was called `Spontaneous Scalarization' and causes the (Einstein
frame) scalar field inside a neutron star to rapidly become inhomogeneous once
the star's mass increases above some critical value. For a star whose mass is
below the threshold, the field is instead nearly uniform (a state which
minimises the star's energy) and the configuration is similar to the General
Relativity one. Here, we show that the spontaneous scalarization phenomenon is
linked to another strong field effect: a spontaneous violation of the weak
energy condition.Comment: 10 pages, 1 figure, accepted for publication in The Astrophysical
Journal Letter
Brans-Dicke wormholes in nonvacuum spacetime
Analytical wormhole solutions in Brans-Dicke theory in the presence of matter
are presented. It is shown that the wormhole throat must not be necessarily
threaded with exotic matter.Comment: Minor corrections, to be published in Phys. Rev.
Radiation from a uniformly accelerated charge in the outskirts of a wormhole throat
Using traversable wormholes as theoretical background, we revisit a deep
question of general relativity: Does a uniformly accelerated charged particle
radiate? We particularize to the recently proposed gravitational \v{C}erenkov
radiation, that happens when the spatial part of the Ricci tensor is negative.
If , the matter threading the gravitational
field violates the weak energy condition. In this case, the effective
refractive index for light is bigger than 1, i.e. particles propagates, in that
medium, faster than photons. This leads to a violation of the equivalence
principle.Comment: 6 pages revtex, 1 eps figure. To be published in Modern Physics
Letters
An inquiry into the nature of the gamma-ray source 3EG J1828+0142
The unidentified, low-latitude, -ray source 3EG J1828+0142 presents
high levels of variability and a steep spectral index . Here we
propose a model for this source where the high-energy emission is produced by a
galactic Kerr-Newman black hole. The model takes into account electron-positron
annihilation losses in the calculation of the expected spectral energy
distribution and can reproduce the observational features, including the
absence of a strong radio counterpart. We also report the discovery of a nearby
supernova remnant that could be associated with the original supernova
explosion that created the black hole. Several faint radio sources were also
detected in the radio field within the inner -ray confidence contour
and their spectral index estimated. Some of these sources could be the expected
weak radio counterpart.Comment: 11 pages, 2 figure
Variable gamma-ray emission from the Be/X-ray transient A0535+26?
We present a study of the unidentified gamma-ray source 3EG J0542+2610. This
source is spatially superposed to the supernova remnant G180.0-1.7, but its
time variability makes unlikely a physical link. We have searched into the
EGRET location error box for compact radio sources that could be the low energy
counterpart of the gamma-ray source. Although 29 point-like radio sources were
detected and measured, none of them is strong enough as to be considered the
counterpart of a background gamma-ray emitting AGN. We suggest that the only
object within the 95 % error box capable of producing the required gamma-ray
flux is the X-ray transient A0535+26. We show that this Be/accreting pulsar can
produce variable hadronic gamma-ray emission through the mechanism originally
proposed by Cheng & Ruderman (1989), where a proton beam accelerated in a
magnetospheric electrostatic gap impacts the transient accretion disk.Comment: 6 pages, 3 figures, Astronomy & Astrophysics in pres
Gamma Ray Bursts with peculiar temporal asymmetry
Based on the study of temporal asymmetry of 631 gamma ray bursts from the
BATSE 3B catalog by Link and Epstein [Ap J 466, 764 (1996)], we identify the
population of bursts whose rising times are longer than their decays, thus
showing atypical profiles. We analyse their sky distribution, morphology,
time-space clustering and other average properties and compare them with those
associated with the bulk of the bursts. We show how most of the peculiar bursts
analysed are consistent with recent fireball models, but a fraction of bursts
(% of the total sample) appear to be inconsistent.Comment: mn style (included in the submission), 4 figures that must be printed
separately. Submitted to Monthly Notices of RA
Adiabatic quantum metrology with strongly correlated quantum optical systems
We show that the quasiadiabatic evolution of a system governed by the Dicke Hamiltonian can be described in terms of a self-induced quantum many-body metrological protocol. This effect relies on the sensitivity of the ground state to a small symmetry-breaking perturbation at the quantum phase transition, which leads to the collapse of the wave function into one of two possible ground states. The scaling of the final-state properties with the number of atoms and with the intensity of the symmetry-breaking field can be interpreted in terms of the precession time of an effective quantum metrological protocol. We show that our ideas can be tested with spin-phonon interactions in trapped ion setups. Our work points to a classification of quantum phase transitions in terms of the capability of many-body quantum systems for parameter estimation
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