4,541 research outputs found
Quantum Non-Gravity and Stellar Collapse
Observational indications combined with analyses of analogue and emergent
gravity in condensed matter systems support the possibility that there might be
two distinct energy scales related to quantum gravity: the scale that sets the
onset of quantum gravitational effects (related to the Planck scale) and
the much higher scale signalling the breaking of Lorentz symmetry. We
suggest a natural interpretation for these two scales: is the energy
scale below which a special relativistic spacetime emerges, is the scale
below which this spacetime geometry becomes curved. This implies that the first
`quantum' gravitational effect around could simply be that gravity is
progressively switched off, leaving an effective Minkowski quantum field theory
up to much higher energies of the order of . This scenario may have
important consequences for gravitational collapse, inasmuch as it opens up new
possibilities for the final state of stellar collapse other than an evaporating
black hole.Comment: 6 pages, 2 figures. v2: Partially restructured; potentially
observable consequence added. Several clarifications + 3 new references. To
appear in Found. of Phy
Quasi-normal mode analysis in BEC acoustic black holes
We perform a quasi-normal mode analysis of black hole configurations in
Bose-Einstein condensates (BEC). In this analysis we use the full Bogoliubov
dispersion relation, not just the hydrodynamic or geometric approximation. We
restrict our attention to one-dimensional flows in BEC with step-like
discontinuities. For this case we show that in the hydrodynamic approximation
quasi-normal modes do not exist. The full dispersion relation, however, allows
the existence of quasi-normal modes. Remarkably, the spectrum of these modes is
not discrete but continuous.Comment: 7 pages, 3 figure
Quantum evolution according to real clocks
We characterize good clocks, which are naturally subject to fluctuations, in
statistical terms. We also obtain the master equation that governs the
evolution of quantum systems according to these clocks and find its general
solution. This master equation is diffusive and produces loss of coherence.
Moreover, real clocks can be described in terms of effective interactions that
are nonlocal in time. Alternatively, they can be modeled by an effective
thermal bath coupled to the system.Comment: RevTeX 3.01, 6 page
Assessing molecular outflows and turbulence in the protostellar cluster Serpens South
Molecular outflows driven by protostellar cluster members likely impact their
surroundings and contribute to turbulence, affecting subsequent star formation.
The very young Serpens South cluster consists of a particularly high density
and fraction of protostars, yielding a relevant case study for protostellar
outflows and their impact on the cluster environment. We combined CO
observations of this region using the Combined Array for Research in
Millimeter-wave Astronomy (CARMA) and the Institut de Radioastronomie
Millim\'{e}trique (IRAM) 30 m single dish telescope. The combined map allows us
to probe CO outflows within the central, most active region at size scales of
0.01 pc to 0.8 pc. We account for effects of line opacity and excitation
temperature variations by incorporating CO and CO data for the
and transitions (using Atacama Pathfinder Experiment and
Caltech Submillimeter Observatory observations for the higher CO transitions),
and we calculate mass, momentum, and energy of the molecular outflows in this
region. The outflow mass loss rate, force, and luminosity, compared with
diagnostics of turbulence and gravity, suggest that outflows drive a sufficient
amount of energy to sustain turbulence, but not enough energy to substantially
counter the gravitational potential energy and disrupt the clump. Further, we
compare Serpens South with the slightly more evolved cluster NGC 1333, and we
propose an empirical scenario for outflow-cluster interaction at different
evolutionary stages.Comment: 26 pages, 15 figures, accepted for publication in the Astrophysical
Journa
Testing Relativity at High Energies Using Spaceborne Detectors
(ABRIDGED) The Gamma-ray Large Area Space Telescope (GLAST) will measure the
spectra of distant extragalactic sources of high energy gamma-rays. GLAST can
look for energy dependent propagation effects from such sources as a signal of
Lorentz invariance violation (LIV). Such sources should also exhibit high
energy spectral cutoffs from pair production interactions with low energy
photons. The properties of such cutoffs can also be used to test LIV. Detectors
to measure gamma-ray polarization can look for the depolarizing effect of
space-time birefingence predicted by loop quantum gravity. A spaceborne
detector array looking down on Earth to study extensive air showers produced by
ultrahigh energy cosmic rays can study their spectral properties and look for a
possible deviation from the predicted GZK effect as another signal of LIV.Comment: 14 pages, Text of invitated talk presented at the "From Quantum to
Cosmos: Fundamental Physics Studies from Space" meeting. More references
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The Circumstellar Structure and Excitation Effects around the Massive Protostar Cepheus A HW 2
We report SMA 335 GHz continuum observations with angular resolution of
~0.''3, together with VLA ammonia observations with ~1'' resolution toward Cep
A HW 2. We find that the flattened disk structure of the dust emission observed
by Patel et al. is preserved at the 0.''3 scale, showing an elongated structure
of ~$0.''6 size (450 AU) peaking on HW 2. In addition, two ammonia cores are
observed, one associated with a hot-core previously reported, and an elongated
core with a double peak separated by ~1.''3 and with signs of heating at the
inner edges of the gas facing HW 2. The double-peaked ammonia structure, as
well as the double-peaked CH3CN structure reported previously (and proposed to
be two independent hot-cores), surround both the dust emission as well as the
double-peaked SO2 disk structure found by Jimenez-Serra et al. All these
results argue against the interpretation of the elongated dust-gas structure as
due to a chance-superposition of different cores; instead, they imply that it
is physically related to the central massive object within a disk-protostar-jet
system.Comment: 12 pages, 3 figures; accepted for publication in the Astrophysical
Journa
Inhomogeneous Loop Quantum Cosmology: Hybrid Quantization of the Gowdy Model
The Gowdy cosmologies provide a suitable arena to further develop Loop
Quantum Cosmology, allowing the presence of inhomogeneities. For the particular
case of Gowdy spacetimes with the spatial topology of a three-torus and a
content of linearly polarized gravitational waves, we detail a hybrid quantum
theory in which we combine a loop quantization of the degrees of freedom that
parametrize the subfamily of homogeneous solutions, which represent Bianchi I
spacetimes, and a Fock quantization of the inhomogeneities. Two different
theories are constructed and compared, corresponding to two different schemes
for the quantization of the Bianchi I model within the {\sl improved dynamics}
formalism of Loop Quantum Cosmology. One of these schemes has been recently put
forward by Ashtekar and Wilson-Ewing. We address several issues including the
quantum resolution of the cosmological singularity, the structure of the
superselection sectors in the quantum system, or the construction of the
Hilbert space of physical states.Comment: 16 pages, version accepted for publication in Physical Review
Stability analysis of sonic horizons in Bose-Einstein condensates
We examine the linear stability of various configurations in Bose-Einstein
condensates with sonic horizons. These configurations are chosen in analogy
with gravitational systems with a black hole horizon, a white hole horizon and
a combination of both. We discuss the role of different boundary conditions in
this stability analysis, paying special attention to their meaning in
gravitational terms. We highlight that the stability of a given configuration,
not only depends on its specific geometry, but especially on these boundary
conditions. Under boundary conditions directly extrapolated from those in
standard General Relativity, black hole configurations, white hole
configurations and the combination of both into a black hole--white hole
configuration are shown to be stable. However, we show that under other (less
stringent) boundary conditions, configurations with a single black hole horizon
remain stable, whereas white hole and black hole--white hole configurations
develop instabilities associated to the presence of the sonic horizons.Comment: 14 pages, 7 figures (reduced resolution
Sensitivity of Hawking radiation to superluminal dispersion relations
We analyze the Hawking radiation process due to collapsing configurations in
the presence of superluminal modifications of the dispersion relation. With
such superluminal dispersion relations, the horizon effectively becomes a
frequency-dependent concept. In particular, at every moment of the collapse,
there is a critical frequency above which no horizon is experienced. We show
that, as a consequence, the late-time radiation suffers strong modifications,
both quantitative and qualitative, compared to the standard Hawking picture.
Concretely, we show that the radiation spectrum becomes dependent on the
measuring time, on the surface gravities associated with different frequencies,
and on the critical frequency. Even if the critical frequency is well above the
Planck scale, important modifications still show up.Comment: 14 pages, 7 figures. Extensive paragraph added in conclusions to
clarify obtained result
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