46 research outputs found

### Polytropic stars in bootstrapped Newtonian gravity

We study self-gravitating stars in the bootstrapped Newtonian picture for
polytropic equations of state. We consider stars that span a wide range of
compactness values. Both matter density and pressure are sources of the
gravitational potential. Numerical solutions show that the density profiles can
be well approximated by Gaussian functions. Later we assume Gaussian density
profiles to investigate the interplay between the compactness of the source,
the width of the Gaussian density profile and the polytropic index. We also
dedicate a section to comparing the pressure and density profiles of the
bootstrapped Newtonian stars to the corresponding General Relativistic
solutions. We also point out that no Buchdahl limit is found, which means that
the pressure can in principle support a star of arbitrarily large compactness.
In fact, we find solutions representing polytropic stars with compactness above
the Buchdhal limit.Comment: 21 pages, 10 figure

### Horizon Quantum Mechanics of collapsing shells

We study the probability that a horizon appears when concentric shells of
matter collide, by computing the horizon wave-function of the system. We mostly
consider the collision of two ultra-relativistic shells, both shrinking and
expanding, at the moment their radii are equal, and find a probability that the
system is a black hole which is in qualitative agreement with what one would
expect according to the hoop conjecture and the uncertainty principle of
quantum physics, and parallels the results obtained for simpler sources. One
new feature however emerges, in that this probability shows a modulation with
the momenta of the shells and the radius at which the shells collide, as a
manifestation of quantum mechanical interference. Finally, we also consider the
case of one light shell collapsing into a larger central mass.Comment: 21 pages, 11 figure

### Bootstrapped Newtonian stars and black holes

We study equilibrium configurations of a homogenous ball of matter in a
bootstrapped description of gravity which includes a gravitational
self-interaction term beyond the Newtonian coupling. Both matter density and
pressure are accounted for as sources of the gravitational potential for test
particles. Unlike the general relativistic case, no Buchdahl limit is found and
the pressure can in principle support a star of arbitrarily large compactness.
By defining the horizon as the location where the escape velocity of test
particles equals the speed of light, like in Newtonian gravity, we find a
minimum value of the compactness for which this occurs. The solutions for the
gravitational potential here found could effectively describe the interior of
macroscopic black holes in the quantum theory, as well as predict consequent
deviations from general relativity in the strong field regime of very compact
objects.Comment: 27 pages, 17 figures. Version accepted for publication in EPJ

### Inner Horizon of the Quantum Reissner-Nordstr\"om Black Holes

We study the nature of the inner Cauchy horizon of a Reissner-Nordstr\"om
black hole in a quantum context by means of the horizon wave-function obtained
from modelling the electrically charged source as a Gaussian wave-function. Our
main finding it that there is a significant range of black hole mass (around
the Planck scale) and specific charge for which the probability of realizing
the inner horizon is negligible. This result suggests that any semiclassical
instability one expects near the inner horizon may not be occur in quantum
black holes.Comment: RevTeX4, 7 pages, 4 figures: new section about HWF added for clarity,
references updated, results unchanged. Version to appear in JHE

### Horizon Wave-Function and the Quantum Cosmic Censorship

We investigate the Cosmic Censorship Conjecture by means of the horizon
wave-function (HWF) formalism. We consider a charged massive particle whose
quantum mechanical state is represented by a spherically symmetric Gaussian
wave-function, and restrict our attention to the superxtremal case (with
charge-to-mass ratio $\alpha>1$), which is the prototype of a naked singularity
in the classical theory. We find that one can still obtain a normalisable HWF
for $\alpha^2<{2}$, and this configuration has a non-vanishing probability of
being a black hole, thus extending the classically allowed region for a charged
black hole. However, the HWF is not normalisable for $\alpha^2 > 2$, and the
uncertainty in the location of the horizon blows up at $\alpha^2=2$, signalling
that such an object is no more well-defined. This perhaps implies that a
quantum Cosmic Censorhip might be conjectured by stating that no black holes
with charge-to-mass ratio greater than a critical value (of the order of
$\sqrt{2}$) can exist.Comment: RevTeX4, 6 pages, 2 figures. Typos corrected and comments added.
Version to appear in PL

### Neutrino-antineutrino oscillations as a possible solution for the LSND and MiniBooNE anomalies?

We investigate resonance structures in CPT and Lorentz symmetry violating
neutrino-antineutrino oscillations in a two generation framework. We work with
four non-zero CPT-violating parameters that allow for resonant enhancements in
neutrino-antineutrino oscillation phenomena in vacuo which are suitably
described in terms of charge conjugation eigenstates of the system. We study
the relation between the flavor, charge conjugation and mass eigenbasis of
neutrino-antineutrino oscillations and examine the interplay between the
available CPT-violating parameter space and possible resonance structures.
Eventually we remark on the consequences of such scenarios for neutrino
oscillation experiments, namely possible solutions for the LSND and MiniBooNE
anomalies.Comment: 14 pages, 3 figure

### Explaining LSND and MiniBooNE using altered neutrino dispersion relations

We investigate the possibility to explain the MiniBooNE anomaly by CPT and
Lorentz symmetry violating neutrino-antineutrino oscillations in a two
generation framework. We work with four non-zero CPT-violating parameters that
allow for resonant enhancements in neutrino-antineutrino oscillation phenomena
in vacuo which are suitably described in terms of charge conjugation
eigenstates of the system. We study the relation between the flavor, charge
conjugation and mass eigenbasis of neutrino-antineutrino oscillations and
examine the interplay between the available CPT-violating parameter space and
possible resonance structures.Comment: 3 pages, 1 figure, Proceedings for Erice 2009 Neutrinos in Cosmology,
in Astro-, Particle- and Nuclear Physic

### Horizon Quantum Mechanics of Rotating Black Holes

The Horizon Quantum Mechanics is an approach that was previously introduced
in order to analyse the gravitational radius of spherically symmetric systems
and compute the probability that a given quantum state is a black hole. In this
work, we first extend the formalism to general space-times with asymptotic
(ADM) mass and angular momentum. We then apply the extended Horizon Quantum
Mechanics to a harmonic model of rotating corpuscular black holes. We find that
simple configurations of this model naturally suppress the appearance of the
inner horizon and seem to disfavour extremal (macroscopic) geometries.Comment: 22 pages, 6 figures. Final version to appear in EPJ

### Boundaries and the Casimir effect in non-commutative space-time

We calculate modifications to the scalar Casimir force between two parallel
plates due to space-time non-commutativity. We devise a heuristic approach to
overcome the difficulties of describing boundaries in non-commutative theories
and predict that boundary corrections are of the same order as non-commutative
volume corrections. Further, both corrections have the form of more
conventional finite surface effects.Comment: 9 pages, 2 figure

### Black holes as self-sustained quantum states, and Hawking radiation

We employ the recently proposed formalism of the "horizon wave-function" to
investigate the emergence of a horizon in models of black holes as
Bose-Einstein condensates of gravitons. We start from the Klein-Gordon equation
for a massless scalar (toy graviton) field coupled to a static matter current.
The (spherically symmetric) classical field reproduces the Newtonian potential
generated by the matter source, and the corresponding quantum state is given by
a coherent superposition of scalar modes with continuous occupation number.
Assuming an attractive self-interaction that allows for bound states, one finds
that (approximately) only one mode is allowed, and the system can be confined
in a region of the size of the Schwarzschild radius. This radius is then shown
to correspond to a proper horizon, by means of the horizon wave-function of the
quantum system, with an uncertainty in size naturally related to the expected
typical energy of Hawking modes. In particular, this uncertainty decreases for
larger black hole mass (with larger number of light scalar quanta), in
agreement with semiclassical expectations, a result which does not hold for a
single very massive particle. We finally speculate that a phase transition
should occur during the gravitational collapse of a star, ideally represented
by a static matter current and Newtonian potential, that leads to a black hole,
again ideally represented by the condensate of toy gravitons, and suggest an
effective order parameter that could be used to investigate this transition.Comment: 25 pages, 6 figures. Improved text and typos fixed. Final version to
appear in PR