615 research outputs found
Self-completeness and spontaneous dimensional reduction
A viable quantum theory of gravity is one of the biggest challenges facing
physicists. We discuss the confluence of two highly expected features which
might be instrumental in the quest of a finite and renormalizable quantum
gravity -- spontaneous dimensional reduction and self-completeness. The former
suggests the spacetime background at the Planck scale may be effectively
two-dimensional, while the latter implies a condition of maximal compression of
matter by the formation of an event horizon for Planckian scattering. We
generalize such a result to an arbitrary number of dimensions, and show that
gravity in higher than four dimensions remains self-complete, but in lower
dimensions it is not. In such a way we established an "exclusive disjunction"
or "exclusive or" (XOR) between the occurrence of self-completeness and
dimensional reduction, with the goal of actually reducing the unknowns for the
scenario of the physics at the Planck scale. Potential phenomenological
implications of this result are considered by studying the case of a
two-dimensional dilaton gravity model resulting from dimensional reduction of
Einstein gravity.Comment: 12 pages, 3 figures; v3: final version in press on Eur. Phys. J. Plu
Effective Polymer Dynamics of D-Dimensional Black Hole Interiors
We consider two different effective polymerization schemes applied to
D-dimensional, spherically symmetric black hole interiors. It is shown that
polymerization of the generalized area variable alone leads to a complete,
regular, single-horizon spacetime in which the classical singularity is
replaced by a bounce. The bounce radius is independent of rescalings of the
homogeneous internal coordinate, but does depend on the arbitrary fiducial cell
size. The model is therefore necessarily incomplete. It nonetheless has many
interesting features: After the bounce, the interior region asymptotes to an
infinitely expanding Kantowski-Sachs spacetime. If the solution is analytically
continued across the horizon, the black hole exterior exhibits asymptotically
vanishing quantum-corrections due to the polymerization. In all spacetime
dimensions except four, the fall-off is too slow to guarantee invariance under
Poincare transformations in the exterior asymptotic region. Hence the
four-dimensional solution stands out as the only example which satisfies the
criteria for asymptotic flatness. In this case it is possible to calculate the
quantum-corrected temperature and entropy. We also show that polymerization of
both phase space variables, the area and the conformal mode of the metric,
generically leads to a multiple horizon solution which is reminiscent of
polymerized mini-superspace models of spherically symmetric black holes in Loop
Quantum Gravity.Comment: 14 pages, 4 figures. Added discussion about the dependency on
auxiliary structures. Matches with the published versio
The Hawking-Page crossover in noncommutative anti-deSitter space
We study the problem of a Schwarzschild-anti-deSitter black hole in a
noncommutative geometry framework, thought to be an effective description of
quantum-gravitational spacetime. As a first step we derive the noncommutative
geometry inspired Schwarzschild-anti-deSitter solution. After studying the
horizon structure, we find that the curvature singularity is smeared out by the
noncommutative fluctuations. On the thermodynamics side, we show that the black
hole temperature, instead of a divergent behavior at small scales, admits a
maximum value. This fact implies an extension of the Hawking-Page transition
into a van der Waals-like phase diagram, with a critical point at a critical
cosmological constant size in Plank units and a smooth crossover thereafter. We
speculate that, in the gauge-string dictionary, this corresponds to the
confinement "critical point" in number of colors at finite number of flavors, a
highly non-trivial parameter that can be determined through lattice
simulations.Comment: 24 pages, 6 figure, 1 table, version matching that published on JHE
Newtonian gravity as an entropic force: Towards a derivation of G
It has been suggested that the Newtonian gravitational force may emerge as an
entropic force from a holographic microscopic theory. In this framework, the
possibility is reconsidered that Newton's gravitational coupling constant G can
be derived from the fundamental constants of the underlying microscopic theory.Comment: 10 pages. v6: published versio
Sub-Planckian black holes and the Generalized Uncertainty Principle
The Black Hole Uncertainty Principle correspondence suggests that there could
exist black holes with mass beneath the Planck scale but radius of order the
Compton scale rather than Schwarzschild scale. We present a modified, self-dual
Schwarzschild-like metric that reproduces desirable aspects of a variety of
disparate models in the sub-Planckian limit, while remaining Schwarzschild in
the large mass limit. The self-dual nature of this solution under naturally implies a Generalized Uncertainty Principle
with the linear form . We also
demonstrate a natural dimensional reduction feature, in that the gravitational
radius and thermodynamics of sub-Planckian objects resemble that of -D
gravity. The temperature of sub-Planckian black holes scales as rather than
but the evaporation of those smaller than g is suppressed by
the cosmic background radiation. This suggests that relics of this mass could
provide the dark matter.Comment: 12 pages, 9 figures, version published in J. High En. Phy
Gravitational collapse in loop quantum gravity
In this paper we study the gravitational collapse in loop quantum gravity. We
consider the space-time region inside the Schwarzschild black hole event
horizon and we divide this region in two parts, the first one where the matter
(dust matter) is localized and the other (outside) where the metric is
Kantowski-Sachs type. We calculate the state solving Hamiltonian constraint and
we obtain a set of three difference equations that give a regular and natural
evolution beyond the classical singularity point in "r=0" localized.Comment: 16 pages, 2 figure
Spinning Loop Black Holes
In this paper we construct four Kerr-like spacetimes starting from the loop
black hole Schwarzschild solutions (LBH) and applying the Newman-Janis
transformation. In previous papers the Schwarzschild LBH was obtained replacing
the Ashtekar connection with holonomies on a particular graph in a
minisuperspace approximation which describes the black hole interior. Starting
from this solution, we use a Newman-Janis transformation and we specialize to
two different and natural complexifications inspired from the complexifications
of the Schwarzschild and Reissner-Nordstrom metrics. We show explicitly that
the space-times obtained in this way are singularity free and thus there are no
naked singularities. We show that the transformation move, if any, the
causality violating regions of the Kerr metric far from r=0. We study the
space-time structure with particular attention to the horizons shape. We
conclude the paper with a discussion on a regular Reissner-Nordstrom black hole
derived from the Schwarzschild LBH and then applying again the Newmann-Janis
transformation.Comment: 18 pages, 18 figure
Space-Time Structure of Loop Quantum Black Hole
In this paper we have improved the semiclassical analysis of loop quantum
black hole (LQBH) in the conservative approach of constant polymeric parameter.
In particular we have focused our attention on the space-time structure. We
have introduced a very simple modification of the spherically symmetric
Hamiltonian constraint in its holonomic version. The new quantum constraint
reduces to the classical constraint when the polymeric parameter goes to
zero.Using this modification we have obtained a large class of semiclassical
solutions parametrized by a generic function of the polymeric parameter. We
have found that only a particular choice of this function reproduces the black
hole solution with the correct asymptotic flat limit. In r=0 the semiclassical
metric is regular and the Kretschmann invariant has a maximum peaked in
L-Planck. The radial position of the pick does not depend on the black hole
mass and the polymeric parameter. The semiclassical solution is very similar to
the Reissner-Nordstrom metric. We have constructed the Carter-Penrose diagrams
explicitly, giving a causal description of the space-time and its maximal
extension. The LQBH metric interpolates between two asymptotically flat
regions, the r to infinity region and the r to 0 region. We have studied the
thermodynamics of the semiclassical solution. The temperature, entropy and the
evaporation process are regular and could be defined independently from the
polymeric parameter. We have studied the particular metric when the polymeric
parameter goes towards to zero. This metric is regular in r=0 and has only one
event horizon in r = 2m. The Kretschmann invariant maximum depends only on
L-Planck. The polymeric parameter does not play any role in the black hole
singularity resolution. The thermodynamics is the same.Comment: 17 pages, 19 figure
Background independence in a nutshell
We study how physical information can be extracted from a background
independent quantum system. We use an extremely simple `minimalist' system that
models a finite region of 3d euclidean quantum spacetime with a single
equilateral tetrahedron. We show that the physical information can be expressed
as a boundary amplitude. We illustrate how the notions of "evolution" in a
boundary proper-time and "vacuum" can be extracted from the background
independent dynamics.Comment: 19 pages, 19 figure
Entropic corrections to Newton's law
In this short letter we calculate separately the generalized uncertainty
principle (GUP) and self gravitational corrections to the Newton's
gravitational formula. We show that for a complete description of the GUP and
self-gravity effects, both temperature and the entropy must be modified.Comment: 4 pages, Accepted for publication in "Physica Scripta",Title changed,
Major revisio
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