7,437 research outputs found

### Interpreting doubly special relativity as a modified theory of measurement

In this article we develop a physical interpretation for the deformed
(doubly) special relativity theories (DSRs), based on a modification of the
theory of measurement in special relativity. We suggest that it is useful to
regard the DSRs as reflecting the manner in which quantum gravity effects
induce Planck-suppressed distortions in the measurement of the "true" energy
and momentum. This interpretation provides a framework for the DSRs that is
manifestly consistent, non-trivial, and in principle falsifiable. However, it
does so at the cost of demoting such theories from the level of "fundamental"
physics to the level of phenomenological models -- models that should in
principle be derivable from whatever theory of quantum gravity one ultimately
chooses to adopt.Comment: 18 pages, plain LaTeX2

### Growing Perfect Decagonal Quasicrystals by Local Rules

A local growth algorithm for a decagonal quasicrystal is presented. We show
that a perfect Penrose tiling (PPT) layer can be grown on a decapod tiling
layer by a three dimensional (3D) local rule growth. Once a PPT layer begins to
form on the upper layer, successive 2D PPT layers can be added on top resulting
in a perfect decagonal quasicrystalline structure in bulk with a point defect
only on the bottom surface layer. Our growth rule shows that an ideal
quasicrystal structure can be constructed by a local growth algorithm in 3D,
contrary to the necessity of non-local information for a 2D PPT growth.Comment: 4pages, 2figure

### Entropy of gravitationally collapsing matter in FRW universe models

We look at a gas of dust and investigate how its entropy evolves with time
under a spherically symmetric gravitational collapse. We treat the problem
perturbatively and find that the classical thermodynamic entropy does actually
increase to first order when one allows for gravitational potential energy to
be transferred to thermal energy during the collapse. Thus, in this situation
there is no need to resort to the introduction of an intrinsic gravitational
entropy in order to satisfy the second law of thermodynamics.Comment: 9 pages, 4 figures. Major changes from previous version. We consider
only thermodynamic entropy in this version. Published in PR

### The inequality between mass and angular momentum for axially symmetric black holes

In this essay I first discuss the physical relevance of the inequality $m\geq
\sqrt{|J|}$ for axially symmetric (non-stationary) black holes, where m is the
mass and J the angular momentum of the spacetime. Then, I present a proof of
this inequality for the case of one spinning black hole. The proof involves a
remarkable characterization of the extreme Kerr black hole as an absolute
minimum of the total mass. Finally, I conjecture on the physical implications
of this characterization for the non linear stability problem for black holes.Comment: 8 pages, Honorable Mention in the Gravity Research Foundation Essay
Competition 200

### Spacetime structure of static solutions in Gauss-Bonnet gravity: charged case

We have studied spacetime structures of static solutions in the
$n$-dimensional Einstein-Gauss-Bonnet-Maxwell-$\Lambda$ system. Especially we
focus on effects of the Maxwell charge. We assume that the Gauss-Bonnet
coefficient $\alpha$ is non-negative and $4{\tilde \alpha}/\ell^2\leq 1$ in
order to define the relevant vacuum state. Solutions have the
$(n-2)$-dimensional Euclidean sub-manifold whose curvature is $k=1,~0$, or -1.
In Gauss-Bonnet gravity, solutions are classified into plus and minus branches.
In the plus branch all solutions have the same asymptotic structure as those in
general relativity with a negative cosmological constant. The charge affects a
central region of the spacetime. A branch singularity appears at the finite
radius $r=r_b>0$ for any mass parameter. There the Kretschmann invariant
behaves as $O((r-r_b)^{-3})$, which is much milder than divergent behavior of
the central singularity in general relativity $O(r^{-4(n-2)})$. Some charged
black hole solutions have no inner horizon in Gauss-Bonnet gravity. Although
there is a maximum mass for black hole solutions in the plus branch for $k=-1$
in the neutral case, no such maximum exists in the charged case. The solutions
in the plus branch with $k=-1$ and $n\geq6$ have an "inner" black hole, and
inner and the "outer" black hole horizons. Considering the evolution of black
holes, we briefly discuss a classical discontinuous transition from one black
hole spacetime to another.Comment: 20 pages, 10 figure

### Energy Estimates and Gravitational Collapse

We study the cancelations in the energy estimates for Einstein vacuum
equations in order to prove the formation of black holes along evolutions. The
novelty of the paper is that, we completely avoid using rotation vector fields
to establish the global existence theorem of the solution. More precisely, we
use only canonical null directions as commutators to derive energy estimates at
the level of one derivatives of null curvature components.Comment: 2 figures, 40 page

### On the exact evaluation of spin networks

We introduce a fully coherent spin network amplitude whose expansion
generates all SU(2) spin networks associated with a given graph. We then give
an explicit evaluation of this amplitude for an arbitrary graph. We show how
this coherent amplitude can be obtained from the specialization of a generating
functional obtained by the contraction of parametrized intertwiners a la
Schwinger. We finally give the explicit evaluation of this generating
functional for arbitrary graphs

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