434 research outputs found
Rainbow universe
The formalism of rainbow gravity is studied in a cosmological setting. We
consider the very early universe which is radiation dominated. A novel
treatment in our paper is to look for an ``averaged'' cosmological metric
probed by radiation particles themselves. Taking their cosmological evolution
into account, we derive the modified Friedmann-Robertson-Walker(FRW) equations
which is a generalization of the solution presented by Magueijo and Smolin.
Based on this phenomenological cosmological model we argue that the spacetime
curvature has an upper bound such that the cosmological singularity is absent.
These modified equations can be treated as effective equations in the
semi-classical framework of quantum gravity and its analogy with the one
recently proposed in loop quantum cosmology is also discussed.Comment: 5 page
Holographic Formulation of Quantum Supergravity
We show that supergravity with a cosmological constant can be
expressed as constrained topological field theory based on the supergroup
. The theory is then extended to include timelike boundaries with
finite spatial area. Consistent boundary conditions are found which induce a
boundary theory based on a supersymmetric Chern-Simons theory. The boundary
state space is constructed from states of the boundary supersymmetric
Chern-Simons theory on the punctured two sphere and naturally satisfies the
Bekenstein bound, where area is measured by the area operator of quantum
supergravity.Comment: 30 pages, no figur
Disordered locality in loop quantum gravity states
We show that loop quantum gravity suffers from a potential problem with
non-locality, coming from a mismatch between micro-locality, as defined by the
combinatorial structures of their microscopic states, and macro-locality,
defined by the metric which emerges from the low energy limit. As a result, the
low energy limit may suffer from a disordered locality characterized by
identifications of far away points. We argue that if such defects in locality
are rare enough they will be difficult to detect.Comment: 11 pages, 4 figures, revision with extended discussion of result
Hidden symmetries for thermodynamics and emergence of relativity
Erik Verlinde recently proposed an idea about the thermodynamic origin of
gravity. Though this is a beautiful idea which may resolve many long standing
problems in the theories of gravity, it also raises many other problems. In
this article I will comment on some of the problems of Verlinde's proposal with
special emphasis on the thermodynamical origin of the principle of relativity.
It is found that there is a large group of hidden symmetries of thermodynamics
which contains the Poincare group of the spacetime for which space is emergent.
This explains the thermodynamic origin of the principle of relativity.Comment: V1: 4 pages, comments/criticisms welcomed; V2: references added; V3:
typos and minor corrections? V4? substantial changes in Section 3 and other
parts mad
Mixture of multiple copies of maximally entangled states is quasi-pure
Employing the general BXOR operation and local state discrimination, the
mixed state of the form
\rho^{(k)}_{d}=\frac{1}{d^{2}}\sum_{m,n=0}^{d-1}(|\phi_{mn}><\phi_{mn}|)^{\otim
es k} is proved to be quasi-pure, where is the canonical set
of mutually orthogonal maximally entangled states in . Therefore
irreversibility does not occur in the process of distillation for this family
of states. Also, the distillable entanglement is calculated explicitly.Comment: 6 pages, 1 figure. The paper is subtantially revised and the general
proof is give
2-Form Gravity of the Lorentzian Signature
We introduce a new spinorial, BF-like action for the Einstein gravity. This
is a first, up to our knowledge, 2-form action which describes the real,
Lorentzian gravity and uses only the self-dual connection. In the generic case,
the corresponding classical canonical theory is equivalent to the
Einstein-Ashtekar theory plus the reality conditions
Infinite Degeneracy of States in Quantum Gravity
The setting of Braided Ribbon Networks is used to present a general result in
spin-networks embedded in manifolds: the existence of an infinite number of
species of conserved quantities. Restricted to three-valent networks the number
of such conserved quantities in a given network is shown to be invariant
barring a single case. The implication of these conserved quantities is
discussed in the context of Loop Quantum Gravity.Comment: 10 pages, 14 figures, v2: some clarifications, no substantial change
de Sitter gravity from lattice gauge theory
We investigate a lattice model for Euclidean quantum gravity based on
discretization of the Palatini formulation of General Relativity. Using Monte
Carlo simulation we show that while a naive approach fails to lead to a vacuum
state consistent with the emergence of classical spacetime, this problem may be
evaded if the lattice action is supplemented by an appropriate counter term. In
this new model we find regions of the parameter space which admit a ground
state which can be interpreted as (Euclidean) de Sitter space.Comment: 16 pages, 11 figures. email address update
A Note on Temperature and Energy of 4-dimensional Black Holes from Entropic Force
We investigate the temperature and energy on holographic screens for
4-dimensional black holes with the entropic force idea proposed by Verlinde. We
find that the "Unruh-Verlinde temperature" is equal to the Hawking temperature
on the horizon and can be considered as a generalized Hawking temperature on
the holographic screen outside the horizons. The energy on the holographic
screen is not the black hole mass but the reduced mass , which is
related to the black hole parameters. With the replacement of the black hole
mass by the reduced mass , the entropic force can be written as
, which could be tested by experiments.Comment: V4: 13 pages, 4 figures, title changed, discussions for experiments
added, accepted by CQ
Implications of Spacetime Quantization for the Bahcall-Waxman Neutrino Bound
There is growing interest in quantum-spacetime models in which small
departures from Lorentz symmetry are governed by the Planck scale. In
particular, several studies have considered the possibility that these small
violations of Lorentz symmetry may affect various astrophysical observations,
such as the evaluation of the GZK limit for cosmic rays, the interaction of TeV
photons with the Far Infrared Background and the arrival time of photons with
different energies from cosmological sources. We show that the same
Planck-scale departures from Lorentz symmetry that lead to a modification of
the GZK limit which would be consistent with the observations reported by
AGASA, also have significant implications for the evaluation of the
Bahcall-Waxman bound on the flux of high-energy neutrinos produced by
photo-meson interactions in sources of size not much larger than the proton
photo-meson mean free path.Comment: 10 pages, Late
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