834 research outputs found
Wormholes in spacetimes with cosmological horizons
A generalisation of the asymptotic wormhole boundary condition for the case
of spacetimes with a cosmological horizon is proposed. In particular, we
consider de Sitter spacetime with small cosmological constant. The wave
functions selected by this proposal are exponentially damped in WKB
approximation when the scale factor is large but still much smaller than the
horizon size. In addition, they only include outgoing gravitational modes in
the region beyond the horizon. We argue that these wave functions represent
quantum wormholes and compute the local effective interactions induced by them
in low-energy field theory. These effective interactions differ from those for
flat spacetime in terms that explicitly depend on the cosmological constant.Comment: 10 pages, LaTeX 2.O9, no figure
Hawking-like radiation does not require a trapped region
We discuss the issue of quasi-particle production by ``analogue black holes''
with particular attention to the possibility of reproducing Hawking radiation
in a laboratory. By constructing simple geometric acoustic models, we obtain a
somewhat unexpected result: We show that in order to obtain a stationary and
Planckian emission of quasi-particles, it is not necessary to create a trapped
region in the acoustic spacetime (corresponding to a supersonic regime in the
fluid flow). It is sufficient to set up a dynamically changing flow
asymptotically approaching a sonic regime with sufficient rapidity in
laboratory time.Comment: revtex4, 4 pages, 1 figur
Modelling gravity on a hyper-cubic lattice
We present an elegant and simple dynamical model of symmetric, non-degenerate
(n x n) matrices of fixed signature defined on a n-dimensional hyper-cubic
lattice with nearest-neighbor interactions. We show how this model is related
to General Relativity, and discuss multiple ways in which it can be useful for
studying gravity, both classical and quantum. In particular, we show that the
dynamics of the model when all matrices are close to the identity corresponds
exactly to a finite-difference discretization of weak-field gravity in harmonic
gauge. We also show that the action which defines the full dynamics of the
model corresponds to the Einstein-Hilbert action to leading order in the
lattice spacing, and use this observation to define a lattice analogue of the
Ricci scalar and Einstein tensor. Finally, we perform a mean-field analysis of
the statistical mechanics of this model.Comment: 5 page
Quasi-particle creation by analogue black holes
We discuss the issue of quasi-particle production by ``analogue black holes''
with particular attention to the possibility of reproducing Hawking radiation
in a laboratory. By constructing simple geometric acoustic models, we obtain a
somewhat unexpected result: We show that in order to obtain a stationary and
Planckian emission of quasi-particles, it is not necessary to create an
ergoregion in the acoustic spacetime (corresponding to a supersonic regime in
the flow). It is sufficient to set up a dynamically changing flow either
eventually generating an arbitrarily small sonic region v=c, but without any
ergoregion, or even just asymptotically, in laboratory time, approaching a
sonic regime with sufficient rapidity.Comment: 30 pages, 16 figure
Anàlisi dels aspectes socials de la caça a Mallorca: distribució territorial, anàlisi per edats i gèneres, tipologies de llicències i comparatives
[cat] L’activitat cinegètica constitueix un dels principals moviments que vertebren el medi rural de les Illes Balears. A partir de la informació facilitada pel Servei de Caça del Consell de Mallorca s’avalua el ressò social del col·lectiu venatori, indicador bàsic per determinar qüestions relaciones amb el potencial de gestió dels espais cinegètics o per avaluar l’impacte econòmic de l’activitat, entre d’altres. S'analitza la distribució, l'edat, el gènere i les modalitats que practiquen les persones titulars de llicències de caça a Mallorca. En un marc socioeconòmic hipertrofitzat pel sector terciari i territorialment limitat, s'observa com la caça persisteix amb força a la majoria de municipis amb emergència de les pràctiques tradicionals i el turisme cinegètic. La manca de relleu generacional, els efectes de la crisi econòmica i la introgressió d’usos urbans són alguns dels factors limitants. Així mateix, la variable gènere marca diferències essencials entre la caça i altres activitats de lleure atesa l’escassa presència de la dona.[eng] The hunting activity is one of the main movements that structure the rural environment of the Balearic Islands. From the information provided by the Hunting Service of the Mallorcan Council the following are assessed among others: the social impact of the hunting collective, basic indicator to determine potential issues related to the management of hunting areas or to evaluate the economic impact of the activity. It analyzes the distribution, age, gender and types of people who practice hunting license holders of Mallorca. A geographically limited framework with a socioeconomical environment based on the tertiary sector, shows how hunting strongly persists in most municipalities with emergence of practices traditional hunting and cynegetic tourism. The lack of generational change, the effects of the economic crisis and the introgression of urban uses are some of the limiting factors. Also, gender marks an essential difference between hunting and other recreational activities given the limited presence of women
Acoustic geometry for general relativistic barotropic irrotational fluid flow
"Acoustic spacetimes", in which techniques of differential geometry are used
to investigate sound propagation in moving fluids, have attracted considerable
attention over the last few decades. Most of the models currently considered in
the literature are based on non-relativistic barotropic irrotational fluids,
defined in a flat Newtonian background. The extension, first to special
relativistic barotropic fluid flow, and then to general relativistic barotropic
fluid flow in an arbitrary background, is less straightforward than it might at
first appear. In this article we provide a pedagogical and simple derivation of
the general relativistic "acoustic spacetime" in an arbitrary (d+1) dimensional
curved-space background.Comment: V1: 23 pages, zero figures; V2: now 24 pages, some clarifications, 2
references added. This version accepted for publication in the New Journal of
Physics. (Special issue on "Classical and Quantum Analogues for Gravitational
Phenomena and Related Effects"
Twilight for the energy conditions?
The tension, if not outright inconsistency, between quantum physics and
general relativity is one of the great problems facing physics at the turn of
the millennium. Most often, the problems arising in merging Einstein gravity
and quantum physics are viewed as Planck scale issues (10^{19} GeV, 10^{-34} m,
10^{-45} s), and so safely beyond the reach of experiment. However, over the
last few years it has become increasingly obvious that the difficulties are
more widespread: There are already serious problems of deep and fundamental
principle at the semi-classical level, and worse, certain classical systems
(inspired by quantum physics, but in no sense quantum themselves) exhibit
seriously pathological behaviour. One manifestation of these pathologies is in
the so-called ``energy conditions'' of general relativity. Patching things up
in the gravity sector opens gaping holes elsewhere; and some ``fixes'' are more
radical than the problems they are supposed to cure.Comment: Honourable mention in the 2002 Gravity Research Foundation essay
contest. 12 pages. Plain LaTeX 2
Black hole radiance, short distances, and TeV gravity
Using a derivation of black hole radiance in terms of two-point functions one
can provide a quantitative estimate of the contribution of short distances to
the spectrum. Thermality is preserved for black holes with .
However, deviations from the Planckian spectrum can be found for mini black
holes in TeV gravity scenarios, even before reaching the Planck phase.Comment: LaTeX, 4 pages, 1 figure. Misprints correcte
Hawking Radiation from an Acoustic Black Hole on an Ion Ring
In this article we propose to simulate acoustic black holes with ions in
rings. If the ions are rotating with a stationary and inhomogeneous velocity
profile, regions can appear where the ion velocity exceeds the group velocity
of the phonons. In these regions phonons are trapped like light in black holes,
even though we have a discrete field theory and a nonlinear dispersion
relation. We study the appearance of Hawking radiation in this setup and
propose a scheme to detect it.Comment: 4 pages, 3 figures, minor revisions, published versio
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