145 research outputs found
Numerical analysis of backreaction in acoustic black holes
Using methods of Quantum Field Theory in curved spacetime, the first order in
hbar quantum corrections to the motion of a fluid in an acoustic black hole
configuration are numerically computed. These corrections arise from the non
linear backreaction of the emitted phonons. Time dependent (isolated system)
and equilibrium configurations (hole in a sonic cavity) are both analyzed.Comment: 7 pages, 5 figure
Backreaction in Acoustic Black Holes
The backreaction equations for the linearized quantum fluctuations in an
acoustic black hole are given. The solution near the horizon, obtained within a
dimensional reduction, indicates that acoustic black holes, unlike
Schwarzschild ones, get cooler as they radiate phonons. They show remarkable
analogies with near-extremal Reissner-Nordstrom black holes.Comment: 4 pages, revtex, 1 figure. revised version, published in pr
The depletion in Bose Einstein condensates using Quantum Field Theory in curved space
Using methods developed in Quantum Field Theory in curved space we can
estimate the effects of the inhomogeneities and of a non vanishing velocity on
the depletion of a Bose Einstein condensate within the hydrodynamical
approximation.Comment: 4 pages, no figure. Discussion extended and references adde
Semiclassical Gravity in the Far Field Limit of Stars, Black Holes, and Wormholes
Semiclassical gravity is investigated in a large class of asymptotically
flat, static, spherically symmetric spacetimes including those containing
static stars, black holes, and wormholes. Specifically the stress-energy
tensors of massless free spin 0 and spin 1/2 fields are computed to leading
order in the asymptotic regions of these spacetimes. This is done for spin 0
fields in Schwarzschild spacetime using a WKB approximation. It is done
numerically for the spin 1/2 field in Schwarzschild, extreme
Reissner-Nordstrom, and various wormhole spacetimes. And it is done by finding
analytic solutions to the leading order mode equations in a large class of
asymptotically flat static spherically symmetric spacetimes. Agreement is shown
between these various computational methods. It is found that for all of the
spacetimes considered, the energy density and pressure in the asymptotic region
are proportional to 1/r^5 to leading order. Furthermore, for the spin 1/2 field
and the conformally coupled scalar field, the stress-energy tensor depends only
on the leading order geometry in the far field limit. This is also true for the
minimally coupled scalar field for spacetimes containing either a static star
or a black hole, but not for spacetimes containing a wormhole.Comment: 43 pages, 2 figures. Reference added, minor changes, PRD versio
On the quantum stress tensor for extreme 2D Reissner-Nordstrom black holes
Contrary to previous claims, it is shown that the expectation values of the
quantum stress tensor for a massless scalar field propagating on a
two-dimensional extreme Reissner-Nordstrom black hole are indeed regular on the
horizon.Comment: 5 pages, revtex, 1 figur
Back-reaction effects in acoustic black holes
Acoustic black holes are very interesting non-gravitational objects which can
be described by the geometrical formalism of General Relativity. These models
can be useful to experimentally test effects otherwise undetectable, as for
example the Hawking radiation. The back-reaction effects on the background
quantities induced by the analogue Hawking radiation could be the key to
indirectly observe it. We briefly show how this analogy works and derive the
backreaction equations for the linearized quantum fluctuations in the
background of an acoustic black hole. A first order in hbar solution is given
in the near horizon region. It indicates that acoustic black holes, unlike
Schwarzschild ones, get cooler as they radiate phonons. They show remarkable
analogies with near-extremal Reissner-Nordstrom black holes.Comment: 10 pages, 1 figure; Talk given at the conference ``Constrained
Dynamics and Quantum Gravity (QG05)", Cala Gonone (Italy), September 200
Hawking Radiation from Acoustic Black Holes, Short Distance and Back-Reaction Effects
Using the action principle we first review how linear density perturbations
(sound waves) in an Eulerian fluid obey a relativistic equation: the d'Alembert
equation. This analogy between propagation of sound and that of a massless
scalar field in a Lorentzian metric also applies to non-homogeneous flows. In
these cases, sound waves effectively propagate in a curved four-dimensional
''acoustic'' metric whose properties are determined by the flow. Using this
analogy, we consider regular flows which become supersonic, and show that the
acoustic metric behaves like that of a black hole. The analogy is so good that,
when considering quantum mechanics, acoustic black holes should produce a
thermal flux of Hawking phonons.
We then focus on two interesting questions related to Hawking radiation which
are not fully understood in the context of gravitational black holes due to the
lack of a theory of quantum gravity. The first concerns the calculation of the
modifications of Hawking radiation which are induced by dispersive effects at
short distances, i.e., approaching the atomic scale when considering sound. We
generalize existing treatments and calculate the modifications caused by the
propagation near the black hole horizon. The second question concerns
backreaction effects. We return to the Eulerian action, compute second order
effects, and show that the backreaction of sound waves on the fluid's flow can
be expressed in terms of their stress-energy tensor. Using this result in the
context of Hawking radiation, we compute the secular effect on the background
flow.Comment: 60 pages, 6 figures. Review submitted to "La Rivista del Nuovo
Cimento
Analogue Cosmological Particle Creation: Quantum Correlations in Expanding Bose Einstein Condensates
We investigate the structure of quantum correlations in an expanding Bose
Einstein Condensate (BEC) through the analogue gravity framework. We consider
both a 3+1 isotropically expanding BEC as well as the experimentally relevant
case of an elongated, effectively 1+1 dimensional, expanding condensate. In
this case we include the effects of inhomogeneities in the condensate, a
feature rarely included in the analogue gravity literature. In both cases we
link the BEC expansion to a simple model for an expanding spacetime and then
study the correlation structure numerically and analytically (in suitable
approximations). We also discuss the expected strength of such correlation
patterns and experimentally feasible BEC systems in which these effects might
be detected in the near future.Comment: Reference adde
Quantum effects in Acoustic Black Holes: the Backreaction
We investigate the backreaction equations for an acoustic black hole formed
in a Laval nozzle under the assumption that the motion of the fluid is
one-dimensional. The solution in the near-horizon region shows that as phonons
are (thermally) radiated the sonic horizon shrinks and the temperature
decreases. This contrasts with the behaviour of Schwarzschild black holes, and
is similar to what happens in the evaporation of (near-extremal)
Reissner-Nordstrom black holes (i.e. infinite evaporation time). Finally, by
appropriate boundary conditions the solution is extended in both the asymptotic
regions of the nozzle.Comment: 23 pages, latex, 1 figure; revised version, to appear in Phys. Rev.
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