73 research outputs found

### Massive phonon modes from a BEC-based analog model

Two-component BECs subject to laser-induced coupling exhibit a complicated
spectrum of excitations, which can be viewed as two interacting phonon modes.
We study the conditions required to make these two phonon modes decouple. Once
decoupled, the phonons not only can be arranged travel at different speeds, but
one of the modes can be given a mass -- it exhibits the dispersion relation of
a massive relativistic particle: omega = sqrt{omega_0^2 + c^2 k^2}.
This is a new and unexpected excitation mode for the coupled BEC system.
Apart from its intrinsic interest to the BEC community, this observation is
also of interest for the ``analogue gravity'' programme, as it opens the
possibility for using BECs to simulate massive relativistic particles in an
effective ``acoustic geometry''.Comment: 4 pages; uses revtex

### An acoustic probe for quantum vorticity in Bose-Einstein condensates

We investigate the deformation of wavefronts of sound waves in rotating
Bose-Einstein condensates. In irrational fluid flows Berry et al. identified
this kind of deformation as the hydrodynamic analogue of the Aharonov-Bohm
effect. We study this effect in Bose-Einstein condensates and obtain the
Aharonov-Bohm phase shift at all wavelengths. We show that this deformation of
wave fronts is seen in both phase and density fluctuations. For wavelengths
larger than the healing length, the phase fluctuations experience a phase shift
of the order of $2\pi$ times the winding number. We also consider lattices of
vortices. If the angular momentum of the vortices are aligned, the total phase
shift is $2\pi$ times the number of vortices in the condensate. Because of this
behaviour the hydrodynamic Aharonov-Bohm can be utilized as a probe for quantum
vorticity, whose experimental realization could offers an alternative route to
investigate quantum turbulence in the laboratory.Comment: 4 pages, 2 figure

### Massive Klein--Gordon equation from a BEC-based analogue spacetime

We extend the "analogue spacetime" programme by investigating a
condensed-matter system that is in principle capable of simulating the massive
Klein--Gordon equation in curved spacetime. Since many elementary particles
have mass, this is an essential step in building realistic analogue models, and
a first step towards simulating quantum gravity phenomenology. Specifically, we
consider the class of two-component BECs subject to laser-induced transitions
between the components. This system exhibits a complicated spectrum of normal
mode excitations, which can be viewed as two interacting phonon modes that
exhibit the phenomenon of "refringence". We study the conditions required to
make these two phonon modes decouple. Once decoupled, the two distinct phonons
generically couple to distinct effective spacetimes, representing a bi-metric
model, with one of the modes acquiring a mass. In the eikonal limit the massive
mode exhibits the dispersion relation of a massive relativistic particle: omega
= sqrt[omega_0^2 + c^2 k^2], plus curved-space modifications. Furthermore, it
is possible to tune the system so that both modes can be arranged to travel at
the same speed, in which case the two phonon excitations couple to the same
effective metric. From the analogue spacetime perspective this situation
corresponds to the Einstein equivalence principle being satisfied.Comment: V1: 10 pages; uses revtex4; V2: two references and brief comments
adde

### The fluid/gravity correspondence: Lectures notes from the 2008 Summer School on Particles, Fields, and Strings

This is a paper compiled by students of the 2008 Summer School on Particles,
Fields, and Strings held at the University of British Columbia on lectures
given by Veronika Hubeny as understood and interpreted by the authors. We start
with an introduction to the AdS/CFT duality. More specifically, we discuss the
correspondence between relativistic, conformal hydrodynamics and Einstein's
theory of gravity. Within our framework the Einstein equations are an effective
description for the string theory in the bulk of AdS_5 spacetime and the
hydrodynamic fluid equations represent the conformal field theory near thermal
equilibrium on the boundary. In particular we present a new technique for
calculating properties in fluid dynamics using the stress-energy tensor induced
on the boundary, by the gravitational field in the bulk, and comparing it with
the form of the stress-energy tensor from hydrodynamics. A detailed treatment
can be found in [JHEP 02 (2008) 045] and [arXiv:0803.2526].Comment: This is a paper compiled by students of the 2008 Summer School on
Particles, Fields, and Strings held at the University of British Columbia on
lectures given by Veronika Hubeny as understood and interpreted by the
authors; 21 pages, 5 figure

### Solution generating theorems: perfect fluid spheres and the TOV equation

We report several new transformation theorems that map perfect fluid spheres
into perfect fluid spheres. In addition, we report new ``solution generating''
theorems for the TOV, whereby any given solution can be ``deformed'' to a new
solution.Comment: 3 pages, 0 figures, to appear in the proceedings of the Eleventh
Marcel Grossmann Meeting on General Relativity (MG11), 23 - 29 July, 2006,
Berlin, German

### Phenomenologically viable Lorentz-violating quantum gravity

Horava's "Lifschitz point gravity" has many desirable features, but in its
original incarnation one is forced to accept a non-zero cosmological constant
of the wrong sign to be compatible with observation. We develop an extension of
Horava's model that abandons "detailed balance", and in 3+1 dimensions exhibit
all five marginal (renormalizable) and four relevant (super-renormalizable)
operators, as determined by power counting. We also consider the classical
limit of this theory, evaluate the Hamiltonian and super-momentum constraints,
and extract the classical equations of motion in a form similar to the ADM
formulation of general relativity. This puts the model in a framework amenable
to developing detailed precision tests.Comment: 4 pages; revtex4; V2: 2 references added; minor edits; no physics
changes; V3: references added; minor edits; no physics changes. This version
published in PR

### Lower-dimensional Horava-Lifshitz gravity

We consider Horava-Lifshitz gravity in both 1+1 and 2+1 dimensions. These
lower-dimensional versions of Horava-Lifshitz gravity are simple enough to be
explicitly tractable, but still complex enough to be interesting. We write the
most general (non-projectable) action for each case and discuss the resulting
dynamics. In the 1+1 case we utilize the equivalence with 2-dimensional
Einstein-aether theory to argue that, even though non-trivial, the theory does
not have any local degrees of freedom. In the 2+1 case we show that the only
dynamical degree of freedom is a scalar, which qualitatively has the same
dynamical behaviour as the scalar mode in (non-projectable) Horava-Lifshitz
gravity in 3+1 dimensions. We discuss the suitability of these
lower-dimensional theories as simpler playgrounds that could help us gain
insight into the 3+1 theory. As special cases we also discuss the projectable
limit of these theories. Finally, we present an algorithm that extends the
equivalence with (higher order) Einstein-aether theory to full Horava-Lifshitz
gravity (instead of just the low energy limit), and we use this extension to
comment on the apparent naturalness of the covariant formulation of the latter.Comment: v2: 9 pages, discussion about projectable version as a limiting case
added; v3: minor changes to match published versio

### Comment on: Detecting Vanishing Dimensions Via Primordial Gravitational Wave Astronomy

It has been recently claimed [arXiv:1102.3434] that quantum gravity models
where the number of dimensions reduces at the ultraviolet exhibit a potentially
observable cutoff in the primordial gravitational wave spectrum, and that this
is a "generic" and "robust" test for such models, since "(2+1)-dimensional
spacetimes have no gravitational degrees of freedom". We argue that such a
claim is misleading.Comment: 1 page, comment to Phys. Rev. Lett. 106, 101101 (2011)
[arXiv:1102.3434

### Superradiant scattering of orbital angular momentum beams

We consider the wave-structure coupling between an orbital angular momentum
beam and a rapidly rotating disk, and present a new configuration exhibiting
the wave amplification effect known as rotational superradiance. While
initially envisioned in terms of the scattering of an incident wave directed
perpendicular to an object's rotation axis, we demonstrate in the context of
acousto-mechanics that superradiant amplification can also occur with a vortex
beam directed parallel to the rotation axis. We propose two different
experimental routes: one must either work with rotations high enough that the
tangential velocity at the outer edge of the disk exceeds the speed of sound,
or use evanescent sound waves. We argue that the latter possibility is more
promising, and provides the opportunity to probe a previously unexamined
parameter regime in the acoustics of rotating porous media.Comment: 11 pages, 3 figure

### On the phenomenon of emergent spacetimes: An instruction guide for experimental cosmology

We present a toy model where spacetime is emergent from a more fundamental
microscopic system, and investigate the gray area interpolating between the
collective and free-particle regimes. For a period of rapid exponential growth
in the analogue universe, we argue that the intermediate regime is best
described by a coloured potpourri of geometries -- a "rainbow geometry". This
can be viewed as an alternative approach towards understanding quantum field
theories in the presence of Lorentz-symmetry breaking at ultraviolet scales.
Firstly, it is pointed out that cosmological particle production in our
emergent FRW-type analogue universe, when compared to conventional
semi-classical quantum gravity, is only temporarily robust against
model-specific deviations from Lorentz invariance. Secondly, it is possible to
carry out a straightforward quantitative analysis to estimate a suitable
parameter regime for experimental (analogue) cosmology.Comment: 19 pages; Based on a talk at the conference: From Quantum to Emergent
Gravity: Theory and Phenomenology, June 11-15 2007, Trieste, Ital

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