69 research outputs found
A Modern Approach to Superradiance
In this paper, we provide a simple and modern discussion of rotational
superradiance based on quantum field theory. We work with an effective theory
valid at scales much larger than the size of the spinning object responsible
for superradiance. Within this framework, the probability of absorption by an
object at rest completely determines the superradiant amplification rate when
that same object is spinning. We first discuss in detail superradiant
scattering of spin 0 particles with orbital angular momentum , and then
extend our analysis to higher values of orbital angular momentum and spin.
Along the way, we provide a simple derivation of vacuum friction---a "quantum
torque" acting on spinning objects in empty space. Our results apply not only
to black holes but to arbitrary spinning objects. We also discuss superradiant
instability due to formation of bound states and, as an illustration, we
calculate the instability rate for bound states with massive spin 1
particles. For a black hole with mass and angular velocity , we
find when the particle's Compton wavelength
is much greater than the size of the spinning object. This rate is
parametrically much larger than the instability rate for spin 0 particles,
which scales like . This enhanced instability rate can be
used to constrain the existence of ultralight particles beyond the Standard
Model.Comment: 39 pages (v2 contains many added details and corrects an error in v1.
In particular, the instability rates for leading vector bound states are
computed exactly in the large Compton wavelength limit.
Zoology of condensed matter: Framids, ordinary stuff, extra-ordinary stuff
We classify condensed matter systems in terms of the spacetime symmetries
they spontaneously break. In particular, we characterize condensed matter
itself as any state in a Poincar\'e-invariant theory that spontaneously breaks
Lorentz boosts while preserving at large distances some form of spatial
translations, time-translations, and possibly spatial rotations. Surprisingly,
the simplest, most minimal system achieving this symmetry breaking
pattern---the "framid"---does not seem to be realized in Nature. Instead,
Nature usually adopts a more cumbersome strategy: that of introducing internal
translational symmetries---and possibly rotational ones---and of spontaneously
breaking them along with their space-time counterparts, while preserving
unbroken diagonal subgroups. This symmetry breaking pattern describes the
infrared dynamics of ordinary solids, fluids, superfluids, and---if they
exist---supersolids. A third, "extra-ordinary", possibility involves replacing
these internal symmetries with other symmetries that do not commute with the
Poincar\'e group, for instance the galileon symmetry, supersymmetry or gauge
symmetries. Among these options, we pick the systems based on the galileon
symmetry, the "galileids", for a more detailed study. Despite some similarity,
all different patterns produce truly distinct physical systems with different
observable properties. For instance, the low-energy scattering
amplitudes for the Goldstone excitations in the cases of framids, solids and
galileids scale respectively as , , and . Similarly the energy
momentum tensor in the ground state is "trivial" for framids (),
normal for solids () and even inhomogenous for galileids.Comment: 58 pages, 1 table, 1 free cut-and-paste project for rainy days in
Appendi
Effective Field Theories in Cosmology
During the last 20 years, a large amount of detailed cosmological observations have promoted cosmology to the rank of high-precision science. Remarkably, all the observations currently available can be accounted for by assuming that (i) the universe is approximately homogeneous and isotropic on large scales, (ii) gravitational interactions are described by General Relativity with a non-vanishing cosmological constant and (ii) 85% of the matter content of the universe is in the form of dark matter, a presently unknown type of matter which interacts with ordinary matter only gravitationally. Current theoretical efforts are focused on gaining a deeper understanding of the small departures from perfect homogeneity and isotropy observed in our universe, the nature of dark matter and the physical origin of the cosmological constant. Effective field theory methods provide a natural framework to try to address such outstanding questions. For instance, such methods have been extensively used to study alternative theories of gravity which mimic a non-vanishing cosmological constant and to build models of the early universe which generate the observed anisotropies and inhomogeneities through a period of accelerated cosmic expansion. In this thesis, we study effective field theories of gravity which violate some basic tenets of General Relativity such as Lorentz invariance and the weak equivalence principle. We also employ effective field theory methods to explore the imprint that high energy physics can leave on the small departures from homogeneity and isotropy generated in the early universe
UV completion without symmetry restoration
We show that it is not possible to UV-complete certain low-energy effective
theories with spontaneously broken space-time symmetries by embedding them into
linear sigma models, that is, by adding "radial" modes and restoring the broken
symmetries. When such a UV completion is not possible, one can still raise the
cutoff up to arbitrarily higher energies by adding fields that transform
non-linearly under the broken symmetries, that is, new Goldstone bosons.
However, this (partial) UV completion does not necessarily restore any of the
broken symmetries. We illustrate this point by considering a concrete example
in which a combination of space-time and internal symmetries is broken down to
a diagonal subgroup. Along the way, we clarify a recently proposed
interpretation of inverse Higgs constraints as gauge-fixing conditions.Comment: 6 page
Effective string theory for vortex lines in fluids and superfluids
We discuss the effective string theory of vortex lines in ordinary fluids and
low-temperature superfluids, by describing the bulk fluid flow in terms of a
two-form field to which vortex lines can couple. We derive the most general
low-energy effective Lagrangian that is compatible with (spontaneously broken)
Poincare invariance and worldsheet reparameterization invariance. This
generalizes the effective action developed by Lund and Regge and by Endlich and
Nicolis. By applying standard field-theoretical techniques, we show that
certain low-energy coupling constants -- most notably the string tension --
exhibit RG running already at the classical level. We discuss applications of
our techniques to the study of Kelvin waves, vortex rings, and the coupling to
bulk sound modes.Comment: 62 pages, 6 figure
Radiation of scalar modes and the classical double copy
The double copy procedure relates gauge and gravity theories through
color-kinematics replacements and holds for both scattering amplitudes and in
classical contexts. Moreover, it has been shown that there is a web of theories
whose scattering amplitudes are related through operations that exchange color
and kinematic factors. In this paper, we generalize and extend this procedure
by showing that the classical perturbative double copy of pions corresponds to
special Galileons. We consider point-particles coupled to the relevant scalar
fields, and find the leading and next to leading order radiation amplitudes. By
considering couplings motivated by those that would arise from extracting the
longitudinal modes of the gauge and gravity theories, we are able to map the
non-linear sigma model radiation to that of the special Galileon. We also
construct the single copy by mapping the bi-adjoint scalar radiation to the
non-linear sigma model radiation through generalized color-kinematics
replacements.Comment: 30 pages, 5 figure
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