145 research outputs found
Galilean currents and charges
We derive the Noether currents and charges associated with an internal
galilean invariance---a symmetry recently postulated in the context of
so-called galileon theories. Along the way we clarify the physical
interpretation of the Noether charges associated with ordinary Galileo- and
Lorentz-boosts.Comment: 5 page
Galilean Genesis: an alternative to inflation
We propose a novel cosmological scenario, in which standard inflation is
replaced by an expanding phase with a drastic violation of the Null Energy
Condition (NEC): \dot H >> H^2. The model is based on the recently introduced
Galileon theories, that allow NEC violating solutions without instabilities.
The unperturbed solution describes a Universe that is asymptotically Minkowski
in the past, expands with increasing energy density until it exits the regime
of validity of the effective field theory and reheats. This solution is a
dynamical attractor and the Universe is driven to it, even if it is initially
contracting. The study of perturbations of the Galileon field reveals some
subtleties, related to the gross violation of the NEC and it shows that
adiabatic perturbations are cosmologically irrelevant. The model, however,
suggests a new way to produce a scale invariant spectrum of isocurvature
perturbations, which can later be converted to adiabatic: the Galileon is
forced by symmetry to couple to the other fields as a dilaton; the effective
metric it yields on the NEC violating solution is that of de Sitter space, so
that all light scalars will automatically acquire a nearly scale-invariant
spectrum of perturbations.Comment: 25 pages, 1 figure. v2: minor changes, JCAP published versio
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
- …