13 research outputs found
Generalizing Galileons
The Galileons are a set of terms within four-dimensional effective field
theories, obeying symmetries that can be derived from the dynamics of a
3+1-dimensional flat brane embedded in a 5-dimensional Minkowski Bulk. These
theories have some intriguing properties, including freedom from ghosts and a
non-renormalization theorem that hints at possible applications in both
particle physics and cosmology. In this brief review article, we will summarize
our attempts over the last year to extend the Galileon idea in two important
ways. We will discuss the effective field theory construction arising from
co-dimension greater than one flat branes embedded in a flat background - the
multiGalileons - and we will then describe symmetric covariant versions of the
Galileons, more suitable for general cosmological applications. While all these
Galileons can be thought of as interesting four-dimensional field theories in
their own rights, the work described here may also make it easier to embed them
into string theory, with its multiple extra dimensions and more general
gravitational backgrounds.Comment: 16 pages; invited brief review article for a special issue of
Classical and Quantum Gravity. Submitted to CQ
Galileons as Wess-Zumino Terms
We show that the galileons can be thought of as Wess-Zumino terms for the
spontaneous breaking of space-time symmetries. Wess-Zumino terms are terms
which are not captured by the coset construction for phenomenological
Lagrangians with broken symmetries. Rather they are, in d space-time
dimensions, d-form potentials for (d+1)-forms which are non-trivial co-cycles
in Lie algebra cohomology of the full symmetry group relative to the unbroken
symmetry group. We introduce the galileon algebras and construct the
non-trivial (d+1)-form co-cycles, showing that the presence of galileons and
multi-galileons in all dimensions is counted by the dimensions of particular
Lie algebra cohomology groups. We also discuss the DBI and conformal galileons
from this point of view, showing that they are not Wess-Zumino terms, with one
exception in each case.Comment: 49 pages. v2 minor changes, version appearing in JHE
Decoding the bispectrum of single-field inflation
Galileon fields arise naturally from the decoupling limit of massive
gravities, and possess special self-interactions which are protected by a
spacetime generalization of Galilean symmetry. We briefly revisit the
inflationary phenomenology of Galileon theories. Working from recent
computations of the fluctuation Lagrangian to cubic order in the most general
model with second-order equations of motion, we show that a distinct shape is
present but with suppressed amplitude. A similar shape has been found in other
higher-derivative models. It may be visible in a theory tuned to suppress the
leading-order shapes, or if the overall bispectrum has large amplitude. Using a
partial-wave expansion of the bispectrum, we suggest a possible origin for the
frequent appearance of this shape. It follows that models with very disparate
microphysics can produce very similar bispectra. We argue that it may be more
profitable to distinguish these models by searching for relations between the
amplitudes of these common shapes. We illustrate this method using the example
of DBI and k-inflation.Comment: v1: 25 pages, including tables, an appendix and references. v2: minor
clarifications about the lowest-order consistency relations; matches version
published in JCA
The Worldvolume Action of Kink Solitons in AdS Spacetime
A formalism is presented for computing the higher-order corrections to the
worldvolume action of co-dimension one solitons. By modifying its potential, an
explicit "kink" solution of a real scalar field in AdS spacetime is found. The
formalism is then applied to explicitly compute the kink worldvolume action to
quadratic order in two expansion parameters--associated with the hypersurface
fluctuation length and the radius of AdS spacetime respectively. Two
alternative methods are given for doing this. The results are expressed in
terms of the trace of the extrinsic curvature and the intrinsic scalar
curvature. In addition to conformal Galileon interactions, we find a
non-Galileon term which is never sub-dominant. This method can be extended to
any conformally flat bulk spacetime.Comment: 32 pages, 3 figures, typos corrected and additional comments adde
Aspects of Galileon non-renormalization
We discuss non-renormalization theorems applying to galileon field theories and their generalizations. Galileon theories are similar in many respects to other derivatively coupled effective field theories, including general relativity and P (X) theories. In particular, these other theories also enjoy versions of non-renormalization theorems that protect certain operators against corrections from self-loops. However, we argue that the galileons are distinguished by the fact that they are not renormalized even by loops of other heavy fields whose couplings respect the galileon symmetry