Hinterbichler, Kurt
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Publication Stability and Superluminality of Spherical DBI Galileon Solutions(2011-04-12) Goon, Garrett L.; Hinterbichler, Kurt; Trodden, MarkThe Dirac-Born-Infeld (DBI) Galileons are a generalization of the Galileon terms, which extend the internal Galilean symmetry to an internal relativistic symmetry, and can also be thought of as generalizations of DBI which yield second order field equations. We show that, when considered as local modifications to gravity, such as in the Solar System, there exists a region of parameter space in which spherically symmetric static solutions exist and are stable. However, these solutions always exhibit superluminality, casting doubt on the existence of a standard Lorentz invariant UV completion.Publication Instabilities of spherical solutions with multiple Galileons and SO(N) symmetry(2011-02-24) Andrews, Melinda; Hinterbichler, Kurt; Khoury, Justin; Trodden, MarkThe 4-dimensional effective theory arising from an induced gravity action for a codimension greater than one brane consists of multiple Galileon fields π1, I = 1,...,N, invariant under separate Galilean transformations for each scalar, and under an internal SO(N) symmetry. We study the viability of such models by examining spherically symmetric solutions.We find that for general, nonderivative couplings to matter invariant under the internal symmetry, such solutions exist and exhibit a Vainshtein screening effect. By studying perturbations about such solutions, we find both an inevitable gradient instability and fluctuations propagating at superluminal speeds. These findings suggest that more general, derivative couplings to matter are required for the viability of SOðNÞ Galileon theories.Publication Multifield Galileons and Higher Codimension Branes(2010-12-07) Hinterbichler, Kurt; Trodden, Mark; Wesley, DanielIn the decoupling limit, the Dvali-Gabadadze-Porrati model reduces to the theory of a scalar field π, with interactions including a specific cubic self-interaction—the Galileon term. This term, and its quartic and quintic generalizations, can be thought of as arising from a probe 3-brane in a five-dimensional bulk with Lovelock terms on the brane and in the bulk.We study multifield generalizations of the Galileon and extend this probe-brane view to higher codimensions. We derive an extremely restrictive theory of multiple Galileon fields, interacting through a quartic term controlled by a single coupling, and trace its origin to the induced brane terms coming from Lovelock invariants in the higher codimension bulk. We explore some properties of this theory, finding de Sitter like self-accelerating solutions. These solutions have ghosts if and only if the flat space theory does not have ghosts. Finally, we prove a general nonrenormalization theorem: multifield Galileons are not renormalized quantum mechanically to any loop in perturbation theory.