519 research outputs found
Goldstone's Theorem and Hamiltonian of Multi-galileon Modified Gravity
The galileon model was recently proposed to locally describe a class of
modified gravity theories, including the braneworld DGP model. We discuss
spontaneous symmetry breaking of the self-accelerating branch in a
multi-galileon theory with internal global symmetries. We show a modified
version of Goldstone's theorem is applicable to the symmetry breaking pattern
and discuss its implications. We also derive the Hamiltonian of a general
multi-galileon theory and discuss its implications.Comment: 13 pages, 1 figure; To appear in PR
Black hole hair in generalized scalar-tensor gravity
The most general action for a scalar field coupled to gravity that leads to
second order field equations for both the metric and the scalar --- Horndeski's
theory --- is considered, with the extra assumption that the scalar satisfies
shift symmetry. We show that in such theories the scalar field is forced to
have a nontrivial configuration in black hole spacetimes, unless one carefully
tunes away a linear coupling with the Gauss--Bonnet invariant. Hence, black
holes for generic theories in this class will have hair. This contradicts a
recent no-hair theorem, which seems to have overlooked the presence of this
coupling.Comment: 4+1 pages, PRL versio
Multi-galileons, solitons and Derrick's theorem
The field theory Galilean symmetry, which was introduced in the context of
modified gravity, gives a neat way to construct Lorentz-covariant theories of a
scalar field, such that the equations of motion contain at most second-order
derivatives. Here we extend the analysis to an arbitrary number of scalars, and
examine the restrictions imposed by an internal symmetry, focussing in
particular on SU(N) and SO(N). This therefore extends the possible gradient
terms that may be used to stabilise topological objects such as sigma model
lumps.Comment: 7 pages, 1 figure. Minor change to order of reference
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