1,701 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
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