Galileon as a local modification of gravity
- Department of Physics and ISCAP, Columbia University, New York, New York 10027 (United States)
- Institut de Theorie des Phenomenes Physiques, EPFL, CH1015 Lausanne (Switzerland)
- Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa (Italy)
In the Dvali-Gabadadze-Porrati (DGP) model, the 'self-accelerating' solution is plagued by a ghost instability, which makes the solution untenable. This fact, as well as all interesting departures from general relativity (GR), are fully captured by a four-dimensional effective Lagrangian, valid at distances smaller than the present Hubble scale. The 4D effective theory involves a relativistic scalar {pi}, universally coupled to matter and with peculiar derivative self-interactions. In this paper, we study the connection between self-acceleration and the presence of ghosts for a quite generic class of theories that modify gravity in the infrared. These theories are defined as those that at distances shorter than cosmological, reduce to a certain generalization of the DGP 4D effective theory. We argue that for infrared modifications of GR locally due to a universally coupled scalar, our generalization is the only one that allows for a robust implementation of the Vainshtein effect--the decoupling of the scalar from matter in gravitationally bound systems--necessary to recover agreement with solar-system tests. Our generalization involves an internal Galilean invariance, under which {pi}'s gradient shifts by a constant. This symmetry constrains the structure of the {pi} Lagrangian so much so that in 4D there exist only five terms that can yield sizable nonlinearities without introducing ghosts. We show that for such theories in fact there are ''self-accelerating'' de Sitter solutions with no ghostlike instabilities. In the presence of compact sources, these solutions can support spherically symmetric, Vainshtein-like nonlinear perturbations that are also stable against small fluctuations. We investigate a possible infrared completion of these theories at scales of order of the Hubble horizon, and larger. There are however some features of our theories that may constitute a problem at the theoretical or phenomenological level: the presence of superluminal excitations; the extreme subluminality of other excitations, which makes the quasistatic approximation for certain solar-system observables unreliable due to Cherenkov emission; the very low strong-interaction scale for {pi}{pi} scatterings.
- OSTI ID:
- 21266394
- Journal Information:
- Physical Review. D, Particles Fields, Vol. 79, Issue 6; Other Information: DOI: 10.1103/PhysRevD.79.064036; (c) 2009 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA); ISSN 0556-2821
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
CHERENKOV RADIATION
DE SITTER GROUP
DECOUPLING
DISTURBANCES
FLUCTUATIONS
FOUR-DIMENSIONAL CALCULATIONS
GENERAL RELATIVITY THEORY
GRAVITATION
LAGRANGIAN FUNCTION
MATHEMATICAL SOLUTIONS
MODIFICATIONS
NONLINEAR PROBLEMS
PERTURBATION THEORY
PION-PION INTERACTIONS
RELATIVISTIC RANGE
SCATTERING
SIMULATION
SOLAR SYSTEM
STRONG INTERACTIONS
SYMMETRY