Galileon as a local modification of gravity

By Alberto Nicolis, R. Rattazzi, E. Trincherini. 2008

1,662 citations recorded by the source corpus.

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.

View Paper: https://www.semanticscholar.org/paper/e42b557a1134e16ce8d369ae3e4d205f176c5bbd

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Citations in the corpus

Spherical collapse in quintessence models with zero speed of sound (2009, background): “With the change of coordinates τ = t − 12Hr(2) and ~y = ~x a(1 + 1 4H 2r2) [37], with a and H evaluated at t rather than at τ , one gets at first order in H2r2, ds(2) ≃ − [”
Exploring gravitational theories beyond Horndeski (2014, methodology): “It is straighforward to see that the minimally coupled versions of the original galileons proposed in [6], (5)–(8), are contained in eqs.”
New experimental approaches in the search for axion-like particles (2018, background): “Scalar fields invoked to explain the identity of dark energy, such as quintessence fields [180–183], chameleons [184–186], galileons [187], symmetrons [188] also share some of the peculiar signatures of axions and ALPs.”
Zoology of condensed matter: framids, ordinary stuff, extra-ordinary stuff (2015, background): “The Lagrangian terms for the galileon that are most relevant at low-energies have the schematic form [5] Ln ∼ ∂φ ∂φ (∂∂φ)n−2 , (5.”
TASI Lectures on Scattering Amplitudes (2017, background): “These exceptional theories include the NLSM (σ = 1), DBI scalar theory (σ = 2), and the special Galileon (σ = 3).”
Planck 2015 results. XVII. Constraints on primordial non-Gaussianity (2015, unknown): “2010; Mizuno & Koyama 2010; Ohashi & Tsujikawa 2012) are well motivated models based on the so called “Galilean symmetry” (Nicolis et al. 2009).”
Teleparallel Palatini theories (2018, background): “Again, ambiguities will arise in more general theories like Galileon or Horndeski type of Lagrangians [61, 62].”
Gravity and Large-Scale Nonlocal Bias (2012, background): “In theories of modified gravity, 4D Galileons arise as a remnant of a higher-dimensional Lorentz symmetry and Lagrangians containing Galileon interactions have equations of motion that are precisely second-order [72].”