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Healthy theories beyond Horndeski
Jérôme Gleyzes, 2 David Langlois, Federico Piazza, 4 and Filippo Vernizzi CEA, Institut de Physique Théorique, F-91191 Gif-sur-Yvette cédex, CNRS, Unité de recherche associée-2306, F-91191
Degenerate higher derivative theories beyond Horndeski: evading the Ostrogradski instability
Theories with higher order time derivatives generically suffer from ghost-like instabilities, known as Ostrogradski instabilities. This fate can be avoided by considering "degenerate" Lagrangians,
Exploring gravitational theories beyond Horndeski
We have recently proposed a new class of gravitational scalar-tensor theories free from Ostrogradski instabilities, in arXiv:1404.6495. As they generalize Horndeski theories, or "generalized"
Hamiltonian formalism and gauge invariance for linear perturbations in inflation
A Hamiltonian treatment of gauge-invariant cosmological perturbations is presented. In this framework the gauge-invariant perturbations as well as their evolution equations are obtained with minimal
A unifying description of dark energy
The approach provides a unified treatment of the linear cosmological perturbations about a FLRW universe, obtained by a systematic expansion of the general action up to quadratic order, and shows that the behaviour of these linear perturbation is generically characterized by five time-dependent functions.
Essential Building Blocks of Dark Energy
We propose a minimal description of single field dark energy/modified gravity within the effective field theory formalism for cosmological perturbations, which encompasses most existing models. We
Degenerate higher order scalar-tensor theories beyond Horndeski and disformal transformations
We consider all degenerate scalar-tensor theories that depend quadratically on second-order derivatives of a scalar field, which we have identified in a previous work. These theories, whose
Curvature and isocurvature perturbations in two-field inflation
We study cosmological perturbations in two-field inflation, allowing for non-standard kinetic terms. We calculate analytically the spectra of curvature and isocurvature modes at Hubble crossing, up