Derives primary degeneracy matrix conditions, antisymmetric consistency conditions from constraint preservation, and a final rank condition sufficient to ensure quadratic multi-field higher-derivative scalar-tensor theories propagate only 2+N degrees of freedom without extra Ostrogradsky modes.
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Healthy theories beyond Horndeski
14 Pith papers cite this work. Polarity classification is still indexing.
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abstract
We introduce a new class of scalar-tensor theories that extend Horndeski, or "generalized galileon", models. Despite possessing equations of motion of higher order in derivatives, we show that the true propagating degrees of freedom obey well-behaved second-order equations and are thus free from Ostrogradski instabilities, in contrast to the standard lore. Remarkably, the covariant versions of the original galileon Lagrangians-obtained by direct replacement of derivatives with covariant derivatives-belong to this class of theories. These extensions of Horndeski theories exhibit an uncommon, interesting phenomenology: the scalar degree of freedom affects the speed of sound of matter, even when the latter is minimally coupled to gravity.
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Extends the stochastic inflation formalism to a wide class of scalar-tensor theories by mapping EFT of dark energy coefficients to stochastic equations of motion.
Rotating black holes with primary scalar hair in beyond Horndeski gravity produce shadows whose diameter increases for negative Q and whose distortion increases for positive Q, with EHT bounds on M87* restricting but not ruling out the (a, Q) parameter space.
A metric-affine version of quadratic DHOST theories is derived and reduced to a one-function family that satisfies degeneracy conditions and light-speed gravitational wave propagation.
Requiring stealth Schwarzschild and de Sitter solutions in quadratic/cubic scalar-tensor theories eliminates odd-parity deviations from GR when all solutions are required, while allowing some deviations and non-trivial GW speeds in less restrictive cases.
A master screening equation is derived for luminal Horndeski gravity that recovers Vainshtein and Chameleon mechanisms and introduces Phaedrus screening with screening radius scaling linearly with source mass.
Derives the cubic scalar-scalar-tensor interaction action for perturbations in quadratic DHOST theories on cosmological backgrounds.
Using simulated binary black hole mergers and neutral hydrogen maps, the radio sirens method constrains H0 to 8% precision with 3000 high-SNR events, offering a 90% improvement over standard dark siren analyses.
An effective constrained scalar-Gauss-Bonnet inflation model yields n_s ≃ 0.958 and r ≃ 2.7×10^{-4} while the exact theory has no propagating scalar degree of freedom.
Five explicit polynomial spatially covariant gravity Lagrangians up to total derivative order three are identified that propagate only two degrees of freedom up to cubic order in perturbations around a cosmological background.
Final Planck CMB data confirms the flat 6-parameter ΛCDM model with Ω_c h² = 0.120 ± 0.001, Ω_b h² = 0.0224 ± 0.0001, n_s = 0.965 ± 0.004, τ = 0.054 ± 0.007, H_0 = 67.4 ± 0.5 km/s/Mpc, and no strong evidence for extensions.
Neutron stars with primary scalar hair in a DHOST subfamily are more compact than GR predictions and develop singularities above a critical scalar charge threshold.
Bouncing solutions in quadratic curvature gravity with a scalar field satisfy null, weak, and dominant energy conditions but violate the strong one when using the scalar-field energy-momentum tensor, while all four conditions are violated near the bounce in the effective tensor formulation.
A review thesis covering Mukhanov parametrization, general scalar-tensor theories, and new slow-roll techniques for canonical and noncanonical inflation observables.
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