A new covariant foliation-based construction of scalar-tensor theories up to four derivatives of the scalar field that extends DHOST and U-DHOST without unitary gauge and propagates three degrees of freedom.
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The Effective Theory of Quintessence: the w<-1 Side Unveiled
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abstract
We study generic single-field dark energy models, by a parametrization of the most general theory of their perturbations around a given background, including higher derivative terms. In appropriate limits this approach reproduces standard quintessence, k-essence and ghost condensation. We find no general pathology associated to an equation of state w_Q < -1 or in crossing the phantom divide w_Q = -1. Stability requires that the w_Q < -1 side of dark energy behaves, on cosmological scales, as a k-essence fluid with a virtually zero speed of sound. This implies that one should set the speed of sound to zero when comparing with data models with w_Q < -1 or crossing the phantom divide. We summarize the theoretical and stability constraints on the quintessential plane (1+w_Q) vs. speed of sound squared.
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Non-parametric reconstruction of non-minimally coupled gravity with a smoothness prior on CMB, DESI BAO, supernovae, and DES data yields a 2.8σ hint for coupling and a preference for phantom divide crossing stabilized by the coupling.
Numerical polology framework samples coupling space to discover ghost-free tensor field theories up to rank three for cosmology, then applies resulting priors to black hole superradiance, dynamical dark energy, and GW data.
Develops a short-timescale injection formalism for post-recombination fluctuations, derives CMB impacts from scalar/vector/tensor perturbations, and constrains sequestered dark sector phase transitions to permille-level fractional energy injections.
Numerical simulations show ghost-normal scalar systems can remain dynamically bounded for long times when initial data is ultraviolet-dominated and low-amplitude, with some nonlinear potentials creating transient metastable states.
Early dark energy resolves CMB-BAO tension and, combined with thawing quintessence, reduces overall cosmological tensions without phantom crossing.
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.
Using CMB, SN, BAO and shear data, the work shows dynamical dark energy in MG models correlates with deviations from GR below z=2 at >95% CL, a link that holds for varying sound speed but vanishes for a cosmological constant.
An extended model with decaying dark matter around equality and w0 dark energy yields H0 ≈ 70 km/s/Mpc from Planck+ACT+DESI data, reducing Hubble tension to ~2.2σ while producing Bayesian evidence comparable to ΛCDM.
Modified gravity theories supply viable mathematical frameworks for inflation, bounces, and dark energy eras that match observational data.
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Numerical Investigations of Stable Dynamics in the Presence of Ghosts
Numerical simulations show ghost-normal scalar systems can remain dynamically bounded for long times when initial data is ultraviolet-dominated and low-amplitude, with some nonlinear potentials creating transient metastable states.