Orateur
Description
We introduce Kinessence models where combining K-mouflage and quintessence leads to phantom-
crossing of the dark energy equation of state and sign-switching of the dark energy density. This
is realised by joining a dark scalar sector comprising K-essence kinetic terms and a quintessence
potential together with a Yukawa coupling to matter. We investigate the dynamics of these models
in the Jordan frame both at the background and perturbative levels. We concentrate on the effective
equation of state and the matter fraction for the background. The Cosmic Microwave Background
(CMB) spectra, the linear power spectrum and the growth of structure via fσ8 are also analysed
at the perturbative level. The quintessence potential drives the acceleration of the Universe at low
redshift with an equation of state greater than minus one. The K-essence kinetic term leads to
a phantom crossing behaviour depending on the strength of the Yukawa coupling. The potential
fifth force mediated by the scalar field is screened by the K-mouflage mechanism and passes solar
system tests, i.e. the Cassini bound and the periastron of the Moon. This allows for relatively large
Yukawa couplings to matter β∼0.1, large enough to lead to the crossing of the phantom divide at
small redshift z ∼0.5. Despite the presence of a significant scalar interaction on large scales, we
find that the power spectrum and the growth of structure are reduced compared to ΛCDM. This
is also the case for the matter fraction which deviates from the standard model by a few percent.
Similarly the Hubble rate is larger at low redshift than at recombination whilst the CMB spectra
show small deviations from ΛCDM. In these models, the dark energy fraction becomes negative at
redshifts greater than two. All these features should alleviate the various tensions such as the larger
value of H0 at small redshift, a possible reduction of growth as measured by S8 and the puzzling
bound on the sum of the neutrino masses. Deviations from Λ-CDM of the CMB lensing spectrum
may provide a detectable window for these models.