Alleviating $H_0$ tension in Horndeski gravity
  We show that the $H_0$ tension can be alleviated in the framework of
Horndeski/generalized galileon gravity. In particular, since the terms
depending on $G_5$ control the friction in the Friedmann equation, we construct
specific sub-classes in which it depends only on the field's kinetic energy.
Since the latter is small at high redshifts, namely at redshifts which affected
the CMB structure, the deviations from $\Lambda$CDM cosmology are negligible,
however as time passes it increases and thus at low redshifts the Hubble
function acquires increased values in a controlled way. We consider two Models,
one with quadratic and one with quartic dependence on the field's kinetic
energy. In both cases we show the alleviation of the tension, resulting to $H_0
\approx 74$ km/s/Mpc for particular parameter choices. Finally, we examine the
behavior of scalar metric perturbations, showing that the conditions for
absence of ghost and Laplacian instabilities are fulfilled throughout the
evolution, and we confront the models with SNIa data.
