Higgs-Dilaton cosmology: Universality vs. criticality

Abstract

The Higgs-Dilaton model is able to produce an early inflationary expansion followed by a dark energy dominated era responsible for the late time acceleration of the Universe. At tree level, the model predicts a small tensor-to-scalar ratio ($0.0021\leq r \leq 0.0034$), a tiny negative running of the spectral tilt ($-0.00057 \leq dn_s/d\ln k \leq -0.00034$) and a nontrivial consistency relation between the spectral tilt of scalar perturbations and the dark energy equation of state, which turns out to be close to a cosmological constant ($0 \leq 1+w_{DE} \leq 0.014$). We reconsider the validity of these predictions in the vicinity of the critical value of the Higgs self-coupling giving rise to an inflection point in the inflationary potential. The value of the inflationary observables in this case strongly depends on the parameters of the model. The tensor-to-scalar ratio can be large [$r\sim {\cal O}(0.1)$] and notably exceed its tree-level value. If that happens, the running of the scalar tilt becomes positive and rather big [$dn_s/d\ln k \sim {\cal O}(0.01)$] and the equation-of-state parameter of dark energy can significantly differ from a cosmological constant [$1+w_{DE}\sim {\cal O}(0.1)$].