Higgs-Dilaton Cosmology: From the Early to the Late Universe

TL;DR

This paper presents a scale-invariant extension of the Standard Model with Unimodular Gravity that unifies early universe inflation and late-time dark energy, providing testable predictions for cosmological parameters.

Contribution

It introduces a model linking inflationary physics with dark energy, deriving specific relations between primordial spectrum tilt and dark energy equation of state, with bounds testable by upcoming observations.

Findings

Predicted bounds for scalar tilt: n_s<0.97

Derived relation between n_s and dark energy parameter ω_DE^0

Predicted scalar-to-tensor ratio: 0.0009 to 0.0033

Abstract

We consider a minimal scale-invariant extension of the Standard Model of particle physics combined with Unimodular Gravity formulated in \cite{Shaposhnikov:2008xb}. This theory is able to describe not only an inflationary stage, related to the Standard Model Higgs field, but also a late period of Dark Energy domination, associated with an almost massless dilaton. A number of parameters can be fixed by inflationary physics, allowing to make specific predictions for any subsequent period. In particular, we derive a relation between the tilt of the primordial spectrum of scalar fluctuations, $n_s$, and the present value of the equation of state parameter of dark energy, $\omega_{DE}^0$. We find bounds for the scalar tilt, $n_s<0.97$, the associated running, $-0.0006<d\ln n_s/d\ln k\lesssim-0.00015$, and for the scalar-to-tensor ratio, $0.0009\lesssim r<0.0033$, which will be critically tested by the results of the Planck mission. For the equation of state of dark energy, the model predicts $\omega_{DE}^0>-1$. The relation between $n_s$ and $\omega_{DE}^0$ allows us to use the current observational bounds on $n_s$ to further constrain the dark energy equation of state to $0< 1+\omega_{DE}^0< 0.02$, which is to be confronted with future dark energy surveys.