ACT-Planck data and phase transitions from a viable no-scale Standard Model completion

TL;DR

This paper presents a realistic classically scale-invariant model that explains large hierarchies, predicts first-order phase transitions, and describes a two-stage inflationary cosmology compatible with recent observational data.

Contribution

It constructs a fully realistic, observationally consistent model based on classical scale invariance that features a novel two-stage inflationary scenario.

Findings

Model accounts for all observational evidence of new physics.

Compatible with recent inflationary constraints from Planck, BICEP/Keck, and ACT.

Illustrates a non-standard cosmology with two inflation stages separated by radiation dominance.

Abstract

Classically scale-invariant (and perturbative) theories provide a way to understand large hierarchies, as scales are generated through dimensional transmutation. They always lead to first-order phase transitions, since symmetries are radiatively broken, and they generically feature quasi-flat potentials, which are suitable for inflation. We construct a simple but fully realistic model of this kind that accounts for all observational evidence of new physics and is remarkably compatible with the most recent constraints on inflationary observables from both the Planck/BICEP/Keck and the Atacama Cosmology Telescope (ACT) collaborations. This model illustrates how classical scale invariance generically leads to a non-standard cosmology in which inflation occurs in two stages: a slow-roll stage and a thermal stage, separated by a radiation-dominated era.