Abstract
Quasi-conformal models are an appealing scenario that can offer naturally a strongly supercooled phase transition and a period of thermal inflation in the early Universe. A crucial aspect for the viability of these models is how the Universe escapes from the supercooled state. One possibility is that thermal inflation phase ends by nucleation and percolation of true vacuum bubbles. This route is not, however, always efficient. In such case another escape mechanism, based on the growth of quantum fluctuations of the scalar field that eventually destabilize the false vacuum, becomes relevant. We study both of these cases in detail in a simple yet representative model. We determine the duration of the thermal inflation, the curvature power spectrum generated for the scales that exit horizon during the thermal inflation, and the stochastic gravitational wave background from the phase transition. We show that these gravitational waves provide an observable signal from the thermal inflation in almost the entire parameter space of interest. Furthermore, the shape of the gravitational wave spectrum can be used to ascertain how the Universe escaped from supercooling.
Highlights
Separate periods, in between which the Universe was, for example, radiation dominated
In both cases we find that the gravitational wave (GW) signal is strong and allows almost entirety of the relevant parameter space to be probed with future GW detectors
Two GW sources are relevant for the thermal inflation scenario: First, if thermal inflation finishes with nucleation of true vacuum bubbles, their collisions and the motions they induce in the plasma generate GWs
Summary
One reasonable possibility is that after the primordial inflation, during which the CMB scales exited horizon, the Universe experienced a period of thermal inflation. By quasi-conformal, we refer to models featuring a nearly scale invariant potential, V (φ) = λ(φ)φ4/4, with a slowly scale-dependent effective quartic coupling λ(φ), which leads generically to a nontrivial conformal symmetry breaking minimum φ = 0 This transition can be very strongly supercooled and a long period of thermal inflation can be realized in these models. We study the gravitational wave (GW) signal from the transition, that in the first case, where the transition is completed by bubble nucleation and percolation, arises from bubble collisions and motions in the plasma, and in the second case, where the fluctuations of the scalar field destabilize the false vacuum, is sourced by the scalar fluctuations In both cases we find that the GW signal is strong and allows almost entirety of the relevant parameter space to be probed with future GW detectors
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