Abstract
If the Standard Model (SM) Higgs is weakly coupled to the inflationary sector, the Higgs is expected to be universally in the form of a condensate towards the end of inflation. The Higgs decays rapidly after inflation - via non-perturbative effects - into an out-of-equilibrium distribution of SM species, which thermalize soon afterwards. If the post-inflationary equation of state of the universe is stiff, $w \simeq +1$, the SM species eventually dominate the total energy budget. This provides a natural origin for the relativistic thermal plasma of SM species, required for the onset of the `hot Big Bang' era. The viability of this scenario requires the inflationary Hubble scale $H_*$ to be lower than the instability scale for Higgs vacuum decay, the Higgs not to generate too large curvature perturbations at cosmological scales, and the SM dominance to occur before Big Bang Nucleosynthesis. We show that successful reheating into the SM can only be obtained in the presence of a non-minimal coupling to gravity $\xi \gtrsim 1$, with a reheating temperature of $T_{\rm RH} \gtrsim \mathcal{O}(10^{10})\xi^{3/2}(H_*/10^{14}{\rm GeV})^2~{\rm GeV}$.
Highlights
Compelling evidence supports the idea of an inflationary phase in the early Universe [1]
To reheat the universe successfully, the relativistic thermal plasma dictating the expansion of the universe before Big Bang Nucleosynthesis (BBN), must be dominated by Standard Model (SM) species; this is a physical constraint that cannot be evaded
If the equation of state (EoS) of the inflationary sector become sufficiently stiff after inflation, the Higgs and its decay products will eventually dominate the energy budget of the universe; this provides a natural origin for the thermal plasma of SM species needed for the onset of the hot
Summary
Compelling evidence supports the idea of an inflationary phase in the early Universe [1]. We will argue that in this limit, the Higgs is universally excited in the form of a condensate around the time inflation ends. If the equation of state (EoS) of the inflationary sector become sufficiently stiff after inflation, the Higgs and its decay products will eventually dominate the energy budget of the universe; this provides a natural origin for the thermal plasma of SM species needed for the onset of the hot. In the weak coupling limit the Higgs is always excited in the form of a condensate with a large vacuum expectation value (VEV): either during inflation [case i)]. SM species ends at a time t = tend, when the (conformal) amplitude of the Higgs condensate stabilizes This moment signals as well the onset of energy equipartition and a stationary regime, from where the system is expected to evolve towards equilibrium. In section III.A.3 we will derive a simple estimate of the thermalization time scale, though its precise value will be unimportant in this paper
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