Massless Preheating and Electroweak Vacuum Metastability
  Current measurements of Standard Model parameters suggest that the
electroweak vacuum is metastable. This metastability has important cosmological
implications, because large fluctuations in the Higgs field could trigger
vacuum decay in the early universe. For the false vacuum to survive,
interactions which stabilize the Higgs during inflation -- e.g., inflaton-Higgs
interactions or non-minimal couplings to gravity -- are typically necessary.
However, the post-inflationary preheating dynamics of these same interactions
could also trigger vacuum decay, thereby recreating the problem we sought to
avoid. This dynamics is often assumed catastrophic for models exhibiting scale
invariance since these generically allow for unimpeded growth of fluctuations.
In this paper, we examine the dynamics of such "massless preheating" scenarios
and show that the competing threats to metastability can nonetheless be
balanced to ensure viability. We find that fully accounting for both the
backreaction from particle production and the effects of perturbative decays
reveals a large number of disjoint "islands of (meta)stability" over the
parameter space of couplings. Ultimately, the interplay among Higgs-stabilizing
interactions plays a significant role, leading to a sequence of dynamical
phases that effectively extend the metastable regions to large Higgs-curvature
couplings.
