  The Standard Model Higgs becomes tachyonic at high energy scales according to
current measurements. This unstable regime of the Higgs potential can be
realized in the early Universe during high scale inflation, potentially with
catastrophic consequences. This letter highlights a crucial inherent feature of
such configurations that has so far remained ignored: Higgs particle production
out of vacuum induced by the rapidly evolving Higgs field, which gets
exponentially enhanced due to the tachyonic instability. Such explosive
particle production can rapidly drain energy away from the Higgs field,
sustaining a significant density of Higgs particles even during inflation, and
could initiate a qualitatively different form of preheating in parts of the
post-inflationary Universe. Any study of the Higgs field in its tachyonic
phase, either during or after inflation, must therefore take this substantial
particle energy density into account, which could significantly affect the
subsequent evolution of such systems. This could carry important implications
for high scale inflation, post-inflationary preheating, observable signals in
the cosmic microwave background, gravitational waves, and primordial black
holes, as well as deeper concepts ranging from eternal inflation to the
metastability of the electroweak vacuum.
