  It has been widely believed that, except in very extreme situations, the
influence of gravity on quantum fields should amount to just small,
sub-dominant contributions. This view seemed to be endorsed by the seminal
results obtained over the last decades in the context of renormalization of
quantum fields in curved spacetimes. Here, however, we argue that this belief
is false by showing that there exist well-behaved spacetime evolutions where
the vacuum energy density of free quantum fields is forced, by the very same
background spacetime, to become dominant over any classical energy-density
component. This semiclassical gravity effect finds its roots in the infrared
behavior of fields on curved spacetimes. By estimating the time scale for the
vacuum energy density to become dominant, and therefore for backreaction on the
background spacetime to become important, we argue that this vacuum dominance
may bear unexpected astrophysical and cosmological implications.
