  We compute fluctuation-induced (Casimir) forces for classical systems after a
temperature quench. Using a generic coarse-grained model for fluctuations of a
conserved density, we find that transient forces arise even if the initial and
final states are force-free. In setups reminiscent of Casimir (planar walls)
and van der Waals (small inclusions) interactions, we find comparable exact
universal expressions for the force. Dynamical details only scale the time axis
of transient force curves. We propose that such quenches can be achieved, for
instance, in experiments on active matter, employing tunable activity or
interaction protocols.
