  Understanding strongly correlated phases of matter, from the quark-gluon
plasma to neutron stars, and in particular the dynamics of such systems, $e.g.$
following a Hamiltonian quench, poses a fundamental challenge in modern
physics. Ultracold atomic gases are excellent quantum simulators for these
problems, thanks to tuneable interparticle interactions and experimentally
resolvable intrinsic timescales. In particular, they give access to the unitary
regime where the interactions are as strong as allowed by quantum mechanics.
Following years of experiments on unitary Fermi gases, unitary Bose gases have
recently emerged as a new experimental frontier. They promise exciting new
possibilities, including universal physics solely controlled by the gas density
and novel forms of superfluidity. Here, through momentum- and time-resolved
studies, we explore both degenerate and thermal homogeneous Bose gases quenched
to unitarity. In degenerate samples we observe universal post-quench dynamics
in agreement with the emergence of a prethermal state with a universal nonzero
condensed fraction. In thermal gases, dynamic and thermodynamic properties
generically depend on both the gas density $n$ and temperature $T$, but we find
that they can still be expressed in terms of universal dimensionless functions.
Surprisingly, the total quench-induced correlation energy is independent of the
gas temperature. Our measurements provide quantitative benchmarks and new
challenges for theoretical understanding.
