  Understanding the rich behavior that emerges from systems of interacting
quantum particles, such as electrons in materials, nucleons in nuclei or
neutron stars, the quark-gluon plasma, and superfluid liquid helium, requires
investigation of systems that are clean, accessible, and have tunable
parameters. Ultracold quantum gases offer tremendous promise for this
application largely due to an unprecedented control over interactions.
Specifically, $a$, the two-body scattering length that characterizes the
interaction strength, can be tuned to any value. This offers prospects for
experimental access to regimes where the behavior is not well understood
because interactions are strong, atom-atom correlations are important,
mean-field theory is inadequate, and equilibrium may not be reached or perhaps
does not even exist. Of particular interest is the unitary gas, where $a$ is
infinite, and where many aspects of the system are universal in that they
depend only on the particle density and quantum statistics. While the unitary
Fermi gas has been the subject of intense experimental and theoretical
investigation, the degenerate unitary Bose gas has generally been deemed
experimentally inaccessible because of three-body loss rates that increase
dramatically with increasing $a$. Here, we investigate dynamics of a unitary
Bose gas for timescales that are short compared to the loss. We find that the
momentum distribution of the unitary Bose gas evolves on timescales fast
compared to losses, and that both the timescale for this evolution and the
limiting shape of the momentum distribution are consistent with universal
scaling with density. This work demonstrates that a unitary Bose gas can be
created and probed dynamically, and thus opens the door for further exploration
of this novel strongly interacting quantum liquid.
