  We discuss the physical interpretation of unparticles and review the
constraints from cosmology. Unparticles may be understood in terms of confined
states of a strongly-coupled scale-invariant theory, where scale-invariance
implies that the confined states have continuous masses. This picture is
consistent with the observation that unparticle operators can be represented in
terms of continuous mass fields. Finite results in scattering processes are
obtained by compensating the infinite number of unparticle final states with an
infinitesimal coupling per unparticle. As a result, unparticles are stable with
respect to decay or annihilation to Standard Model particles, implying a
one-way flow of energy from the Standard Model sector to the unparticle sector.
The qualitative properties of unparticles, which result from their continuous
mass nature, are unchanged in the case where scale-invariance is broken by a
mass gap. Unparticles with a mass gap can evade constraints from astrophysical
and 5th force considerations, in which case cosmology provides the strongest
constraints.
