We analyse theories that do not have a de Sitter vacuum and cannot lead to
slow-roll quintessence, but which nevertheless support a transient era of
accelerated cosmological expansion due to interactions between a scalar Ï•
and either a hidden sector thermal bath, which evolves as Dark Radiation, or an
extremely-light component of Dark Matter. We show that simple models can
explain the present-day Dark Energy of the Universe consistently with current
observations. This is possible both when Ï•'s potential has a hilltop form
and when it has a steep exponential run-away, as might naturally arise from
string theory. We also discuss a related theory of multi-field quintessence, in
which Ï• is coupled to a sector that sources a subdominant component of
Dark Energy, which overcomes many of the challenges of slow-roll quintessence.Comment: 30 pages + appendices (two columns), 9 figure