Dark energy with the help of interacting dark sectors

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 $\phi$ 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 $\phi$'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 $\phi$ is coupled to a sector that sources a subdominant component of Dark Energy, which overcomes many of the challenges of slow-roll quintessence.