Multi-field dark energy: cosmic acceleration on a steep potential
  We argue that dark energy with multiple fields is theoretically
well-motivated and predicts distinct observational signatures, in particular
when cosmic acceleration takes place along a trajectory that is highly
non-geodesic in field space. Such models provide novel physics compared to
$\Lambda$CDM and quintessence by allowing cosmic acceleration on steep
potentials. From the theoretical point of view, these theories can easily
satisfy the conjectured swampland constraints and may in certain cases be
technically natural, potential problems which are endemic to standard
single-field dark energy. Observationally, we argue that while such multi-field
models are likely to be largely indistinguishable from the concordance
cosmology at the background level, dark energy perturbations can cluster,
leading to an enhanced growth of large-scale structure that may be testable as
early as the next generation of cosmological surveys.
