Dark energy with the help of interacting dark sectors

We analyze 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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mi>ϕ</a:mi></a:mrow></a:math> 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 <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>ϕ</c:mi></c:math>’s potential has a hilltop form and when it has a steep exponential runaway, as might naturally arise from string theory. We also discuss a related theory of multifield quintessence, in which <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mi>ϕ</e:mi></e:math> is coupled to a sector that sources a subdominant component of dark energy, which overcomes many of the challenges of slow-roll quintessence. Published by the American Physical Society 2024