De Sitter vs Quintessence in String Theory
  De Sitter solutions have been recently conjectured to be incompatible with
quantum gravity. In this paper we critically assess the progress and challenges
of different mechanisms to obtain de Sitter vacua in string compactifications
and compare them to quintessence models. We argue that, despite recent
criticisms, de Sitter models reached a level of concreteness and calculational
control which has been improving over time. On the other hand, building string
models of quintessence appears to be more challenging and requires additional
fine-tuning. We discuss the tension between the swampland conjecture and the
Higgs potential and find examples which can evade fifth-force bounds even if
they seem very hard to realise in string theory. We also comment on the tension
with low-redshift data and explore ultra-light axions from string theory as
dark energy candidates.
