Dark energy two decades after: Observables, probes, consistency tests
  The discovery of the accelerating universe in the late 1990s was a watershed
moment in modern cosmology, as it indicated the presence of a fundamentally
new, dominant contribution to the energy budget of the universe. Evidence for
dark energy, the new component that causes the acceleration, has since become
extremely strong, owing to an impressive variety of increasingly precise
measurements of the expansion history and the growth of structure in the
universe. Still, one of the central challenges of modern cosmology is to shed
light on the physical mechanism behind the accelerating universe. In this
review, we briefly summarize the developments that led to the discovery of dark
energy. Next, we discuss the parametric descriptions of dark energy and the
cosmological tests that allow us to better understand its nature. We then
review the cosmological probes of dark energy. For each probe, we briefly
discuss the physics behind it and its prospects for measuring dark energy
properties. We end with a summary of the current status of dark energy
research.
