¹ ‘Dark energy’ is a common term unifying different models for the ubiquitous form of energy permeating the entire universe (about 70% of the total energy budget of the physical universe) and causing accelerated expansion of space time. The most famous of these models is Einstein’s cosmological constant, but there are others, going under the names of quintessence, phantom energy, and so on. They are all characterized by negative pressure, in sharp contrast to all other forms of energy we see around us.