Dark energy. In the late 1990s, the discovery of the Universe’s accelerated expansion proved the exis- tence of an additional component besides matter and radiation (Xxxxx et al., 1998; Perl- mutter et al., 1999). This discovery came initially as a surprise, as such acceleration is possible only in a Universe dominated by an exotic constituent with negative eective pressure. Over time, however, what we now call dark energy quickly became accepted as an observational fact thanks to numerous supporting observations. Broadly speak- ing, the evidence can be divided into two groups: one related to its role in shaping the expansion of the Universe, that led to its discovery, and the other pertaining to its part in shaping the distribution of structure in the Universe, that appeared only a few years later (Xxxxxxxx et al., 2005; Xxxxxxxxxx et al., 2005). Despite the general belief in its ex- istence, however, very little has been discovered about dark energy apart from the fact that it accounts for about 70 percent of the Universe’s present-day energy content. Cosmological constant In the context of general relativity, the simplest explanation for dark energy is the cosmological constant appearing on the left-hand side of Equation (1.1). When moved to the right-hand side, u can be interpreted as a zero-point energy in addition to the energy-momentum content described by Tµ⌫ . If we assume that this constant is the sole cause of the accelerated expansion, then its value in terms of xxx Xxxxxx length lP is measured to be P u = 2.89 × 10—122 l2 , (1.5) r ! with an uncertainty of a few percentage points. In general, an accelerated expan- sion causes the energy density of matter and radiation to quickly dilute over time and eventually results in a Universe completely dominated by the cosmological constant. Asymptotically, this leads to a de-Sitter Universe where the scale factor can be written as: a(t) a exp uc2 t
