dark energy

For most of the twentieth century cosmologists debated only whether the expansion would slow to a coast or reverse into a collapse; everyone assumed gravity was pulling the brakes. Then in 1998 two teams tracking distant exploding stars found the opposite: the expansion is speeding up. Some component fills all of space, does not dilute as the universe grows, and pushes it apart. We call it dark energy, and it is the largest and most mysterious ingredient of the cosmos.

Dark energy is defined by its gravitational effect: it has an energy density that stays nearly constant as space expands and a strongly negative pressure. In the Friedmann acceleration equation, a_ddot / a = -(4 pi G / 3)(rho + 3 p / c^2), a component with p about equal to minus rho c^2 makes the right-hand side positive, so a_ddot is positive and the expansion accelerates. Its property is captured by an equation-of-state parameter w = p / (rho c^2); observations give w very close to minus 1. Dark energy accounts for roughly 68 percent of the total energy content of the universe today, dominating over both dark matter and ordinary matter.

What is it really? The simplest answer is that it is the cosmological constant Lambda, an intrinsic energy of empty space with exactly w = minus 1. Alternatives, collectively called quintessence, propose a slowly evolving field with w near but not exactly minus 1. The deep embarrassment is the cosmological constant problem: naive quantum field theory estimates the vacuum energy to be many tens of orders of magnitude larger than observed, one of the worst mismatches between theory and experiment in all of physics. So we measure dark energy precisely yet cannot explain its tiny value.