E. Dark energy

The idea that the universe contains close to homogeneous dark energy that approximates a time-variable cosmological "constant" arose in particle physics, through the discussion of phase transitions in the early universe and through the search for a dynamical cancellation of the vacuum energy density; in cosmology, through the discussions of how to reconcile a cosmologically flat universe with the small mass density indicated by galaxy peculiar velocities; and on both sides by the thought that might be very small now because it has been rolling toward zero for a very long time. ⁽⁴³⁾

The idea that the dark energy is decaying by emission of matter or radiation is now strongly constrained by the condition that the decay energy must not significantly disturb the spectrum of the 3 K cosmic microwave background radiation. But the history of the idea is interesting, and decay to dark matter still a possibility, so we comment on both here. The picture of dark energy in the form of defects in cosmic fields has not received much attention in recent years, in part because the computations are difficult, but might yet prove to be productive. Much discussed nowadays is dark energy in a slowly varying scalar field. The idea is reviewed at some length here and in even more detail in the Appendix. We begin with another much discussed approach: prescribe the dark energy by parameters in numbers that seem fit for the quality of the measurements.

⁴³ This last idea is similar in spirit to Dirac's (1937, 1938) attempt to explain the large dimensionless numbers of physics. He noted that the gravitational force between two protons is much smaller than the electromagnetic force, and that that might be because the gravitational constant G is decreasing in inverse proportion to the world time. This is the earliest discussion we know of what has come to be called the hierarchy problem, that is, the search for a mechanism that might be responsible for the large ratio between a possibly more fundamental high energy scale, for example, that of grand unification or the Planck scale (where quantum gravitational effects become significant) and a lower possibly less fundamental energy scale, for example that of electroweak unification (see, for example, Georgi, Quinn, and Weinberg, 1974). The hierarchy problem in particle physics may be rephrased as a search for a mechanism to prevent the light electroweak symmetry breaking Higgs scalar field mass from being large because of a quadratically divergent quantum mechanical correction (see, for example, Susskind, 1979). In this sense it is similar in spirit to the physicists' cosmological constant problem of Sec. III.B. Back.