Physics invites the idea that space contains energy whose gravitational effect approximates that of Einstein's cosmological constant, Λ; nowadays the concept is termed dark energy or quintessence. Physics also suggests the dark energy could be dynamical, allowing the arguably appealing picture that the dark energy density is evolving to its natural value, zero, and is small now because the expanding universe is old. This alleviates the classical problem of the curious energy scale of order a millielectronvolt associated with a constant Λ. Dark energy may have been detected by recent advances in the cosmological tests. The tests establish a good scientific case for the context, in the relativistic Friedmann-Lemaître model, including the gravitational inverse square law applied to the scales of cosmology. We have well-checked evidence that the mean mass density is not much more than one quarter of the critical Einstein-de Sitter value. The case for detection of dark energy is serious but not yet as convincing; we await more checks that may come out of work in progress. Planned observations might be capable of detecting evolution of the dark energy density; a positive result would be a considerable stimulus to attempts to understand the microphysics of dark energy. This review presents the basic physics and astronomy of the subject, reviews the history of ideas, assesses the state of the observational evidence, and comments on recent developments in the search for a fundamental theory.

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Late time motivation (observational, theoretical) Previous cosmological seed magnetic field models Exponential potential scalar field inflation Maxwell L U(1)~ √-g F μν F μν does not work Modify L U(1) (toy model) Inflation, radiation, baryon epoch computations Results, numbers, consequences What remains to be done, generalizations Some observable consequences BR ApJL 391 (1992) (rejected by PLB) + Caltech preprint (rejected by PRD) at www.phys.ksu.edu/personal/ratra (backreaction discussed in detail in Caltech preprint; it is not a problem.) 2 Until recently, only upper bounds on the large-scale (1 Mpc) intergalactic magnetic field (IGMF), < few nG now.

We compile a list of 28 independent measurements of the Hubble parameter between redshifts 0.07 ≤ z ≤ 2.3 and use this to place constraints on model parameters of constant and time-evolving dark energy cosmologies. These H(z) measurements by themselves require a currently accelerating cosmological expansion at about, or better than, 3 σ confidence. The mean and standard deviation of the 6 best-fit model deceleration-acceleration transition redshifts (for the 3 cosmological models and 2 Hubble constant priors we consider) is z da = 0.74 ± 0.05, in good agreement with the recent Busca et al. (2012) determination of z da = 0.82 ± 0.08 based on 11 H(z) measurements between redshifts 0.2 ≤ z ≤ 2.3, almost entirely from BAO-like data.