4.2. Direct detection of dark energy
If dark energy is dynamical rather than simply a constant, it is able to interact with other fields, including those of the Standard Model of particle physics. For the particular example of an ultra-light scalar field, interactions introduce the possibility of two observable phenomena: long-range "fifth forces" and time-dependence of the constants of nature. Even if a dark-energy scalar interacts with ordinary matter only through indirect gravitational-strength couplings, searches for these phenomena should have already enabled us to detect the quintessence field (Carroll 1998, Dvali & Zaldarriaga 2002); to avoid detection, we need to introduce dimensionless suppression factors of order 10-5 or less in the coupling constants. On the other hand, there has been some evidence from quasar absorption spectra that the fine-structure constant was slightly smaller ( / ~ -10-5) at redshifts z ~ 0.5 - 3 (Murphy et al. 2001). On the most optimistic reading, this apparent shift might be direct evidence of a quintessence field; this would place strong constraints on the quintessence potential (Chiba & Kohri 2002). Before such an interpretation is accepted, however, it will be necessary to be certain that all possible sources of systematic error in the quasar measurements are understood, and that models can be constructed which fits the quasar data while remaining consistent with other experimental bounds (Uzan 2003).
More likely, we should work to construct particle physics models of quintessence in which both the mass and the interactions of the scalar field with ordinary matter are naturally suppressed. These requirements are met by Pseudo-Nambu-Goldstone bosons (PNGB's) (Frieman, Hill & Watkins 1992, Frieman, Hill, Stebbins & Waga 1995), which arise in models with approximate global symmetries of the form
Clearly such a symmetry should not be exact, or the potential would be precisely flat; however, even an approximate symmetry can naturally suppress masses and couplings. PNGB's typically arise as the angular degrees of freedom in Mexican-hat potentials that are "tilted" by a small explicitly symmetry breaking, and the PNGB potential takes on a sinusoidal form:
Fields of this type are ubiquitous in string theory, and it is possible that one of them may have the right properties to be the dark energy (Choi 2000; Kim 2000; Kim & Nilles 2003). Supersymmetric versions have been studied by Bi, Li & Zhang (2003). Interestingly, while the symmetry (1.41) suppresses most possible interactions with ordinary matter, it leaves open one possibility - a pseudoscalar electromagnetic interaction in which couples to E . B. The effect of such an interaction would be to gradually rotate the plane of polarization of light from distant sources (Carroll 1998, Lue, Wang & Kamionkowski 1999); current limits on such a rotation are not quite sensitive enough to tightly constrain this coupling. It is therefore very plausible that a pseudoscalar quintessence field will be directly detected by improved polarization measurements in the near future.
Even if we manage to avoid detectable interactions between dark energy and ordinary matter, we may still consider the possibility of nontrivial interactions between dark matter and dark energy. Numerous models along these lines have been proposed (Casas, Garcia-Bellido & Quiros 1992, Wetterich 1995, Anderson & Carroll 1998, Amendola 2000, Bean 2001, Comelli, Pietroni & Riotto 2003; for recent work and further references see Farrar & Peebles 2003, Hoffman 2003, Fardon, Nelson & Weiner 2003). If these two dark components constitute 95% of the universe, the idea that they are separate and non-interacting may simply be a useful starting point. Investigations thus far seem to indicate that some sorts of interactions are possible, but constraints imposed by the cosmic microwave background and large-scale structure are actually able to exclude a wide range of possibilities. It may be that the richness of interaction we observe in the ordinary-matter sector is an exception rather than the rule.
Finally, our natural tendencies toward economy of explanation inspires us to consider models in which the dark energy does more than simply accelerate the universe. Accordingly, models have been proposed in which quintessence is involved in inflation (Peebles & Vilenkin 1999, Copeland, Liddle and Lidsey 2001) as well as in baryogenesis (Li, Wang, Feng & Zhang 2002, De Felice, Nasri & Trodden 2003, Gu, Wang & Zhang 2003). Perhaps the success or failure of these mechanisms will one day provide a clue to a more comprehensive picture of cosmology.