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We describe a natural particle-physics basis for late-time phase transitions in the Universe. Such a transition can seed the formation of large-scale structure while leaving a minimal imprint upon the microwave background anisotropy. The key ingredient is an ultralight pseudo Nambu-Goldstone boson with an astronomically large (-Mpc) Compton wavelength. We analyze the cosmological signatures of and constraints upon a wide class of scenarios which do not involve domain walls. In addition to seeding structure, coherent ultralight bosons may also provide unclustered dark matter in a spatially flat universe, _1.
Frieman et al. (Sat,) studied this question.