Late-time cosmological phase transitions: Particle-physics models and cosmic evolution

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 ({~}kpc-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, Ω_φ1.