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, ${\Omega }_{\phi }\simeq 1$.

• Received 23 March 1992

DOI:https://doi.org/10.1103/PhysRevD.46.1226