Speaker
Description
Dynamical dark energy offers an alternative to a cosmological constant with distinct observational signatures. However, the small energy density scale, Hubble-sized mass, and Planckian excursions make simple models fine-tuned and unnatural. In this talk, we discuss a weak version of the axion that can naturally reproduce the scale hierarchies expected for dark energy. After identifying the axion shift symmetry with $U(1)_{B+L}$, the axion potential is controlled by sources of explicit baryon and lepton number violation. In particular, the proximity between the dark energy scale and neutrino masses is explained if Majorana neutrinos provide the dominant contribution to the axion potential, as we show is generically expected. Other contributions, such as axion couplings to visible matter and weak instantons, give subleading corrections to the potential. We identify the essential ingredients for weak axion dark energy, show that its cosmological dynamics, and discuss possible ultraviolet origins