Varying-constant cosmology from hyperlight, coupled scalars

New scalar fields may couple to the Standard Model by modulating its fundamental parameters in space and time. I will discuss a class of models in which a coupled scalar shifts the electron mass and electromagnetic fine-structure constant before recombination, paying careful attention to the scalar's gravitational effects, the impact of Standard Model matter on its evolution, and its signatures relevant to complementary probes of fundamental constants at other epochs. I will show that a hyperlight, coupled scalar's gravitational effects open up new degeneracies in cosmological parameters and present quantitative constraints on theoretically consistent scenarios using cosmological data.