Screened modified gravity around small, dense sources and tests with hydrogen and muonium

New theories in physics, such as ones explaining dark energy and dark matter, tend to introduce new scalar particles.  Such particles generically couple to Standard Model fermions, and lead to numerous physical observables, most notably a new force law.  Hence, these theories typically fall under the umbrella of modified gravity, and tests of gravity and new forces are capable of searching for them.  A particularly interesting class of new theories exhibit screening, a dynamical mechanism by which the new force is suppressed around extended objects.  I will describe how atomic spectroscopy provides a window into testing screened theories.  The discussion will particularly focus on muonium, as it is a system composed only of fundamental particles, and is therefore immune to some of the effects of screening.  I will also discuss my work on some of the theoretical puzzles that extremely compact sources like fundamental particles and atomic nuclei present in these models.