Abstract: The LIGO/Virgo collaboration has recently announced the detection of GW190521: a heavy binary black hole merger with component masses in the "black hole mass gap," which cannot be explained by standard stellar structure theory. In this talk, I will discuss how new light particles that couple to the Standard Model can act as an additional source of energy loss in the cores of population-III stars, dramatically altering their evolution and potentially explaining GW190521. The black hole mass gap will emerge as a powerful tool for studying beyond the Standard Model particle physics.