  We demonstrate the power of the black hole mass gap as a novel probe of
fundamental physics. 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. We investigate the effects of two
paradigmatic weakly coupled, low-mass particles, axions and hidden photons, and
find that the pulsational pair instability, which causes a substantial amount
of mass loss, is suppressed. As a result, it is possible to form black holes of
$72\msun$ or heavier, deep inside the black hole mass gap predicted by the
Standard Model. The upper edge of the mass gap is raised to $>130{\rm
M}_\odot$, implying that heavier black holes, anticipated to be observed after
LIGO's sensitivity is upgraded, would also be impacted. In contrast, thermally
produced heavy particles would remain in the core, leading to the tantalizing
possibility that they drive a new instability akin to the electron-positron
pair instability. We investigate this effect analytically and find that stars
that avoid the electron-positron pair instability could experience this new
instability. We discuss our results in light of current and upcoming
gravitational wave interferometer detections of binary black hole mergers.
