  In this paper, we provide a physical model for the origin of variations in
the shapes and bump strengths of dust attenuation laws in galaxies by combining
a large suite of cosmological "zoom-in" galaxy formation simulations with 3D
Monte Carlo dust radiative transfer calculations. We model galaxies over 3
orders of magnitude in stellar mass, ranging from Milky Way like systems
through massive galaxies at high-redshift. Critically, for these calculations
we employ a constant underlying dust extinction law in all cases, and examine
how the role of geometry and radiative transfer effects impact the resultant
attenuation curves. Our main results follow. Despite our usage of a constant
dust extinction curve, we find dramatic variations in the derived attenuation
laws. The slopes of normalized attenuation laws depend primarily on the
complexities of star-dust geometry. Increasing fractions of unobscured young
stars flatten normalized curves, while increasing fractions of unobscured old
stars steepen curves. Similar to the slopes of our model attenuation laws, we
find dramatic variation in the 2175 Angstrom ultraviolet (UV) bump strength,
including a subset of curves with little to no bump. These bump strengths are
primarily influenced by the fraction of unobscured O and B stars in our model,
with the impact of scattered light having only a secondary effect. Taken
together, these results lead to a natural relationship between the attenuation
curve slope and 2175 Angstrom bump strength. Finally, we apply these results to
a 25 Mpc/h box cosmological hydrodynamic simulation in order to model the
expected dispersion in attenuation laws at integer redshifts from z=0-6. A
significant dispersion is expected at low redshifts, and decreases toward z=6.
We provide tabulated results for the best fit median attenuation curve at all
redshifts.
