  Empirical constraints on reionization require galactic ionizing photon escape
fractions fesc>20%, but recent high-resolution radiation-hydrodynamic
calculations have consistently found much lower values ~1-5%. While these
models include strong stellar feedback and additional processes such as runaway
stars, they almost exclusively consider stellar evolution models based on
single (isolated) stars, despite the fact that most massive stars are in
binaries. We re-visit these calculations, combining radiative transfer and
high-resolution cosmological simulations from the Feedback in Realistic
Environments (FIRE) project. For the first time, we use a stellar evolution
model that includes a physically and observationally motivated treatment of
binaries (the BPASS model). Binary mass transfer and mergers enhance the
population of massive stars at late times (>3 Myr) after star formation, which
in turn strongly enhances the late-time ionizing photon production (especially
at low metallicities). These photons are produced after feedback from massive
stars has carved escape channels in the ISM, and so efficiently leak out of
galaxies. As a result, the time-averaged "effective" escape fraction (ratio of
escaped ionizing photons to observed 1500 A photons) increases by factors 4-10,
sufficient to explain reionization. While important uncertainties remain, we
conclude that binary evolution may be critical for understanding the ionization
of the Universe.
