  Recent work has shown that the z~2.5 Lyman-alpha forest on large scales
encodes information about the galaxy and quasar populations that keep the
intergalactic medium photoionized. We present the first forecasts for
constraining the populations with data from current and next-generation
surveys. At a minimum the forest should tell us whether galaxies or,
conversely, quasars dominate the photon production. The number density and
clustering strength of the ionising sources might be estimated to sub-10%
precision with a DESI-like survey if degeneracies (e.g., with the photon
mean-free-path, small-scale clustering power normalization and potentially
other astrophysical effects) can be broken by prior information. We demonstrate
that, when inhomogeneous ionisation is correctly handled, constraints on dark
energy do not degrade.
