  We forecast the expected cosmological constraints from a combination of
probes of both the universal expansion rate and matter perturbation growth, in
the form of weak lensing tomography, galaxy tomography, supernovae, and the
cosmic microwave background incorporating all cross-correlations between the
observables for an extensive cosmological parameter set. We allow for non-zero
curvature and parameterize our ignorance of the early universe by allowing for
a non-negligible fraction of dark energy (DE) at high redshifts. We find that
early DE density can be constrained to 0.2% of the critical density of the
universe with Planck combined with a ground-based LSST-like survey, while
curvature can be constrained to 0.06%. However, these additional degrees of
freedom degrade our ability to measure late-time dark energy and the sum of
neutrino masses. We find that the combination of cosmological probes can break
degeneracies and constrain the sum of neutrino masses to 0.04 eV, present DE
density also to 0.2% of the critical density, and the equation of state to 0.01
- roughly a factor of two degradation in the constraints overall compared to
the case without allowing for early DE. The constraints for a space-based
mission are similar. Even a modest 1% dark energy fraction of the critical
density at high redshift, if not accounted for in future analyses, biases the
cosmological parameters by up to 2 sigma. Our analysis suggests that throwing
out nonlinear scales (multipoles > 1000) may not result in significant
degradation in future parameter measurements when multiple cosmological probes
are combined. We find that including cross-correlations between the different
probes can result in improved constraints by up to a factor of 2 for the sum of
neutrino masses and early dark energy density.
