  A new generation of powerful dark energy experiments will open new vistas for
cosmology in the next decade. However, these projects cannot reach their utmost
potential without data from other telescopes. This white paper focuses in
particular on the compelling benefits of ground-based spectroscopic and
photometric observations to complement the Vera C. Rubin Observatory, as well
as smaller programs in aid of a DESI-2 experiment and CMB-S4. These additional
data sets will both improve dark energy constraints from these flagship
projects beyond what would possible on their own and open completely new
windows into fundamental physics. For example, additional photometry and
single-object spectroscopy will provide necessary follow-up information for
supernova and strong lensing cosmology, while highly-multiplexed spectroscopy
both from smaller facilities over wide fields and from larger facilities over
narrower regions of sky will yield more accurate photometric redshift estimates
for weak lensing and galaxy clustering measurements from the Rubin Observatory,
provide critical spectroscopic host galaxy redshifts for supernova Hubble
diagrams, provide improved understanding of limiting astrophysical systematic
effects, and enable new measurements that probe the nature of gravity. A common
thread is that access to complementary data from a range of
telescopes/instruments would have a substantial impact on the rate of advance
of dark energy science in the coming years.
