  Cosmological observations in the new millennium have dramatically increased
our understanding of the Universe, but several fundamental questions remain
unanswered. This topical group report describes the best opportunities to
address these questions over the coming decades by extending observations to
the $z<6$ universe. The greatest opportunity to revolutionize our understanding
of cosmic acceleration both in the modern universe and the inflationary epoch
would be provided by a new Stage V Spectroscopic Facility (Spec-S5) which would
combine a large telescope aperture, wide field of view, and high multiplexing.
Such a facility could simultaneously provide a dense sample of galaxies at
lower redshifts to provide robust measurements of the growth of structure at
small scales, as well as a sample at redshifts $2<z<5$ to measure cosmic
structure at the largest scales, spanning a sufficient volume to probe
primordial non-Gaussianity from inflation, to search for features in the
inflationary power spectrum on a broad range of scales, to test dark energy
models in poorly-explored regimes, and to determine the total neutrino mass and
effective number of light relics. A number of compelling opportunities at
smaller scales should also be pursued alongside Spec-S5. The science
collaborations analyzing DESI and LSST data will need funding for a variety of
activities, including cross-survey simulations and combined analyses. The
results from these experiments can be greatly improved by smaller programs to
obtain complementary data, including follow-up studies of supernovae and
spectroscopy to improve photometric redshift measurements. The best future use
of the Vera C. Rubin Observatory should be evaluated later this decade after
the first LSST analyses have been done. Finally, investments in pathfinder
projects could enable powerful new probes of cosmology to come online in future
decades.
