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Cosmological observations in the new millennium have dramatically increasedour understanding of the Universe, but several fundamental questions remainunanswered. This topical group report describes the best opportunities toaddress these questions over the coming decades by extending observations tothe $z<6$ universe. The greatest opportunity to revolutionize our understandingof cosmic acceleration both in the modern universe and the inflationary epochwould be provided by a new Stage V Spectroscopic Facility (Spec-S5) which wouldcombine a large telescope aperture, wide field of view, and high multiplexing.Such a facility could simultaneously provide a dense sample of galaxies atlower redshifts to provide robust measurements of the growth of structure atsmall scales, as well as a sample at redshifts $2<z<5$ to measure cosmicstructure at the largest scales, spanning a sufficient volume to probeprimordial non-Gaussianity from inflation, to search for features in theinflationary power spectrum on a broad range of scales, to test dark energymodels in poorly-explored regimes, and to determine the total neutrino mass andeffective number of light relics. A number of compelling opportunities atsmaller scales should also be pursued alongside Spec-S5. The sciencecollaborations analyzing DESI and LSST data will need funding for a variety ofactivities, including cross-survey simulations and combined analyses. Theresults from these experiments can be greatly improved by smaller programs toobtain complementary data, including follow-up studies of supernovae andspectroscopy to improve photometric redshift measurements. The best future useof the Vera C. Rubin Observatory should be evaluated later this decade afterthe first LSST analyses have been done. Finally, investments in pathfinderprojects could enable powerful new probes of cosmology to come online in futuredecades.

Snowmass2021 Cosmic Frontier: Report of the CF04 Topical Group on Dark Energy and Cosmic Acceleration in the Modern Universe