In this white paper, we present the MegaMapper concept. The MegaMapper is a proposed ground-based experiment to measure Inflation parameters and Dark Energy from galaxy redshifts at 2 < z < 5. In order to achieve path-breaking results with a mid-scale investment, the MegaMapper combines existing technologies for critical path elements and pushes innovative development in other design areas. To this aim, we envision a 6.5-m Magellan-like telescope, with a newly designed wide field, coupled with DESI spectrographs, and small-pitch robots to achieve multiplexing of 26,100. This will match the expected achievable target density in the redshift range of interest and provide a 15x capability over the existing state-of the art, without a 15x increase in project budget.

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The mechanism(s) driving the early-and late-time accelerated expansion of the Universe represent one of the most compelling mysteries in fundamental physics today. The path to understanding the causes of early-and late-time acceleration depends on fully leveraging ongoing surveys, developing and demonstrating new technologies, and constructing and operating new instruments. This report presents a multi-faceted vision for the cosmic survey program in the 2030s and beyond that derives from these considerations. Cosmic surveys address a wide range of fundamental physics questions, and are thus a unique and powerful component of the HEP experimental portfolio.Wide-field surveys in the optical/near-infrared have played a critical role in establishing the standard model of cosmology, ΛCDM. We strongly advocate for continuing this extremely successful program into the coming decade and beyond.Regarding photometric imaging surveys, the HEP community sees three options for Rubin Observatory beyond LSST, each of which would require different investments with costs and