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Le Guillou, Manasvee Saraf, Ming Zhu, M. Ishak, M. Landriau, M. Manera, M. Schubnell, O. Lahav, P. Doel, Peng Jiang, R. Joyce, R. Kehoe, R. Miquel, S. Ahlen, S. Juneau, S. Nadathur, T. Claybaugh, Wenlin Ma, W. J. Percival, Xiaohu Yang, Yirong Wang, Yizhou Gu, Y. P. Jing, Zhejie Ding, Zheng Zheng","submitted_at":"2026-07-06T17:02:03Z","abstract_excerpt":"The cosmic star formation rate density (CSFRD) has declined sharply toward the present day, but the roles of the atomic and molecular gas reservoirs remain uncertain. We measure the cosmic HI density, $\\Omega_{\\mathrm{HI}}$, over $0<z<0.41$ by combining HI spectra from the Five-hundred-meter Aperture Spherical Telescope with optical spectroscopy from the Dark Energy Spectroscopic Instrument for $\\sim2.5$ million galaxies across $\\sim12,000\\,{\\rm deg}^2$. We measure a raw decrease in $\\Omega_{\\mathrm{HI}}$ by a factor of $1.35\\pm0.10$ over the past 4.5 Gyr. 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