We present a new bound on the ultralight axion (ULA) dark matter mass $m_{\mathrm{a}}$, using the Lyman-alpha forest to look for suppressed cosmic structure growth: a 95% lower limit $m_{\mathrm{a}}>2\times {10}^{-20}\mathrm{eV}$. This strongly disfavors ($>99.7\%$ credibility) the canonical ULA with ${10}^{-22}\mathrm{eV}<m_{\mathrm{a}}<{10}^{-21}\mathrm{eV}$, motivated by the string axiverse and solutions to possible tensions in the cold dark matter model. We strengthen previous equivalent bounds by about an order of magnitude. We demonstrate the robustness of our results using an optimized emulator of improved hydrodynamical simulations.

• Received 7 August 2020
• Revised 18 November 2020
• Accepted 22 December 2020

DOI:https://doi.org/10.1103/PhysRevLett.126.071302

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Published by the American Physical Society