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The axion is a hypothetical, well-motivated dark-matter particle whose existence would explain the lack of charge-parity violation in the strong interaction. In addition to this original motivation, an `axiverse' of ultra-light axions (ULAs) with masses $10^{-33}\,{\rm eV}\lesssim m_{\rm a}\lesssim 10^{-10}\,{\rm eV}$ also emerges from string theory. Depending on the mass, such a ULA contributes to the dark-matter density, or alternatively, behaves like dark energy. At these masses, ULAs' classical wave-like properties are astronomically manifested, potentially mitigating observational tensions within the $\Lambda$CDM paradigm on local-group scales. ULAs also provide signatures on small scales such as suppression of structure, interference patterns and solitons to distinguish them from heavier dark matter candidates. Through their gravitational imprint, ULAs in the presently allowed parameter space furnish a host of observational tests to target in the next decade, altering standard predictions for microwave background anisotropies, galaxy clustering, Lyman-$\alpha$ absorption by neutral hydrogen along quasar sightlines, pulsar timing, and the black-hole mass spectrum., Comment: 5 pages, 1 figure, Astro2020 Decadal Survey science white paper

Chakrabarty, Sankha "Duration of Classicality of Homogeneous Condensateswith Attractive Interactions" in Proceedings of the 13th "Patras" Workshop on Axions, WIMPs and WISPs, PATRAS 2017 / Maroudas, Marios (eds.), Verlag Deutsches Elektronen-Synchrotron : 2018 ; Patras 2017 : 13th Patras Workshop on Axions, WIMPs and WISPs, 2017-05-15 - 2017-05-19, Thessaloniki 13th Patras Workshop on Axions, WIMPs and WISPs, AXION-WIMP 2017, Thessaloniki, Greece, 15 May 2017 - 19 May 2017; Hamburg : Verlag Deutsches Elektronen-Synchrotron, DESY-PROC, 231-235 (2018). doi:10.3204/DESY-PROC-2017-02/sikivie_pierre, Dark matter axions and other highly degenerate bosonic fluids are commonly described by classical field equations. In a recent paper \cite{BECprop} we calculated the duration of classicality of homogeneous condensates with attractive contact interactions and of self-gravitating homogeneous condensates in critical expansion. According to their classical equations of motion, such condensates persist forever. In their quantum evolution parametric resonance causes quanta to jump in pairs out of the condensate into all modes with wavevector less than some critical value. We estimated in each case the time scale over which the condensate is depleted and after which a classical description is invalid., Published by Verlag Deutsches Elektronen-Synchrotron, Hamburg

Banik, Nilanjan "The Rethermalizing Bose-Einstein Condensate ofDark Matter Axions" in Proceedings, 11th Patras Workshop on Axions, WIMPs and WISPs (Axion-WIMP 2015) / Irastorza, Igor G., Redondo, Javier, Carmona, José Manuel, Cebrian, Susana, Dafni, Theopisti, Iguaz, Francisco J., Luzon, Gloria (eds.), Verlag Deutsches Elektronen-Synchrotron : 2015 ; AXION-WIMP 2015 : 11th Patras Workshop on Axions, WIMPs and WISPs, 2015-06-22 - 2015-06-26, Zaragoza 11th Patras Workshop on Axions, WIMPs and WISPs, AXION-WIMP 2015, Zaragoza, Spain, 22 Jun 2015 - 26 Jun 2015 ; DESY-PROC 134-139(2015). doi:10.3204/DESY-PROC-2015-02/sikivie_pierre, The axions produced during the QCD phase transition by vacuum realignment, string decay and domain wall decay thermalize as a result of their gravitational self-interactions when the photon temperature is approximately 500 eV. They then form a Bose-Einstein condensate (BEC). Because the axion BEC rethermalizes on time scales shorter than the age of the universe, it has properties that distinguish it from other forms of cold dark matter. The observational evidence for caustic rings of dark matter in galactic halos is explained if the dark matter is axions, at least in part, but not if the dark matter is entirely WIMPs or sterile neutrinos., Published by Verlag Deutsches Elektronen-Synchrotron, Hamburg

Contribution to Proceedings, Proc. of PATRAS08; DESY-PROC-2008-02; ISSN 1435-8077