Abstract
The peculiar value of θ is a challenge to the notion of an anthropic landscape. We briefly review the possibility that a suitable axion might arise from an anthropic requirement of dark matter. We then consider an alternative suggestion of Kaloper and Terning that θ might be correlated with the cosmological constant. We note that in a landscape one expects that θ is determined by the expectation value of one or more axions. We discuss how a discretuum of values of θ might arise with an energy distribution dominated by QCD, and find the requirements to be quite stringent. Given such a discretuum, we find no circumstances where small θ might be selected by anthropic requirements on the cosmological constant.
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Dine, M., Haskins, L.S., Ubaldi, L. et al. Some remarks on anthropic approaches to the strong CP problem. J. High Energ. Phys. 2018, 171 (2018). https://doi.org/10.1007/JHEP05(2018)171
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DOI: https://doi.org/10.1007/JHEP05(2018)171