ALAverse: A falsifiable anthropic model from the string landscape

Abstract

We combine the axiverse with a negative cosmological constant $\Lambda$, both originating from the string landscape, and apply an observation-time-weighted anthropic argument. Adopting a uniform prior on $\Lambda$ and a typical string-motivated axion decay constant comparable to the reduced Planck scale, this framework predicts a $\sim 40\%$ probability of observing $0.1 < \Omega_m < 0.9$, thereby naturally resolving the long-standing fine-tuning and coincidence problems of dark energy. Unlike many scalar-field dark energy models, the anthropic Lambda-axion universe (ALAverse) statistically disfavors slow-roll dynamics, as slow-roll requires fine-tuning of the initial field displacement. Moreover, the negative cosmological constant renders fast-roll scenarios anthropically unfavorable, since they typically yield only a very brief observational window with positive dark energy density. Having ruled out both extremes, the ALAverse characteristically predicts a moderate-roll dynamics. We derive a two-parameter parametrization in terms of $\delta_\Omega$ and $|\varepsilon_s|$ that covers ALAverse solutions as well as a broad class of canonical and phantom field models. Current observational data yield $\delta_\Omega = -0.0498 \pm 0.0186$, corresponding to a $2.7\sigma$ rejection of $\Lambda$CDM ($\delta_\Omega = 0$) and phantom models ($\delta_\Omega > 0$). The data also show a mild preference for the ALAverse over slow-roll quintessence, a trend that can be conclusively tested with future high-precision measurements of the Hubble diagram.