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Axiverse Lampposts
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The string axiverse predicts a unique connection between the high scales approachable only through theory and the low energies within reach of experimental verification: a multitude of light, feebly interacting axions. In order to capture the collective effects of such an axion ensemble, we model the string axiverse by $N$ coupled axions with a simple assumption: hierarchical axion masses that arise from hierarchical instantons with statistically distributed axion couplings. In this limit, we find that axion field ranges, which determine late-time cosmological abundances, shrink as $1/\sqrt{N}$ as the number of axions grows. Moreover, the heaviest modes tend to align with the smallest kinetic eigenvalues, further reducing their field ranges. Interactions with the Standard Model (SM) are largely set by the kinetic structure and do not grow with $N$, thus suppressing detection prospects relative to the individual-axion expectation. The main exception is the QCD axion, whose coupling is tied to its potential and is therefore unsuppressed. The heaviest and lightest axions can also avoid the typical suppression in certain limits. We further find that coupled axiverse dark matter has parametrically relaxed tuning on initial conditions when produced via long, low-scale inflation relative to independent axions and high-scale inflation. Taken together, these results sharpen the observational outlook: the most accessible signals typically come from the QCD axion and from heavy axions that make up small dark matter subcomponents. An anthropic plateau of comparable energy density states produces subdominant signals; meanwhile, if light axions have SM interactions independent of QCD, they can also be within reach of future direct-detection experiments.
Forward citations
Cited by 7 Pith papers
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Heterotic String Theory Suggests a QCD Axion Near 0.5 neV
Heterotic string theory implies the QCD axion mass is bounded below by 0.5 neV and typically falls in [0.5, 0.8] neV across most compactifications.
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Fuzzy Dark Matter Halo Mass Functions at Cosmic Dawn
New simulation-based fitting formula for FDM halo mass functions at z=6-11, with ~30% weaker suppression than earlier formulas for m c²=10^{-21} eV at M~3e9 solar masses.
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Testing F-theory GUTs with the Axiverse
In F-theory GUTs, non-universal ALPs induced by hypercharge flux satisfy g_aγ/m_a well below the QCD axion prediction when gauge couplings unify near the string scale.
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The structure of multi-axion solutions to the strong CP problem
Multi-axion solutions to the strong CP problem produce varied mass-coupling patterns set by PQ symmetry breaking structure and QCD-EM anomaly alignment, summarized by a general sum rule for N-axion systems.
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The structure of multi-axion solutions to the strong CP problem
Multi-axion theories solving the strong CP problem produce varied mass-coupling relations via a general sum rule that depends on the details of PQ symmetry breaking and anomaly alignments.
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String Axiverse Enhancement of Superradiant Dark Matter Production
O(100-10^5) string axions enhance PBH superradiance efficiency via increased spin, expanding viable mass-spin regions for micro-boson star dark matter while too many axions cause overly rapid evaporation.
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Stochastic Axion Mixing: A General Mechanism Beyond Decay Constant Constraints
Stochastic axion mixing is a broad mechanism for axion interactions in multi-axion systems that occurs naturally under distinct mass conditions and does not depend on decay constant hierarchies.
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