Identify additional cosmic epochs compatible with appreciable axion contributions

Determine at which epochs between recombination and late-time acceleration, and more generally throughout cosmic history, an additional axion can contribute appreciably to the expansion while remaining consistent with observations, including the dependence on the transient’s amplitude and duration.

Background

The proposed axion ladder contains selected contributions during BBN, near matter–radiation equality, and the late-time accelerated era. The authors ask whether other periods—particularly the interval between recombination and the onset of late-time acceleration—could support additional transient axion energy density without violating constraints from distances, structure growth, CMB lensing, high-redshift BAO, the Lyman-alpha forest, and other probes.

The paper introduces a phenomenological localized expansion profile with adjustable central redshift, width, and amplitude. Scanning this profile would establish observational upper bounds on transient expansion enhancements as a function of epoch and duration. For a specified axion potential, those bounds could then be translated into constraints on the curvature scale, decay constant, and initial angle.

References

The ladder hypothesis also raises a broader question: at what other epochs could an axion contribute appreciably to the expansion? Rather than choosing those epochs from existing observational tensions, we can ask how much additional expansion observations permit throughout cosmic history.

— A Multi-Axion Ladder Across Cosmic History: From Inflation, BBN, and Early Dark Energy to Late-Time Accelerated Expansion  (2609.11907 - Sengupta et al., 10 Sep 2026) in Section 6.4, “Searching for additional rungs”; reiterated in Section 7, Discussion and conclusions