Beyond $Λ$CDM: fundamental constants as cosmological observables

Recent cosmological tensions pose difficulties for $Λ$CDM. Forthcoming facilities will be able to check whether these tensions result from systematic effects or indeed with the $Λ$CDM model itself. However, these new data will primarily probe gravitational interactions and provide only limited information about non-gravitational interactions. Distinguishing between competing models that make similar predictions yet rely on fundamentally different principles, therefore requires suitably diverse physical tests. Observational constraints on spacetime variations of fundamental constants fill this need. The fine-structure constant, $α= e^2/\hbar c$, can be measured using absorption systems towards bright quasars using the Many Multiplet method, and using atomic doublets from line emitting gas in galaxies. A spectroscopic facility such as the WST could produce more than 100,000 new measurements of $α$ from quasars together with a million measurements from galaxies. When combined with other probes, such a large and homogeneous dataset of $α$ measurements would provide unprecedented constraints on physics beyond $Λ$CDM.

💡 Research Summary

The paper argues that the growing tensions within the standard ΛCDM cosmology—most notably the H₀ and σ₈ discrepancies—call for observational probes that go beyond purely gravitational measurements. It proposes that precise, large‑scale measurements of the fine‑structure constant α, a dimensionless quantity, can provide a fundamentally different window onto new physics. In any relativistic theory, a spacetime variation of α necessarily implies the existence of a light scalar field that couples to electromagnetism and possibly to the dark sector. Such a field could drive the observed tensions without leaving a strong imprint on the large‑scale structure or distance–redshift relations that upcoming surveys (Rubin Observatory, Euclid, DESI, SKA, CMB experiments) will map.

Theoretical work predicts that spatial perturbations in α evolve as δα/α ∝ (1+z)⁻¹ during the matter‑dominated era, meaning that a primordial fluctuation of order 10⁻⁹ at recombination would be amplified to ≈10⁻⁶ by redshift z≈3. Consequently, a measurement of α at the 10⁻⁶ level at z≈3 already constrains early‑Universe physics more tightly than the temperature anisotropies of the CMB.

Two spectroscopic techniques are reviewed. The “alkali doublet” (AD) method uses forbidden emission doublets such as