Beyond $Λ$CDM: fundamental constants as cosmological observables

TL;DR

This work argues that testing the constancy of the fine-structure constant $α$ offers a direct, non-gravitational probe of physics beyond $Λ$CDM by constraining potential electromagnetism–dark-sector couplings. It reviews two principal spectroscopic approaches—the alkali doublet and the many multiplet methods—showing that $Δα/α$ can be constrained from $∼10^{-4}$ (AD) to $∼10^{-6}$ per system (MM), with future surveys poised to dramatically increase both the number and quality of measurements. The proposed 2040s program with the Wide-field Spectroscopic Telescope (WST) could deliver ~100,000 MM measurements at $R≈40{,}000$ and ~1 million AD measurements at $z<1$, enabling tomographic mapping of $α$ and cross-checks with large-scale structure and CMB maps. Realizing this potential requires superb wavelength calibration (e.g., laser frequency combs, Fabry-Pérot etalons) and fully automated, bias-aware data analysis pipelines (e.g., AI-VPFIT) to exploit the large, homogeneous datasets and produce robust constraints or detections of $α$ variation.

Abstract

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.

Figures (1)

• Figure 1: $\alpha$ measurements in the 2040s. Left: a comparison of the number of $\Delta\alpha/\alpha$ measurements from two single-object spectrographs (ESPRESSO and ANDES), as well as the WST programmes targeting emission line galaxies (ELG) and quasars (QSO) in this proposal. Instrumental systematics limit DESI measurements to $|\Delta\alpha/\alpha|<10^{-4}$, so it is not shown. The WST ELGs are the only ones covering $z<1$ and the WST QSO sample will represent an unprecedented jump in the number of measurements. Top right: the expected statistical uncertainty on $\Delta\alpha/\alpha$ for a single measurement. Bottom right: the final statistical uncertainty on $\Delta\alpha/\alpha$ from the full measurement sample.