Dimensionless constants and cosmological measurements

TL;DR

This paper discusses how cosmological measurements can only determine dimensionless combinations of fundamental constants, using methods like BAO and supernova observations, and explains the underlying reason related to the expansion of distances and redshift.

Contribution

It demonstrates how pairs of cosmological measurements can isolate information on dimensionless constants, clarifying the fundamental limits of what such measurements can reveal.

Findings

Measurements depend only on dimensionless constants

Distance measurements like BAO and supernova probe these constants

Distances expand with the same function as photon wavelength redshift

Abstract

The laws of physics have a set of fundamental constants, and it is generally admitted that only dimensionless combinations of constants have physical significance. These combinations include the electromagnetic and gravitational fine structure constants, $\alpha=e^2/4\pi\epsilon_0\hbar c$ and $\alpha_G=Gm_p^2/\hbar c$, along with the ratios of elementary-particles masses. Cosmological measurements clearly depend on the values of these constants in the past and can therefore give information on their time dependence if the effects of time-varying constants can be separated from the effects of cosmological parameters. The latter can be eliminated by using pairs of redundant measurements and here we show how such pairs conspire to give information only on dimensionless combinations of constants. Among other possibilities, we will use distance measurements based on Baryon Acoustic Oscillations (BAO) and on type Ia supernova. The fact that measurements yield information only on dimensionless combinations is traced to the fact that distances between co-moving points expand following the same function of time that governs the redshift of photon wavelengths.