Precise limits on cosmological variability of the fine-structure constant with zinc and chromium quasar absorption lines

TL;DR

This study uses high-quality quasar spectra to place the most precise limits yet on possible variations of the fine-structure constant over cosmological time, finding no significant change within a few parts per million.

Contribution

First to provide a large, systematic-error-resistant measurement of $ ablarac{ ext{alpha}}{ ext{alpha}}$ using Zn and CrII lines in quasar spectra, achieving unprecedented precision.

Findings

No significant variation of $ ablarac{ ext{alpha}}{ ext{alpha}}$ detected.

Measurement precision at the 1-2 ppm level.

Systematic errors dominated by short-range distortions.

Abstract

The strongest transitions of Zn and CrII are the most sensitive to relative variations in the fine-structure constant ($\Delta\alpha/\alpha$) among the transitions commonly observed in quasar absorption spectra. They also lie within just 40\AA\ of each other (rest frame), so they are resistant to the main systematic error affecting most previous measurements of $\Delta\alpha/\alpha$: long-range distortions of the wavelength calibration. While Zn and CrII absorption is normally very weak in quasar spectra, we obtained high signal-to-noise, high-resolution echelle spectra from the Keck and Very Large Telescopes of 9 rare systems where it is strong enough to constrain $\Delta\alpha/\alpha$ from these species alone. These provide 12 independent measurements (3 quasars were observed with both telescopes) at redshifts 1.0--2.4, 11 of which pass stringent reliability criteria. These 11 are all consistent with $\Delta\alpha/\alpha=0$ within their individual uncertainties of 3.5--13 parts per million (ppm), with a weighted mean $\Delta\alpha/\alpha = 1.2\pm1.7_{\rm stat}\pm0.9_{\rm sys}$ ppm (1$\sigma$ statistical and systematic uncertainties), indicating no significant cosmological variations in $\alpha$. This is the first statistical sample of absorbers that is resistant to long-range calibration distortions (at the $<$1 ppm level), with a precision comparable to previous large samples of $\sim$150 (distortion-affected) absorbers. Our systematic error budget is instead dominated by much shorter-range distortions repeated across echelle orders of individual spectra.