The value of the fine structure constant over cosmological times

TL;DR

This study measures the fine structure constant over cosmological timescales using SDSS QSO spectra, finding no significant variation and improving previous constraints by a factor of five.

Contribution

It provides the most precise constraints to date on the variation of the fine structure constant using [OIII] lines in QSO spectra over redshifts 0-0.8.

Findings

No evidence of change in alpha over cosmological timescales.

Improved constraints on alpha variation by a factor of five.

Limited usefulness of [NeIII] and [SII] doublets due to faintness and systematics.

Abstract

The optical spectra of objects classified as QSOs in the SDSS DR6 are analyzed with the aim of determining the value of the fine structure constant in the past and then check for possible changes in the constant over cosmological timescales. The analysis is done by measuring the position of the fine structure lines of the [OIII] doublet (4959 and 5008) in QSO nebular emission. From the sample of QSOs at redshifts z < 0.8 a subsample was selected on the basis of the amplitude and width of the [OIII] lines. Two different method were used to determine the position of the lines of the [OIII] doublet, both giving similar results. Using a clean sample containing 1568 of such spectra, a value of Delta alpha /alpha=(+2.4 +-2.5) x 10^{-5} (in the range of redshifts 0-0.8) was determined. The use of a larger number of spectra allows a factor ~5 improvement on previous constraints based on the same method. On the whole, we find no evidence of changes in alpha on such cosmological timescales. The mean variation compatible with our results is 1/ <t> Delta alpha/alpha=(+0.7 +- 0.7) x 10^{-14} yr^{-1}. The analysis was extended to the [NeIII] and [SII] doublets, although their usefulness is limited due to the fact that all these doublets in QSOs tend to be fainter than [OIII], and that some of them are affected by systematics.