[Paper Review] Evidence for spatial variation of the fine structure constant

John K. Webb, Julian A. King+4 more|arXiv (Cornell University)|Aug 23, 2010

Astronomy and Astrophysical Research29 citations

TL;DR

This study presents evidence for spatial variation in the fine structure constant (α), based on Keck and VLT telescope data showing opposing trends: α appears smaller at high redshift in the Keck direction and larger in the VLT direction. The combined dataset reveals a spatial dipole in α with 4.2-sigma significance, pointing toward right ascension 17.5 ± 0.9 hours and declination -58 ± 9 degrees, with no detected systematic effects explaining the pattern.

ABSTRACT

We previously reported Keck telescope observations suggesting a smaller value of the fine structure constant, alpha, at high redshift. New Very Large Telescope (VLT) data, probing a different direction in the universe, shows an inverse evolution; alpha increases at high redshift. Although the pattern could be due to as yet undetected systematic effects, with the systematics as presently understood the combined dataset fits a spatial dipole, significant at the 4.2-sigma level, in the direction right ascension 17.5 +/- 0.9 hours, declination -58 +/- 9 degrees. The independent VLT and Keck samples give consistent dipole directions and amplitudes, as do high and low redshift samples. A search for systematics, using observations duplicated at both telescopes, reveals none so far which emulate this result.

Motivation & Objective

• To investigate potential spatial variation in the fine structure constant (α) across the sky using high-redshift quasar absorption spectra.
• To test whether discrepancies between Keck and VLT observations could be attributed to instrumental or systematic effects.
• To determine if the observed trend in α is consistent across different redshift ranges and sky directions.
• To assess the statistical significance of a spatial dipole in α and evaluate its robustness against known systematics.

Proposed method

• Analyzing high-resolution quasar absorption spectra from the Keck and Very Large Telescope (VLT) to measure shifts in atomic transition lines.
• Using the relative redshifts of different ion transitions to infer variations in α at high redshift (z > 1).
• Combining Keck and VLT datasets to test for spatial patterns in α, particularly a dipole anisotropy.
• Applying statistical modeling to quantify the significance of the observed dipole, assuming a dipole model with amplitude and direction parameters.
• Conducting a systematic error check by analyzing overlapping observations from both telescopes to rule out instrumental or calibration artifacts.
• Using chi-squared and likelihood ratio tests to compare the dipole model against isotropic and null models.

Experimental results

Research questions

• RQ1 Is there evidence for spatial variation in the fine structure constant across the observable universe?
• RQ2 Do the Keck and VLT datasets show consistent directional trends in α variation?
• RQ3 Could the observed dipole in α be explained by undetected systematic errors in the data or instrumentation?
• RQ4 What is the statistical significance of the observed spatial dipole in α?
• RQ5 Do high-redshift and low-redshift samples yield consistent results in the α variation pattern?

Key findings

• The combined Keck and VLT datasets show a spatial dipole in the fine structure constant with a significance of 4.2-sigma.
• The dipole direction is estimated at right ascension 17.5 ± 0.9 hours and declination -58 ± 9 degrees.
• The Keck data suggest a decrease in α at high redshift, while the VLT data indicate an increase, consistent with a dipole pattern.
• High- and low-redshift samples from both telescopes yield consistent dipole directions and amplitudes.
• No systematic effects—identified through duplicated observations at both telescopes—were found to explain the observed α variation.
• The dipole pattern remains robust under current systematics analysis, suggesting a possible cosmological origin for α variation.