Spatial variation in the fine-structure constant - new results from VLT/UVES

Abstract

Quasar absorption lines provide a precise test of whether the fine‐structure constant, α, is the same in different places and through cosmological time. We present a new analysis of a large sample of quasar absorption‐line spectra obtained using the Ultraviolet and Visual Echelle Spectrograph (UVES) on the Very Large Telescope (VLT) in Chile. We apply the many‐multiplet method to derive values of Δα/α≡ (αz−α0)/α0 from 154 absorbers, and combine these values with 141 values from previous observations at the Keck Observatory in Hawaii. In the VLT sample, we find evidence that α increases with increasing cosmological distance from Earth. However, as previously shown, the Keck sample provided evidence for a smaller α in the distant absorption clouds. Upon combining the samples, an apparent variation of α across the sky emerges which is well represented by an angular dipole model pointing in the direction RA = 17.3 ± 1.0 h and Dec. =−61°± 10°, with amplitude . The dipole model is required at the 4.1σ statistical significance level over a simple monopole model where α is the same across the sky (but possibly different from the current laboratory value). The data sets reveal remarkable consistencies: (i) the directions of dipoles fitted to the VLT and Keck samples separately agree; (ii) the directions of dipoles fitted to z < 1.6 and z > 1.6 cuts of the combined VLT+Keck samples agree; and (iii) in the equatorial region of the dipole, where both the Keck and VLT samples contribute a significant number of absorbers, there is no evidence for inconsistency between Keck and VLT. The amplitude of the dipole is clearly larger at higher redshift. Assuming a dipole‐only (i.e. no‐monopole) model whose amplitude grows proportionally with ‘lookback‐time distance’ (r=ct, where t is the lookback time), the amplitude is (1.1 ± 0.2) × 10−6 GLyr−1 and the model is significant at the 4.2σ confidence level over the null model (Δα/α≡ 0). We apply robustness checks and demonstrate that the dipole effect does not originate from a small subset of the absorbers or spectra. We present an analysis of systematic effects, and are unable to identify any single systematic effect which can emulate the observed variation in α. To the best of our knowledge, this result is not in conflict with any other observational or experimental result.

The UVES Large Program for testing fundamental physics – III. Constraints on the fine-structure constant from three telescopes

“…The fitting approach followed that of, e. g., Murphy et al (2003), King et al (2012), and Molaro et al (2013) except that we construct our fits using the blinded spectra (see the description of the blinding procedure in Section 4). The first step in the fitting process was to model the spectra with multiple components for each absorption system.…”

“…When King et al (2012) combined their VLT measurements with the Murphy et al (2004) Keck ones, they found internally consistent evidence for a dipolar variation in α across the sky. In order to have any confidence in this result, we must be assured that the α-dipole is the result of physics varying in different parts of the universe and not the result of systematic errors between observations and/or spectrographs.…”

“…The most surprising ∆α/α results come from a sample of 143 Keck/HIRES-observed absorption systems (Murphy et al 2003;Murphy et al 2004) and 154 VLT/UVES-observed absorption systems (Webb et al 2011;King et al 2012). When King et al (2012) combined their VLT measurements with the Murphy et al (2004) Keck ones, they found internally consistent evidence for a dipolar variation in α across the sky.…”

“…Their 'Direct Comparison' (DC) method can detect velocity distortions between pairs of spectra with a reliable measure of uncertainty in a quick, model independent fashion. Advantages over the King et al (2012) fitting approach include automatization and the inclusion of a greater number of absorption features, which gives a smaller uncertainty on the velocity shifts or distortions found between the spectra. Unlike the asteroid approach, or the similar iodine cell approach of Griest et al (2010) and Whitmore et al (2010), the DC method does not rely on a reference spectrum and cannot provide absolute distortion information.…”

“…King et al (2012) measured velocity shifts between 7 pairs of spectra from Keck and VLT using a Voigt profile fitting approach. Even though only some spectral lines are useful for measuring ∆α/α, any well-defined feature contains information about long-range velocity distortions present, thereby breaking the degeneracy.…”