Fundamental constants and high‐resolution spectroscopy

Abstract

Absorption-line systems detected in high resolution quasar spectra can be used to compare the value of dimensionless fundamental constants such as the fine-structure constant, α , and the proton-to-electron mass ratio, µ = mp/me, as measured in remote regions of the Universe to their value today on Earth. In recent years, some evidence has emerged of small temporal and also spatial variations in α on cosmological scales which may reach a fractional level of ≈ 10 ppm (parts per million). We are conducting a Large Programme of observations with the Very Large Telescope's Ultraviolet and Visual Echelle Spectrograph (UVES), and are obtaining high-resolution (R ≈ 60 000) and high signal-to-noise ratio (S/N ≈ 100) spectra calibrated specifically to study the variations of the fundamental constants. We here provide a general overview of the Large Programme and report on the first results for these two constants, discussed in detail in Molaro et al. and Rahmani et al.A stringent bound for ∆α/α is obtained for the absorber at z abs = 1.6919 towards HE 2217-2818. The absorption profile is complex with several very narrow features, and is modeled with 32 velocity components. The relative variation in α in this system is +1.3 ± 2.4stat ± 1.0sys ppm if Al II λ 1670Å and three Fe II transitions are used, and +1.1 ±2.6stat ppm in a slightly different analysis with only Fe II transitions used. This is one of the tightest bounds on α-variation from an individual absorber and reveals no evidence for variation in α at the 3-ppm precision level (1-σ confidence). The expectation at this sky position of the recently-reported dipolar variation of α is (3.2-5.4) ± 1.7 ppm depending on dipole model used and this constraint of ∆α/α at face value is not supporting this expectation but not inconsistent with it at the 3σ level. For the proton-to-electron mass ratio the analysis of the H2 absorption lines of the z abs ≈ 2.4018 damped Lyα system towards HE 0027-1836 provides ∆µ/µ = (−7.6 ± 8.1stat ± 6.3sys) ppm which is also consistent with a null variation. The cross-correlation analysis between individual exposures taken over three years and comparison with almost simultaneous asteroid observations revealed the presence of a possible wavelength dependent velocity drift as well as of inter-order distortions which probably dominate the systematic error and are a significant obstacle to achieve more accurate measurements.

Planckintermediate results

“…The claim that the fine structure constant may have been smaller in the past (Webb et al 2001), drawn from observations of specific quasar absorption spectra by the Keck telescope, has not been confirmed by independent studies. All VLT observations of quasar absorption spectra (Srianand et al 2004(Srianand et al , 2007 and observations of molecular absorption lines (Kanekar et al 2005), are compatible with no variation (see also Bonifacio et al 2014, and references therein). Despite the lack of definitive empirical evidence of any constants having a different value in the past, there are many theoretical ideas to motivate the continued search for variations in cosmological data.…”

Fine-structure constant constraints on Bekenstein-type models

“…(See [10] for a brief overview of ongoing research.) In particular, there have been recent indications of spacetime variations of α at the level of a few parts per million, in the approximate redshift range 1 < z < 4.…”

“…In recent years this has proven to be a very active area of observational research. In addition to the theoretical motivation to carry out these tests, there has been some recent evidence for such a variation [7], and a significant effort is being put into independently confirming this result, including through a dedicated VLT/UVES Large Program [8][9][10].…”

The status of varying constants: a review of the physics, searches and implications

“…These are particularly useful for testing for hypothetical dependencies on look-back time. A more detailed description of the Large Program sample may be found in [55].…”