The spectrum of molecular hydrogen with the prominent Lyman and Werner band systems may be used to detect a change in Mp/me over time, because each individual spectral line depends in a different way on this mass ratio. In recent years several observations have been made on the Lyman and Werner bands in quasars, taking advantage of the fact that the Earth's atmosphere is transparent for these spectral regions at high redshift (z), and that ground-based echelle-grating spectrometers can be employed in high-resolution studies.
In previous comparisons the laboratory data of the Lyman and Werner band Tables of the Meudon Observatoire (Paris) were used, which represent an extensive and accurate database. Nevertheless it was emphasized that improved spectroscopic data are desired, because the statistical errors from astronomical and laboratory data contributed equally in the estimates on the variation of Mp/me. This is illustrated by the fact that in previous studies rather different constraints on on the variation of Mp/me were found, when comparing quasar data with different laboratory data sets.
With the use of the XUV-laser system at the Laser Centre VU spectral recordings of about 150 rotational lines in the Lyman and Werner band systems of molecular hydrogen were performed leading to absolute calibrations at the 5 x 10-8 accuracy level. In the figure below such a recording is displayed of the B-X (4,0) R(0) line with the simultaneously recorded I2 spectrum for calibration purposes.
Left: First results of a comparison based on the laboratory data and the then available quasar data (from quasars PKS 0528-250, Q0347-382 and Q1232+082) displayed in the plot on the left.
Right: This study yields a fractional change in the proton-to-electron mass-ratio of (-0.5 +/- 3.6) x 10-5 (at the 2 sigma accuracy level), which would correspond to a temporal change of (-0.4 +/- 3.0 x 10-15 per year (2 sigma) if a linear cosmological expansion model is assumed. These results were published in [Phys. Rev. Lett. 92 (2004) 101302].