Variation of Mp/Me on a cosmological time scale?

Our starting point is the highly accurate study of the hydrogen molecule in the laboratory, with the aim of testing the understanding of molecular structure in the benchmark system: the smallest neutral molecule H2. Recently we have determined accurate values of ionization and dissociation energies of H2 and its isotopomers (HD and D2), which challenge the most advanced theoretical calculations, even including subtle QED-effects. We have calibrated the transition wavelengths of the characteristic Lyman and Werner lines to an accuracy of Dl/l = 5 x 10-9. In addition we study the dissociation dynamics in the excited level structure. For these studies we use advanced lasers, including extreme ultraviolet lasers, as well as the Soleil synchrotron in Paris. 

The H2 molecule, the most abundant molecular species in the universe, can be observed at high redshift, so looking back in time toward the origin of the universe. By comparing molecular spectra (of the Lyman and Werner molecular bands) one can address the question whether the proton-electron mass ratio, a dimensionless fundamental constant (m=1836.1526726[8]) has changed over cosmological times. Such a discovery would have vast implications on fundamental physics theories; for one it would violate Einstein’s Equivalence Principle of general relativity. It would change our perspective on the universe, as well as the perspective of physical law.

We are involved in observing, and analyzing, spectra obtained with the UVES spectrometer (attached to the Very Large Telescope VLT-2 at Paranal, Chile) and with the HIRES spectrometer (attached to the Keck telescope at Mauna Kea, Hawai). Thusfar we have observed some indications for such a change, that request further confirmation, however. 

Ubachs1 Ubachs2

Top: The Pulsed-Dye-Amplifier used to generate narrowband Extreme Ultraviolet radiation for the investigation of hydrogen in the laboratory; Bottom: View on the Very Large Telescope (Paranal, Chile) where quasar spectra were obtained. 

Contact: Wim Ubachs, email:

Physics of light and matter