Unchanged in over 12 billion years

OBSERVATIONS of a distant quasar have revealed that the mass ratio of the proton to the electron has not changed over 12 billion years. This work was published in a recent issue of “Physical Review Letters”.

The standard model of physics is built on the fundamental constants of nature. But the model does not require that there should be no variation at all in their values over time. For instance, the proton-electron mass ratio could be changing in response to time dependence in the coupling strengths of the strong and electromagnetic forces. Certain theories predict that the dark energy that accelerates the universe’s expansion is a field that evolves over cosmological time. This could mean that certain fundamental quantities related to forces and masses were different in the infant universe.

For the proton-electron mass ratio, astronomers look for unexpected shifts in the absorption wavelengths of certain molecules. Most molecules can only be seen in relatively nearby objects, but hydrogen (H ₂ ) is abundant and can be observed to great distances. A research team led by Wim Ubachs of VU University Amsterdam, the Netherlands, analysed the spectrum of a very distant quasar (J1443+2724) and identified H absorption lines from a galaxy in front of the quasar. This absorption signal was etched into the spectrum when the universe was just 1.5 billion years old. The lines showed no shift (beyond the normal redshift) compared with values measured on the earth, allowing the authors to place an upper bound of a few parts per million on a varying proton-electron mass ratio.

The results imply that a dark energy scalar field, if it exists, has evolved very little over 90 per cent of the age of the universe, nor have the coupling constants of the electromagnetic and strong force changed over this cosmological time scale.