The spectrum of a distant quasar reveals no sign of changes in the mass ratio of the proton and the electron over 12 billion years, constraining dark energy theories.

Certain models predict that the dark energy that accelerates the Universe’s expansion is a field that evolves over cosmological times. This could mean that certain fundamental quantities related to forces and masses were different long ago. However, a new analysis of the spectrum from a very distant quasar finds no evidence of deviation in molecular lines produced 12 billion years ago, thus implying no change in the mass ratio of the proton to the electron

see:

Particle Weighing in the Early Universe