Proton-electron mass ratio change limits grand unified theories
Some grand unified theories predict that the ratio of the proton to electron mass (about 1836) varies on cosmological timescales. Now new astronomical measurements have limited that variation to no more than eight parts per million over the past 10.6 billion years.
About a decade ago, some physicists detected what looks like a variation in the constant alpha, the strength of the electromagnetic force, over the age of the universe. (Yes, it seems odd that a constant could vary but that’s because we always just assumed that it is a constant–perhaps it isn’t.) It is a small change but a significant one as it tells us something fundamental is happening to the forces of nature.
Until now, there had been few tests of other supposed constants. A new paper in Physical Review Letters describes a measurement of the change in the proton mass to electron mass ratio. The measurements are performed by looking at distant quasars through intervening gas clouds and seeing what frequencies of light are absorbed in the cloud. Those frequencies should have particular values based on the electron to proton mass ratio.
In this case, physicists looked at the spectrum of hydrogen molecules in a gas cloud obscuring the quasar J2123-005 with the Very Large Telescope (VLT) in Chile. They could see 90 spectral lines from H2 molecules and another six from hydrogen-deuterium molecules. By doing a statistical fit, they arrived at a limit of 8.5 parts per million variation up to redshift 2.06, which corresponds to about 10.6 billion years ago.
A previous observation of the same quasar with the Keck telescope saw a variation of no more than 5.6 parts per million. Averaging, this leads to a limit of about 7.6 +/- 3.5 parts per million.
Most grand unified theories define a relationship between any variation in the fine structure constant, alpha, and the ratio of proton to electron mass. The mass ratio should be 10-40 times larger than the alpha variation in some of the theories. With a measured variation in alpha of 5.4 parts per million, this rules out theories that predict a significantly larger variation in the mass ratio. There are, however, some variants of grand unified theories that predict a factor of 5 or less difference between the variations so they could be compatible with these measurements.
If you assume that grand unifed theories are correct and that there consequently must be some variation in our fundamental constants, then these measurements help distinguish between which grand unified theories might be right and which must be wrong.