The ALPHA experiment (no relation) at the CERN antiproton decelerator has just published the first measurement of the Lyman-alpha lines of anti-hydrogen (a positron bound to an antiproton)¹. 

There is an excellent summary of the measurement here, written by Ana Lopez.

Ana mentions the importance of the Lyman-alpha line to astrophysics. As light of multiple frequencies travels to us from distant objects, it passes through clouds of interstellar and intergalactic hydrogen. The Lyman-alpha energy transition of hydrogen “takes out” various frequencies, depending on the red shift, when a photon of the right energy hits a hydrogen atom and makes the electron jump up the level. There’s a video from Andrew Pontzen above, illustrating how, because of the different red shifts, a single energy transition leads to a “forest” of absorption lines.

There are multiple interesting reasons for measuring the anti-hydrogen equivalent of this energy transition (see Ana’s article). But one thing that tickles me about it is that there are still (fringe, but serious) ideas that the universe may have more antimatter in it than it appears, there may be antimatter galaxies, and presumably also anti-hydrogen gas clouds. This idea is called the Dirac-Milne universe and featured in this article. If you want to see whether there is any chance this idea is viable² then seeing whether this energy transition is the same for antihydrogen and hydrogen is an important thing to do.

To within the precision of this first measurement, they are the same. This was expected, but not – until now – known.

¹Note added 23/12/18 I also made this my entry for the Observer’s science review of 2018.

² Probably not but it should be considered, since the Standard Model predicts the universe should contain equal amounts of matter and antimatter.