One mystery that keeps getting harder and harder to explain is the expansion rate of the universe. Since everything that is once occupied a single point and now occupies an undefined and vast volume, we know expansion happened. By using objects of known luminosity, we can measure the distance to galaxies in the near to middle-distance of our universe and then use Doppler shift measurements to get their velocity the same way a speed trap clocks your car’s velocity. Using complex models and measurements, we can also get at the expansion rate by looking at various facets in the early universe, including the cosmic microwave background radiation. Back when our measurements had large error bars, all the measurements were consistent, but as our data has gotten better, the agreement between the expansion rate measured from things going on in the early universe and the rate measured from things going more recently have fallen into a disagreement. This could mean our ability to measure distances is wrong, or that our physics is wrong. I’m not going to lie – I’ve been hoping we’d find a measurement error.
Today, however, a new paper in the Astrophysical Journal Letters by lead author Dom Pesce finds that measurements of the expansion rate that use masers as standard candles are consistent with measurements from supernovae and gravitationally lensed quasar geometry, but do not match the early universe measurements. This means there is something wrong with our physics. This is terribly exciting – we have a new problem – and also kind of terrible because we have a new problem. This isn’t to say our physics is necessarily wrong – although that’s always an option. In all likelihood, we have some term missing or some assumption is just wrong and when we run the numbers… it’s like forgetting to include the dog in the household budget, things just don’t work out as expected and everything falls a little short. Add in that missing term, and everything works.
“The Megamaser Cosmology Project. XIII. Combined Hubble Constant Constraints,” D. W. Pesce, 2020 Feb. 26, Astrophysical Journal Letters (Preprint on arxiv.org)