One of the weird things that can happen as measurements get more precise is unknown errors crop up. Need two pieces of pipe to be the same length? Well, in building my old house, that would call for a pencil, a ruler, a fancy blade used only with goggles, and a bit of trepidation. The pieces may differ by a millimeter here or there, but the house was built in 1893, and anything better than a centimeter? I’ll take it.

If you are building something for LIGO, however, digital calipers and lasers are likely to be involved and differences may be measured in wavelengths of light.

This means that a bunch of pipes I see as being the same size, to LIGO, may all appear pretty different. And it may turn out that the differences between cutting on the inside of the pencil line and on the outside of the pencil line can lead to more sensitive measurements to reveal two different sets of noisy measurements.

In trying to measure the expansion rate of our universe, we’ve used a lot of different rulers, and we’ve even strung them together awkwardly since no one tool can measure the full distance back to the beginning of the universe. Any issues in those rulers can lead to errors that get multiplied across everything else.

As hard as these measurements are to make, our ability to make accurate measurements has been getting better and better, and over the past several years, we’ve started to notice that one set of measurements – those that look at the cosmic background light – yield one value, and measurements that look at nearby and middle distance stars and supernovae appear to have a slightly different value, with those values not quite matching.

Now, a review paper by Wendy Freedman, one of the scientific heroes of these kinds of measurements, reveals that it looks like while our cosmic microwave background-based measurements seem to be more like the high-precision digital caliper measurements we’d like, the measurements from stars and supernovae may have been made with not entirely accurate rulers that added errors. 

Specifically, we calibrate our supernova distance measurement using red giant stars and Cepheid variable stars, and she finds that improving understanding of red giants is shifting the distance scale just a bit and in the direction that brings supernovae measurements in line with cosmic microwave background measurements. A similar error hasn’t yet been found in Cepheid variable stars, but Freedman points out that those measurements could easily be affected by dust that exists in the vicinity of these young stars. Simply adding a bit of dust, and the changes in apparent brightness it would cause, is enough to also bring things into alignment.

I’m sure this won’t be the final word on this science. If the discrepancy is real, it would indicate there is new physics that could lead to a Nobel prize, and folks are always willing to chase Nobel prizes into dark scientific alleys. For now though, I will actually sleep a bit better knowing there probably isn’t weird new physics to complicate our universe.

More Information

University of Chicago press release

“Measurements of the Hubble Constant: Tensions in Perspective,” Wendy L. Freedman, to be published in The Astrophysical Journal (preprint on arxiv.org)