The Universe in Meters: How Units of Measurement Set Astronomers at Odds express
Two astronomers look at the same universe but see it differently. One measures the expansion rate using ancient light that has been traveling toward us for nearly 14 billion years. The other uses recent stellar explosions in nearby galaxies. The first gets 67 kilometers per second per megaparsec, the second gets 73. The difference seems small, but it is five times larger than can be explained by chance. It's like two thermometers in the same room reading 20 and 24 degrees.
The dispute dragged on for years. Each team of scientists was confident in its data and could not figure out what was going on. It turned out that the culprits were not telescopes or physics but ordinary units of measurement. One team used miles, the other kilometers, and nobody noticed this when comparing results. When all measurements were converted to the metric system, the numbers matched: the expansion rate of the universe is 69 kilometers per second per megaparsec. It's like two builders arguing about the height of a house, only to discover that one was measuring in feet and the other in meters.
Now astronomers can breathe a sigh of relief. The puzzle that had plagued them for several years turned out to be a simple translation error. But this story reminds us that even in science about the farthest reaches of the universe, it's important not to forget earthly details. After all, if you mix up miles and kilometers, you can set whole generations of scientists at odds.
Original abstract
as written by the authors arXiv:2603.29879
Recent cosmological surveys and datasets have highlighted a variety of tensions to the concordance model of our universe, $$\Lambda$$CDM. Of particular interest is the Hubble tension, the $$5.5\sigma$$ discrepancy between measurements of the Hubble constant $$H_0$$ using high redshift CMB data from Planck ($$67.27\pm0.60$$km$$\text{s}^{-1}\text{Mpc}^{-1}$$) and low redshift supernovae from SH0ES ($$73.2\pm1.3$$km$$\text{s}^{-1}\text{Mpc}^{-1}$$). To avoid stepping on any toes, we have initiated the CROCS collaboration to resolve this tension, gathering experts from across many fields of cosmology, astrophysics, astronomy, machine learning, data science, philosophy, and astrology. In this paper, we present findings from CROCS Data Release 1, corresponding to the first $$\sim3$$ days and 27 minutes (rest frame) of observation. We perform a robust statistical analysis, showing that Planck and SH0ES both suffer from imperial biasing systematics (IBS) at $$5\sigma$$ significance. Accounting for these errors by converting to metric units reconciles the high and low redshift data, with $$H_0 = 69.00\pm0.420$$km$$\text{s}^{-1}\text{Mpc}^{-1}$$. We thus report that our results are sufficient to end the Hubble tension for good.
If you were driving a car at 100 km/h, it would take you 40 million years to reach the nearest star after the Sun. And that distance is less than one parsec!