A Ladder with No Top Rung: Cepheids vs. Supernovae ✛
The cosmological community has been living in a state of quiet civil war for several years. Two camps measure the expansion rate of the Universe and get different answers. Planck, peering at the cosmic microwave background, says 67.4 km/s/Mpc. SH0ES, climbing the distance ladder from Cepheids to supernovae, insists on 73.2. The 5.8-sigma gap does not look like a fluke. Now comes a paper that takes a bold step: it throws out supernovae altogether and asks what Cepheids alone have to say.
A distance ladder without its top rung: that is one way to describe this experiment. Usually Cepheids serve only as a calibration for Type Ia supernovae, which carry the measurement into deep space. Here, the authors stopped at the second rung: they took 35 galaxies with Cepheids, measured their redshifts, and built a Bayesian model that directly links the apparent brightness of pulsating stars to the expansion of space. Peculiar velocities, the galaxies' own motions through space, were accounted for using the Manticore-Local reconstruction, which produces an entire ensemble of possible velocity fields. Galaxy selection was not left to chance either: it was modeled as a smooth threshold based on supernova brightness.
The result is sobering. If the sample is assumed to be selected by supernova brightness, the Hubble constant is 71.1 ± 1.4 km/s/Mpc. If selected by redshift, it is 72.5 ± 1.4. Both values are lower than SH0ES but still higher than Planck's. The tension has not gone away: the discrepancy with Planck's data is 2.8 sigma. However, the uncertainty compared to a previous analysis of the same data has shrunk by 41 percent, and using Manticore-Local instead of the older reconstruction reduced the error by another 18 percent. The ladder without its top rung has held up.
What does this mean for the future? The method can be extended to other distance indicators: the tip of the red giant branch, Mira-type stars, surface brightness fluctuations. With more Cepheid galaxies and better velocity models, the precision could reach one percent. The proposed Habitable Worlds Observatory could add 24 new Cepheid galaxies, enough to reach the target precision. Meanwhile, Bayesian reconstructions of the local velocity field, such as BORG, will keep reducing systematics from peculiar motions. The ladder, stripped of its top rung, turned out not to be deficient but simply different, and this different ladder leads to new questions about dark energy and the physics of the early Universe.
Cepheids pulsate with periods ranging from a few days to months, and from the period one can infer their true luminosity. This law was discovered by Henrietta Leavitt in 1912, working with photographic plates of the Magellanic Clouds.
In Carl Sagan's novel "Contact," the protagonist uses Cepheids to calibrate distances while searching for a signal from space. Here, Cepheids also serve as a calibration, but for testing cosmology itself.