Cosmologists have measured how fast distant galaxies are receding in different parts of the sky. To do this, they used two kinds of "cosmic rulers": galaxy clusters and a certain type of supernova explosion. Both methods showed the same thing: in one direction, the Universe is expanding slightly faster than in the opposite direction.
The difference is small — about two sigma, meaning it could be random, but it persists across all checks. The most intriguing part: the direction of this unevenness nearly matches the direction in which the entire sky appears to be pulled due to the motion of Earth and the Sun. This might not be a coincidence but rather a trace of some deep process in the early Universe.
If the directional difference in expansion rate is confirmed, it would crack the standard cosmological model. So far, all data fit the standard picture, but such "irregularities" could be the first warning sign.
as written by the authors arXiv:2604.04408
Using $$\Lambda$$CDM and Pad\'e-(2,1) cosmography, we study directional variations in the Hubble constant, $$H_0$$, using galaxy cluster and Type Ia Supernovae (from Pantheon Plus) by the hemisphere decomposition method. Since there is a degeneracy between $$H_0$$ and absolute magnitude $$M_B$$ for Supernovae, Cepheid host calibration is usually required to constrain $$H_0$$. Hence, in this work in order to complement the Cepheid host calibration in Supernovae, we also use calibrations based on galaxy cluster scaling relations. We find that there is a $$\lesssim 1\sigma$$ difference in $$H_0$$ variations when using galaxy clusters as calibrators compared to Cepheids highlighting that the variations in $$H_0$$ are robust across different calibration methods. Across all combinations of models and data sets used, we obtain a consistent deviation $$\sim 2\sigma$$ from isotropy. In nearly all cases, we notice that the maximum $$\Delta H_0$$ aligns with the CMB dipole direction.
The most precise way to measure the distance to a distant galaxy is to look at a Type Ia supernova explosion. Such explosions always have nearly the same brightness, which is why they are called "standard candles."