Veronica's hair turned the ongoing tension with Hubble into a full-blown cosmological crisis.

Scientists have reliably established that our universe is expanding. This has been documented through various methods, such as analyzing the characteristics of the cosmic microwave background radiation (which is 13.8 billion years old) or by assessing the brightness of "standard candles," specifically type Ia supernovae, which almost always possess the same explosion energy. Since the energy is uniform, so too is the brightness. By comparing this brightness with the distance to distant galaxies, astronomers can determine the rate of expansion of space between Earth and the observed galaxy.

The challenge is that these methods yield very different rates of universal expansion. According to the cosmic microwave background radiation, the universe expands at a rate of 67.4 kilometers per second per megaparsec (one megaparsec is 3.26 million light-years). However, based on supernovae observations, it expands at least at 73 kilometers per second. The discrepancy of a few kilometers per second may seem minor. Yet, considering that the diameter of the universe is around 100 billion light-years (tens of thousands of megaparsecs), this amounts to a significant divergence.

In scientific literature, this discrepancy is referred to as the "Hubble tension." For a long time, the divergence was attributed to the limitations of the supernova measurement method. It was suggested that when a supernova is located far away, the distance to that region may not be accurately determined, leading to discrepancies with the method based on the analysis of cosmic microwave background radiation.

Now, a group of American astronomers led by Dan Scolnic has published results in The Astrophysical Journal Letters from observations of supernovae in the galaxy cluster known as the Hair of Veronica. This is a relatively large cluster, encompassing three thousand cubic megaparsecs.

The authors studied the brightness of 13 type Ia supernovae in this cluster. They then turned to the distances to these supernovae, determined by other methods (using the Dark Energy Spectroscopic Instrument). As a result, they obtained a rate of universal expansion of 76.5 kilometers per second, which is consistent with earlier studies on supernovae. The fundamental difference between this new study and previous ones is that it pertains to galaxies and supernovae that are relatively close to us, with distances that astronomers have accurately established. Questioning the calculated rate of universal expansion based on this data is now extremely challenging.

The authors of this new study are aware of this. Dan Scolnic commented to the press noted: “The Hubble tension has now turned into a crisis… This suggests, to some extent, that the current cosmological model may be broken. We are at a point where we are putting serious pressure on the models that have been used for the last 25 years, and we see that the ends do not meet. This situation may change how we perceive the universe, and that is exciting.”

Scolnic and his co-authors do not question the rate of universal expansion calculated based on cosmic microwave background radiation. The scientists merely point out that the rate of expansion of spacetime recorded for cosmic microwave background radiation does not match the rate recorded for the Hair of Veronica, which is located approximately one hundred megaparsecs (about 320 million light-years) away from us.

Previously, Naked Science noted that the so-called Hubble tension concerns cosmologists primarily because the Standard Cosmological Model assumes that the rate of universal expansion should not change significantly over time. However, other models—particularly Nikolai Gorkavy's pulsating universe—not only do not contradict the Hubble tension but also require its existence. In Gorkavy's model of the universe, the rate of expansion will increase up to a certain point. Thus, within this framework, it makes perfect sense that the expansion of spacetime 13.8 billion years ago occurred at a slower rate than it did three hundred million years ago, in the Hair of Veronica.