Webb’s latest observations pointed to the unknown physics of the expansion of the Universe.

The Universe is expanding at an accelerating rate and scientists cannot explain what causes stars and galaxies not bound by gravity to rush away from each other. But what’s even worse is that the magnitude of the acceleration is by no means constant: shortly after the Big Bang, objects flew away at one acceleration, and now they fly away at another. This hints that physics may have differed at different stages in the development of the Universe, which further confuses the situation. “James Webb” might have helped, but no.

Galaxy NGC 5468 is 130 million light years away from Earth. Image source: NASA

Scientists hoped that a series of observations of stars using the Hubble telescope contained errors that could eliminate the so-called Hubble intensity from the equations for estimating the rate of expansion of the Universe. This tension arose from a discrepancy between measurements of the expansion rate based on CMB data and estimates of the distances to beacon stars (Cepheids, supernovae, red giants and others). The CMB and the LambdaCDM model based on its characteristics give a value of 66.93 ± 0.62 (km/s)/Mpc, while data on stars give a value of 73.24 ± 1.74 (km/s)/Mpc. The difference of about 8% suggests that there is something we don’t know about the early Universe.

With the advent of the space observatory. James Webb hoped that this instrument would either disprove or prove the validity of the Hubble tension. Actually, for now he both refutes and proves that there is a difference in the expansion rate of the Universe, which suggests that it is too early for theorists to get involved. Thanks to Webb’s high sensitivity, scientists are improving the calibration of the distance ladder, ranging from Cepheids to supernovae. The new work is aimed at clarifying the errors in the estimates of distances to stars previously made by Hubble.

In total, scientists examined 1,000 Cepheids in five galaxies at a distance of 130 million light years using Webb. Hubble collected similar data. Webb’s observations turned out to be more accurate, as they eliminated errors such as the influence of interstellar dust on the brightness of Cepheids, as well as eliminated the effect of mixing the light of stars, which made it difficult to determine their true luminosity and, therefore, the distances to objects.

With a probability of 8 sigma, it was shown that “unidentified crowding of Cepheid photometry” cannot serve as an explanation for the Hubble intensity. In other words, the Hubble tension is not an error and remains unexplained. Observational data from Hubble and Webb continue to confirm a significant difference in the rate of expansion of the Universe in the early stages of its existence and in its maturity.

«One possible explanation for the Hubble tension could be that something is missing from our understanding of the early universe, such as a new component of matter—early dark energy—that gave the universe an unexpected boost after the Big Bang, said JHU cosmologist Marc Kamionkowski. “There are also other ideas, such as unusual properties of dark matter, exotic particles, changes in electron mass, or primordial magnetic fields that could play a role. There is scope for theorists to be creative.”