A new possible explanation for the Hubble tension

The cosmos is growing larger. The Hubble-Lemaitre constant describes how quickly it does so. However, there is disagreement on the true value of this constant: Conflicting results are obtained using various measuring techniques.

For cosmologists, this "Hubble tension" is a riddle. A fresh approach is now being put out by researchers from the Universities of Bonn and St. Andrews: The Hubble tension vanishes when a different theory of gravity is used, explaining the observed differences. The research has now been released in the Royal Astronomical Society's Monthly Notices (MNRAS).

Galaxies separate from one another due to the universe's expansion. The distance between them determines how quickly they do this. For example, galaxy A's distance from Earth increases twice as quickly if it is twice as far away as galaxy B. Edwin Hubble, an American astronomer, was among the first to notice this link.

Consequently, knowing the distance between two galaxies is essential to determining how quickly they are traveling apart. But in order to double this distance, a constant is also needed. This is a key cosmological parameter known as the Hubble-Lemaitre constant. One way to ascertain its importance is to examine the furthest reaches of the cosmos. This results in a speed of around 244,000 km/h for every megaparsec of distance (three million light-years equals one megaparsec).

244.000 mph (megapascals per hour) or 264,000?

"However, you can also examine celestial objects that are considerably closer to us—so-called category 1a supernovae, which are a specific kind of burst star," says Prof. Dr. Pavel Kroupa of the University of Bonn's Helmholtz Institute of Radiation and Nuclear Physics. The precise distance of a 1a supernova from Earth may be ascertained. Additionally, we know that as sparkling things travel away from us, their color changes, and the stronger the change, the quicker they go. This sounds like an ambulance, which gets farther away from us and makes a deeper siren.

We get a different value for the Hubble-Lemaitre constant if we now compute the speed of the 1a supernovae based on their color shift and connect this with their distance: little less than 264,000 km/s. "The universe, therefore, appears to be expanding faster in our vicinity—that is, up to a distance of around three billion light years—than in its entirety," Kroupa explains. "And that shouldn't really be the case."

But a new finding could provide an explanation for this. This suggests that the Earth is situated in an area of space where matter is scarce, like an air bubble in a cake. The area surrounding the bubble has a greater matter density. This surrounding stuff emits gravitational forces that push the galaxies within the bubble toward the cavity's boundaries. According to Dr. Indranil Banik of St. Andrews University, "that's why they are moving away from us faster than would actually be expected." Thus, a simple explanation for the variations would be a local "under-density."

Indeed, the average speed of a significant number of galaxies that are 600 million light years away from us was recently observed by another study team. As Sergij Mazurenko from Kroupa's research group puts it, "it was found that these galaxies are moving away from us four times faster than the standard model of cosmology allows."

A bubble in the universe's dough

This is due to the fact that such under-densities or "bubbles" are not allowed for in the usual model and shouldn't genuinely exist. Rather, matter ought to be dispersed uniformly throughout space. But if that were the true, it would be challenging to explain what factors cause the galaxies to accelerate to such a great pace.

"The standard model is based on a theory of the nature of gravity put forward by Albert Einstein," Kroupa explains. "However, the gravitational forces may behave differently than Einstein expected." In a computer simulation, the working groups from the Universities of Bonn and St. Andrews have used a modified theory of gravity.

The term "modified Newtonian dynamics" (abbreviated: MOND) was first used forty years ago by Prof. Dr. Mordehai Milgrom, an Israeli psychologist. Even now, it is still regarded as a fringe hypothesis. "In our calculations, however, MOND does accurately predict the existence of such bubbles," Kroupa states.

The Hubble tension would vanish if gravity truly operated in accordance with Milgrom's theories: the universe's expansion would be governed by a single constant, and the observable variations would result from anomalies in the distribution of matter.