The universe is expanding. How quickly this happens is explained by what is called the Hubble-Lemaitre constant. But there is disagreement about how large this constant actually is: different measurement methods give conflicting values.

This so-called “Hubble tension” is a mystery to cosmologists. Bonn and St. Researchers from the universities of St. Andrews now offer a new solution: The discrepancy in the measured values ​​​​can be easily explained using an alternative theory of gravity – the Hubble voltage disappears. The study was published on: Monthly Notices of the Royal Astronomical Society ( MNRAS ).

The expansion of the universe causes galaxies to move away from each other. The speed at which they do this is proportional to the distance between them. For example, if galaxy A is twice as far from Earth as galaxy B, its distance from us is increasing twice as fast. American astronomer Edwin Hubble was one of the first to notice this connection.

Therefore, in order to calculate how fast two galaxies are moving away from each other, it is necessary to know the distance between them. But this also requires a constant to multiply this distance by. This is the Hubble-Lemaitre constant, a fundamental parameter in cosmology. Its value can be determined, for example, by looking at very distant regions of the universe. This gives a speed of almost 244,000 kilometers per hour per megaparsec of distance (one megaparsec is just over three million light-years).

244,000 kilometers per hour or 264,000 per megaparsec?

Professor Dr. from the Helmholtz Institute for Radiation and Nuclear Physics at the university. “But you can also look at celestial objects that are much closer to us, called category 1a supernovae, which are a specific type of exploding stars,” explains Pavel Krupa. of Bonn. The distance of Supernova 1a to the Earth can be determined very accurately. We also know that glowing objects change color as they move away from us, and the faster they move, the stronger the change. It looks like an ambulance whose siren gets deeper as it moves away from us.

If we now calculate the speed of 1a supernovae based on their color shifts and relate this to their distance, we obtain a different value for the Hubble-Lemaitre constant, which is just under 264,000 kilometers per hour per megaparsec distance. “So the universe appears to be expanding faster near us, about three billion light-years away,” says Krupa. “And it really shouldn’t be like that.”

However, an observation was recently made that may explain this. Accordingly, the Earth is located in a region of space where there is relatively little matter, about the size of an air bubble in a cake. The density of the substance is higher around the bubble. Gravitational forces emanate from this surrounding matter, pulling the galaxies inside the bubble to the edges of the void. “So they’re moving away from us faster than we expected,” explains Dr Indranil Banik from the University of St Andrews. The deviation can therefore simply be explained by local “insufficient density”.

In fact, another research group recently measured the average speed of multiple galaxies 600 million light-years away. “These galaxies were found to be moving away from us four times faster than the standard cosmology model allows,” explains Serhii Mazurenko from Krupa’s research group, who participated in the current study.

A bubble in the dough of the universe

This is because the Standard Model does not predict such low densities or “bubbles”; These shouldn’t actually exist. Instead, matter must be evenly distributed in space. But if this were the case, it would be difficult to explain what forces are pushing galaxies to high speeds.

“The Standard Model is based on Albert Einstein’s theory about the nature of gravity,” says Krupa. “But gravitational forces may behave differently than Einstein expected.” Bonn and St. Working groups from Andrews University used modified gravity theory in computer simulations.

This “modified Newtonian dynamics” (abbreviation: MOND) was developed by Israeli physicist Professor Dr. Suggested by Mordechai Milgrom. Today it is still considered an external theory. “But in our calculations, MOND accurately predicts the existence of such bubbles,” says Krupa.

If we assume that gravity really behaves according to Milgrom’s assumptions, the Hubble tension will disappear: there will be effectively a single constant for the expansion of the universe, and the observed deviations will result from irregularities in the distribution of matter.