Possibility of new explanation of Hubble tension

The universe is expanding. How fast this happens is determined by the so-called Hubble-Lemaître constant. However, there is debate about how large this constant actually is. Different measurement methods will give inconsistent values.

This so-called “Hubble tension” poses a puzzle for cosmologists. Researchers from the University of Bonn and the University of St. Andrews are now proposing a new solution. The discrepancy in the measurements can be easily explained using an alternative theory of gravity: the Hubble tension disappears. This research is currently Royal Astronomical Society Monthly Notices (MNRAS).

As the universe expands, galaxies 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 away from Earth as she is in galaxy B, it will also grow in distance from us twice as fast as she does. American astronomer Edwin Hubble was one of the first to recognize this connection.

Therefore, to calculate how fast two galaxies are moving away from each other, we need to know how far apart they are. However, this also requires a constant to multiply this distance. This is the so-called Hubble-Lemaître constant, a fundamental parameter in cosmology. Its value can be determined, for example, by looking at very distant regions of space. This gives us a speed of about 244,000 kilometers per hour per megaparsec of distance (one megaparsec is just over 3 million light years).

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

“But we can also see objects much closer to us, so-called category 1a supernovae, which are some kind of exploding stars,” explains Professor Pavel Krupa from the Helmholtz Institute for Radiation and Nuclear Physics. University of Bonn. 1a It is possible to measure the distance from a supernova to Earth with great precision. We also know that shiny objects change color as they move away from us. And the faster they move, the stronger the changes. This resembles an ambulance, with its siren getting louder as it gets farther away.

Now, if we calculate the speed of the 1a supernova from the color shift and correlate it with the distance, we get a different value for the Hubble-Lemaître constant: just under 264,000 kilometers per hour per megaparsec distance. “So the universe appears to be expanding faster near us, up to about 3 billion light-years away, than the universe as a whole,” Krupa says. “And actually, it shouldn’t be.”

However, we recently made an observation that could explain this. According to this, Earth is located in a region of space that is relatively devoid of matter, comparable to bubbles in a cake. The density of the substance is higher around the bubble. This surrounding material creates a gravitational force that pulls the galaxy inside the bubble toward the edge of the cavity. “That’s why they’re actually moving away from us faster than expected,” explains Dr Indranil Banik of the University of St Andrews. Therefore, the deviation may be simply explained by a local “density deficit”.

In fact, another research group recently measured the average velocity of a large number of galaxies 600 million light-years from us. “It turns out that these galaxies are moving away from us four times faster than the standard model of cosmology allows,” said Sergizh Mazurenko of the Krupa Research Group, who was involved in the study. He explains.

Bubbles in the fabric of space

This is because the standard model does not provide for such density deficiencies or “bubbles”, and they really should not exist. Instead, matter must be evenly distributed in space. But if that were the case, it would be difficult to explain which forces propel galaxies to high speeds.

“The Standard Model is based on Albert Einstein’s theory of the nature of gravity,” Krupa says. “But gravity may behave differently than Einstein predicted.” The working group at the University of Bonn and the University of St. Andrews used a modified theory of gravity in computer simulations.

This “Modified Newtonian Mechanics” (abbreviation: MOND) was proposed by Israeli physicist Dr. Mordechai Milgrom 40 years ago. It is still considered an outsider theory today. “However, in our calculations, MOND accurately predicts the existence of such bubbles,” says Kroupa.

Assuming that gravity actually behaves according to Milgrom’s assumptions, the Hubble tension vanishes. In reality, there is only one constant for the expansion of the universe, and the observed deviations are due to irregularities in the distribution of matter.