A galactic test to clarify the existence of dark matter

Researchers from the University of Bonn and the University of California at Irvine have used sophisticated computer simulations to design a test that could answer a burning question in astrophysics: does dark matter actually exist? ? Or does Newton's gravitational law need to be modified? The new study, now published in the Letters of physical examination, shows that the answer is hidden in the movement of stars within small satellite galaxies swirling around the Milky Way.

Using one of the fastest supercomputers in the world, scientists simulated the distribution of matter from so-called "dwarf" satellite galaxies. These are small galaxies that orbit larger galaxies like the Milky Way or Andromeda.

The researchers focused on a relationship called radial acceleration relationship (RAR). In disk galaxies, stars move in circular orbits around the galactic center. The acceleration that forces them to change direction is caused by the attraction of matter in the galaxy. The RAR describes the relationship between this acceleration and that caused by the only visible matter. It gives an overview of the structure of galaxies and the distribution of their material.

"We have now simulated, for the first time, the RAR of dwarf galaxies assuming that dark matter exists," says Professor Cristiano Porciani of the Argelander Institute of Astronomy at the University of California. University of Bonn. "It's turned out that they behave like smaller versions of larger galaxies." But what about there is no dark matter and that gravity works differently than what Newton thought? "In this case, the RAR of the dwarf galaxies depends strongly on the distance of their parent galaxy, whereas this does not happen if the dark matter exists", explains the researcher Emilio Romano-Díaz.

This difference makes satellites a powerful probe for testing whether dark matter actually exists. The Gaia satellite, launched by the European Space Agency (ESA) in 2013, could provide an answer. It was designed to study the stars of the Milky Way and its satellite galaxies in unprecedented detail and collected a large amount of data.

However, it will probably take years to analyze the data. "The individual measurements are not enough to test the small differences that we found in our simulations," explains Enrico Garaldi, PhD student. "But repeated examination of the same stars improves measurements every time: sooner or later, it should be possible to determine whether dwarf galaxies behave as in a universe with dark matter – or not."

The cement that holds galaxies together

This question is one of the most pressing issues of cosmology today. The existence of dark matter was suggested more than 80 years ago by the Swiss astronomer Fritz Zwicky. He realized that galaxies move so fast within clusters of galaxies that they should actually move away. He therefore postulated the presence of invisible matter which, because of its mass, exerts a gravity sufficient to maintain the galaxies on their observed orbits. In the 1970s, her American colleague Vera Rubin discovered a similar phenomenon in spiral galaxies such as the Milky Way: they turn so fast that the centrifugal force must tear them if only visible matter is present.

Today, most physicists are convinced that dark matter represents about 80% of the mass of the universe. As it does not interact with light, it is invisible to telescopes. Yet, assuming that its existence offers an excellent fit to a number of other observations – such as the distribution of background radiation, the persistence of the Big Bang. Dark matter also provides a good explanation for the arrangement and rate of formation of galaxies in the universe. However, despite many experimental efforts, there is no direct evidence that dark matter exists. This has led astronomers to the hypothesis that the gravitational force itself could behave differently than previously thought. According to the so-called modified Newtonian Dynamics (MOND) theory, the attraction between two masses obeys Newton's laws only up to a certain point. At very small accelerations, like those prevalent in galaxies, gravity becomes considerably stronger. As a result, galaxies do not tear due to their rotation speed and the MOND theory can do without dark matter.

The new study opens the possibility for astronomers to test these two hypotheses in an unprecedented regime.

Explore further:
Dancing with Giants: Dynamics of Dwarf Satellite Galaxies

More information:
Enrico Garaldi et al., Radial acceleration relationship of satellite galaxies ΛCDM, Letters of physical examination (2018). DOI: 10.1103 / PhysRevLett.120.261301