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Astrophysics

Students challenge 40-year-old theory of galaxy dynamics and the dark universe

No known physical laws can explain how stars move in galaxies. Now physics students from SDU show that a 40-year-old theory can be ruled out.

By Birgitte Svennevig, , 1/18/2022

Galaxies consist of billions of stars, and in many galaxies the stars converge in a disk-like structure, orbiting the center of the galaxy.

The speeds of the stars' movements around the center of the galaxies – the so-called rotation curves – are far too high for the known gravity of the visible matter in the galaxies to hold the stars in place in the orbits measured by astronomers.

In fact, the speed is so high that galaxies should fall apart, but they do not.

Dark matter or a new law of gravity

There are two possible explanations for this:

Either Newton's and Einstein's laws of gravity are not correct and must, therefore, be modified. This is a radical hypothesis, which goes by the name 'MOdified Newtonian Dynamics (MOND)'.

Or the galaxies are filled with more matter than we can see – the so-called dark matter. In that case, it is the gravity of the dark matter that holds the stars in place in their orbits, and then the law of gravity is correct.

Analysis of 176 galaxies

Dark matter has for long, and for many reasons, been the preferred hypothesis. But in 2016, the study The Radial Acceleration Relation in Rotationally Supported Galaxies (published in Physical Review Letters by McGaugh, Lelli, and Schombert) renewed focus on the 40-year-old MOND alternative. The study showed that the rotation curves in a group of 176 galaxies apparently agreed with the MOND hypothesis.

This study has now been challenged by Mikkel Have Eriksen, Mogens Henrik From et al. The work took place when they were studying Physics at SDU. Today, Mogens works as DevOps at Ordbogen.com, and Mikkel is a PhD student at the Faculty of Engineering at SDU.

Their work is published in the journal Astronomy and Astrophysics: A cusp-core-like challenge for modified Newtonian dynamics.

Existing hypotheses are challenged

Together with Mads Toudal Frandsen, astro- and particle physicist at SDU, Eriksen and From based their work on a new geometric representation of rotation curves, which had been developed by Mads Toudal Frandsen and PhD student Jonas Petersen: A Method for Discriminating Between Dark Matter Models and MOND Modified Inertia via Galactic Rotation Curves.

Their work demonstrates that the consequences of the MOND hypothesis for galaxies in the innermost part of galaxies do not agree with data, and they thus believe that Newton's and Einstein's law of gravity must still apply in galaxies – and that large amounts of dark matter helps keep the stars in their orbits in the galaxies.

More specifically, their work shows that the MOND models leave a completely unique mark in the part of the stars that rotates closest to the center of the galaxy. Eriksen, From and Frandsen show that this mark cannot be found in the measurements of rotation curves made by astronomers.

The mysterious dark matter

– Although new dark matter particles have long been considered the most likely explanation for the observed rotation curves, we have not yet directly detected these particles. Therefore, it is important to still keep alternative explanations in mind and seek as definitively as possible to test these alternatives, says Mads Toudal Frandsen, continuing:

– But with Eriksen's and Fromm's work, the most common MOND models are now very strongly challenged.

The work is supported by the Danish Council for Independent Research.

Meet the researcher

Mads Toudal Frandsen is an associate professor at the Department of Physics, Chemistry and Pharmacy.

Contact

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Editing was completed: 18.01.2022