Since the 1960s, there has been a general consensus among astronomers and cosmologists that the majority of the Universe is made up of an invisible, mysterious mass (known as Dark Matter). While scientists still haven’t identified the candidate particle that makes up this mass, indirect tests and simulations have shown that Dark Matter must exist in order for the Universe to be the way it is.

In a fascinating twist, a team of European researchers conducted a simulation that looked at a Universe without Dark Matter. Using an alternative theory known as MOdified Newtonian Dynamics (MOND), the team created a computer simulation in which the galaxies were actually very similar to what we see in the Universe today. These findings could help to resolve one of the most enduring mysteries of modern cosmology.

The study that describes their findings (recently published in the Astrophysical Journal) was conducted by the Stellar Populations and Dynamics Research Group (SPODYR) – led by Prof. Pavel Kroupa of the Helmholtz Institue for Radiation and Nuclear Physics at the University of Bonn. He was joined by Nils Wittenburg, a doctoral member of SPODYR, and Benoit Famaey – the Research Director at the University of Strasbourg.

This theory that gravity behaves differently than previously thought (depending on the scale) was first proposed by Israeli physicist Prof. Dr. Mordehai Milgrom – hence the alternative name “Milgromian gravity.” According to this theory, the attraction between two masses obeys Newton’s Laws of Motion (aka. Universal Gravitation) only up to a certain point.

At lower accelerations, as is the case with galaxies, the influence of gravity becomes considerably stronger. In short, the attraction of a body depends not only on its own mass but also on whether other objects are in its vicinity. This theory is a possible explanation for why galaxies do not break apart as a result of their rotational speed.

MOND is also attractive because it makes the existence of Dark Matter (which remains unconfirmed) entirely superfluous. Nevertheless, MOND remains a largely unproven and untested theory, which is what Wittenberg and his colleagues sought to address. With the help of Famaey, the team employed computational software that conducts gravitational computations (which they designed) to simulate a cosmos where MOND exists.

This consisted of simulating the birth of the first stars and galaxies – which are believed to have formed between 100,000 and 300,000 years after the Big Bang – and how they have evolved since. What they found, interestingly enough, was that the distribution and velocity of the stars in the computer-generated galaxies followed the same pattern as those that are visible in the Universe today.

As Wittenburg, who was the lead author on the study, explained:

“In many aspects, our results are remarkably close to what we actually observe with telescopes. Furthermore, our simulation resulted mostly in the formation of rotating disk galaxies like the Milky Way and almost all other large galaxies we know. Dark matter simulations, on the other hand, predominantly create galaxies without distinct matter disks – a discrepancy to the observations that is difficult to explain.”

In addition, the MOND simulation was virtually immune to changes in parameters, like the frequency of supernovae and their effect on the distribution of matter in galaxies. In the case of simulations where the existence of Dark Matter is assumed, however, changes in these parameters have a considerable effect. This is not to say that the MOND simulations were correct on all points.

For example, the simulations relied on some rather simple assumptions about the distribution of matter and the conditions present during the early Universe. “Our simulation is only a first step,” Prof. Kroupa emphasized. “We now have to repeat the calculations and include more complex influencing factors. Then we will see if the MOND theory actually explains reality.”

Invariably, when it comes to the dynamics and behavior of the Universe on the grandest of scales and longest of time periods, the jury is still out. While the existence of Dark Matter remains unproven, it is the only cosmological theory that is consistent with General Relativity – an endlessly proven theory and the only working hypothesis for how gravity behaves on cosmological scales.

The Big Bang timeline of the Universe. Cosmic neutrinos affect the CMB at the time it was emitted, and physics takes care of the rest of their evolution until today. Credit: NASA/JPL-Caltech/A. Kashlinsky (GSFC)

And while MOND provides some resolution to theoretical problems presented by Dark Matter, it presents problems of its own. In the near future, a number of next-generation observatories that could help resolve this mystery will be going into space – including the James Webb Space Telescope (JWST) and the ESA’s Euclid mission.

These and other missions will offer a better picture of the geometry of the Universe and improved measurements of the cosmic expansion. From this, scientists hope to gain a better understanding of how Dark Matter could have affected cosmic evolution – not to mention Dark Energy, another cosmological mystery that is also the subject of debate!

Further Reading: University of Bonn, arXiv