It’s a natural human instinct to want to understand our cosmic surroundings, but the universe doesn’t always (or even often) make it easy. For instance, scientists have spent decades building a standard model of cosmology that explains much of the weird phenomena that we see in space, yet many outlier observations still challenge this established framework.
One example can be found in barred galaxies, which are extremely common spiral galaxies that are named after the radiant bars of stars running through their central regions. According to the standard model, the rotation of these bars should be slowed down over time by the friction caused by cold dark matter haloes, which are immense hypothetical structures that envelop galaxies.
However, new research concludes that these common features of the universe actually challenge our grasp of physics and may point to more exotic explanations of dark matter, an invisible material that is known by its gravitational effects. Scientists led by Mahmood Roshan, an associate professor of physics at Ferdowsi University of Mashhad in Iran, demonstrate that fast galaxy bars are far more common than suggested by the standard model, which is also known as the Lambda cold dark matter (ΛCDM) model.
These “dramatic disagreements cast serious doubt on whether galaxies actually have massive cold dark matter haloes, with their associated dynamical friction acting on galactic bars,” according to the team’s study, which has been accepted for publication in the Monthly Notices of the Royal Astronomical Society and is available on the preprint server arXiv.
While the results pose an interesting challenge to the standard model, the researchers do not argue that it refutes the entire well-corroborated ΛCDM framework.
“Of course, replacing the standard cosmological paradigm is an incredibly big move,” said Roshan in an email. “A simple reason for this claim is that replacing or dramatically modifying ΛCDM is almost equivalent to implementing a new theory of gravitation. I think that our investigation by no means implies this conclusion, in and by itself.”
“This study, however, strongly implies that the current version of dark matter particles is unlikely to be viable,” he noted.
Scientists have known that fast galaxy bars might be a thorn in the side of the standard model for decades, but constraining this problem has required sharper astronomical observations as well as more sophisticated simulations of galaxy formation and evolution. Researchers hoped the contradictory evidence might resolve itself as we learned more about normal “baryonic” matter, which is the visible stuff that makes up stars, planets, and life-forms, and dark matter.
“Naturally, this challenge has been treated as a minor one up to date: in other words, it was wise to be optimistic about solving this problem by taking into account more physics and baryonic feedbacks to the cosmological hydrodynamical simulations,” Roshan said.
“Improving the resolution of the cosmological simulations has been proposed as another solution,” he continued. “Now, we are in the era of high-resolution cosmological simulations of galaxy formation. So it is necessary to explore if this challenge is still alive and quantify its significance.”
To that end, Roshan and his colleagues used the state-of-the-art simulations developed by the IllustrisTNG and Evolution and Assembly of GaLaxies and their Environments (EAGLE) collaborations, which are advanced programs used to run scenarios within the standard model framework. The team then compared the galaxies generated in these ΛCDM model simulations with recent observations of real barred galaxies.
The results confirmed a clear tension between the models and observations, in which the ΛCDM simulations predicted that there would be far fewer fast-spinning galaxy bars than what scientists actually see when they look into the universe. For some reason, the galactic dark matter haloes theorized by standard cosmology do not seem to be slowing down spinning galaxy bars at anywhere near the predicted rate, which raises new questions about the nature of these haloes and their constituent particles.
“I believe that this is the time to doubt the standard cold dark matter particles,” Roshan said. “In other words, the dark matter particles seem to have more exotic properties compared to what is normally believed. In the specific case related to the dynamical friction, the dark matter particles should be postulated in such a way as to suppress the friction on the galactic scales.”
He noted that some “brilliant ideas” have recently been proposed that could relieve this tension. For instance, scientists have suggested that hypothetical particles known as ultra-light axions could be a key candidate for dark matter. These axions might have quantum mechanical properties that suppress the friction applied to galaxy bars by dark matter haloes, which would explain why so many of these structures maintain fast spins.
Roshan also pointed to efforts to completely nix the idea that dark matter haloes exist in galaxies, and focus on modified theories of gravitation. While there is preliminary research along these lines, a framework that completely throws out dark matter does not remotely rival the cohesiveness of the standard model.
Indeed, though the new study points out that the galaxy bar problem is "in line with several previously documented significant failures of ΛCDM in other respects,” the standard model remains the best explanation for our universe that we have at this point. Roshan and his colleagues hope that ΛCDM, and its challenges, will be further refined by new theories and perhaps even the detection of a dark matter particle, which is a major goal of particle physics.
“Although ΛCDM has failed to explain many local/galactic scale observations, it still has powerful features,” Roshan said. “In my view, it is too soon to conclude that some form of dark matter is unable to resolve problems like the bar pattern speed.”
“In the current context, and given the challenges we uncovered, it is particularly important to keep an open mind on what the actual solutions might be,” he concluded.