There’s a very important piece of research out about dark matter in galaxies that is not getting the attention it deserves. A team of scientists has found that the peculiar rotation rate of galaxies, long thought to be indirect evidence for dark matter, may not be the smoking gun we have been looking for.
The research was led by researchers from Case Western Reserve University in Ohio. Their paper will be published in Physical Review Letters, and a pre-print is available on arXiv.
So, what’s going on exactly? Well, one of the biggest hints for dark matter’s existence is how galaxies spin. Normally, when something large spins, you’d expect the rotation rate near the center to be faster than at the edge. A common analogy is to think of an ice skater spinning with their arms out; if they pull their arms closer to themselves, they’ll spin faster.
The same is not true for galaxies, though. Stars at the edge of the galaxy move just as fast as stars at the center, which is called the “missing mass problem”. The long-held explanation for this was that dark matter surrounding galaxies must be providing an additional gravitational attraction, making the rotation rate the same throughout.
Not so, says this new study. Studying 153 galaxies of all shapes and sizes, they say the visible mass alone is enough to explain this spin-rate, what’s known as a flat rotation curve.
“In our case, we find a relation between what you see in normal matter in galaxies and what you get in their gravity," said lead author Stacy McGaugh in a statement. "This is important because it is telling us something fundamental about how galaxies work."
Using near-infrared images from NASA’s Spitzer Space Telescope, the researchers showed there is a tight correlation between the amount of “normal” matter in a galaxy and the rotation curve. The researchers say this discovery was possible because near-infrared data is better at measuring a star’s mass from its light than other data.
In the statement from Case Western Reserve University, two researchers who were not involved with the study offered their own analysis, and their comments hint at how important it could turn out to be in our understanding of the cosmos.
"The relation discovered by McGaugh et al. is a serious, and possibly fatal, challenge to [the dark matter] hypothesis, since it shows that rotation curves are precisely determined by the distribution of the normal matter alone,” said David Merritt, professor of physics and astronomy at the Rochester Institute of Technology.
"[If] there is a single observation which keeps me awake at night worrying that we might have something essentially wrong [with the standard model of cosmology], this is it," added Arthur Kosowsky, professor of physics and astronomy at the University of Pittsburgh.
"Throughout the history of physics, unexplained regularities in data have often pointed the way towards new discoveries."
If confirmed, this would truly be groundbreaking research. It doesn’t mean that dark matter doesn’t exist, but the researchers say it may hint at a new previously undiscovered law of nature for rotating galaxies. It would undeniably cause a shake-up in our understanding of galaxies, and force a rethink on what role dark matter may have to play.
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