Has Hubble Detected Rogue Clumps of Dark Matter?

By Ryan F. Mandelbaum on at

Scientists using the Hubble Space Telescope have discovered evidence of small clumps of dark matter warping the light from distant quasars.

Regular matter seems to form only a small part of the universe – much more of the matter seems to be “dark” stuff that influences regular matter via gravity but can’t be detected directly. The most widely accepted theory to explain dark matter suggests that it is a slow-moving particle that can form clumps in the universe. New Hubble observations provide evidence of the smallest clumps yet, which will hopefully further guide scientists’ search for the mysterious material.

For decades, scientists have been aware of various discrepancies between the way they observe objects behaving and the way they predicted those objects to behave, discrepancies that would be fixed with an unobserved source of mass. This has led to a prevailing theory featuring “cold”, or slow-moving, dark matter (versus fast, “warm” dark matter), and various searches using telescopes, particle colliders and sensitive particle detectors to find the identity of this mass. Cold dark matter should clump together on scales far smaller than a galaxy, but scientists had not found evidence of that clumping – until now.

The search for clumps relies on a concept called gravitational lensing. Mass warps the shape of spacetime itself, so scientists can hunt for mass sources based on how they alter the appearance of distant background light. This warping can cause bright objects like quasars to appear as multiple copies in the night sky. Scientists Leonidas Moustakas and Ben Metcalf in the early 2000s developed a method in which they focused on objects where an intervening galaxy had created a quad, or four images of the background light source (usually a bright, distant object called a quasar). But smaller intervening objects, such as clumps of dark matter, can change the brightnesses of the images. Comparing each of the four images allows researchers to determine whether there are any smaller dark matter haloes, or clumps, in the way.

Past observations had a problem: other light objects could cause warping, too, and create a signal that mimicked dark matter, called microlensing. Scientist Anna Nierenberg, now at the Jet Propulsion Laboratory at CalTech, realised that quasars release a kind of radiation called “nuclear-narrow line emission” that isn’t affected by microlensing and that Hubble would be able to spot this emission. She and her team submitted a proposal to study these emissions, first in 2014 when she was still a graduate student. After a long wait, data acquisition, and analysis on eight of these “quads”, they’ve released their results in a paper in the Monthly Notices of the Royal Astronomical Society.

“It’s a huge validation for me as an early career scientist,” Nierenberg told Gizmodo, who said that at first she was just excited for her proposal to have been selected and that she’d be able to dictate where Hubble should point.

A second paper in the same journal, led by Daniel Gilman at UCLA, argues that the data better agrees with cold dark matter models that include clumping, rather than models without clumping. The smallest clumps that would produce the observed effects would be between 1 million and 10 million times the Sun’s mass, far smaller than a galaxy and perhaps too small to even hold stars. It would just be a big clump of dark matter.

It’s an exciting observation. “Substructure lensing has long been the dream for revealing the nature of dark matter but was held up for almost two decades because of technical issues – for a long time, there were only seven suitable lenses for substructure lensing studies,” Annika Peter, associate professor in physics at the Ohio State University who was not involved in this study, told Gizmodo in an email. “[Gilman’s] paper shows how great constraints on the nature of dark matter can be with this increased sample size.” Basically, the simplest cold dark matter model still works and will be even better-tested with more of these quads.

This is just further evidence of cold dark matter, and scientists are still hunting for what that cold dark matter might be made of. Nierenberg said we’re not at the point where scientists can say for sure that dark matter is a particle that will be found in an experiment on Earth. But this paper serves as gut check that cold dark matter is the right kind of dark matter for scientists to be hunting. Of course, the possibility that dark matter is something completely wild that scientists haven’t thought of yet isn’t off the table. We’ll have to be patient; after all, particle hunts typically take decades.

Researchers will continue to look for these lenses, which will provide further constraints on dark matter’s properties. There’s more Hubble data to sift through, Nierenberg said, and the upcoming Vera Rubin Observatory will be hopefully be able to find more. The search for dark matter continues.

Featured image: NASA, ESA, A. Nierenberg (JPL) and T. Treu (UCLA)