Illuminating a mysterious universe
By: Saima Sidik
Tags: Virus

Physicists are going above Earth’s atmosphere in a quest to understand the composition of the universe

Spiral Galaxy NGC 7331. Source: NASA Hubble Telescope

On a chilly February morning, Brandon Roach pulled a gold-plated disc, a few inches in diameter, out of a storage chamber in the MIT physics lab where he works as a graduate student. “Detector 744,” the label on the disc read. It’s one of over 1,000 detectors that, together with a gigantic balloon and favorable Antarctic wind patterns, could help Roach peer into the properties of a little-understood substance called dark matter.

As a member of Kerstin Perez’s nuclear science lab, Roach works on the General AntiParticle Spectrometer Project, or GAPS. At the end of 2022, GAPS scientists plan to work with NASA to launch a dark matter-seeking balloon the size of a seven-story building, with 1,000 detectors hanging off, from the McMurdo research station in Antarctica. GAPS is only a recent example of scientists’ attempts to solve a thorny problem that’s been bothering them for almost a century — namely, that they have no idea what the universe is made of.

The problem dates back to the 1930s, when physicist Fritz Zwicky realized that some galaxies spin too fast to hold together. Spinning throws stars apart, the way lettuce is thrown to the walls of a salad spinner, but the gravitational attraction between stars keeps galaxies intact. In the 1970s, astronomer Vera Rubin found several more examples of galaxies that have too few stars to produce the gravity required to stick together. Today, scientist think that the extra mass is made of something they’ve dubbed dark matter that makes up an astounding 80% of the universe. Scientists think even Earth is home to a small amount of dark matter, and yet nobody knows what dark matter is.

“Every bit of science that humans have ever seen, on Earth or in space, is based on one fifth of the matter in the universe,” said Perez. “And for dark matter people, that bugs us. What the heck is the other stuff?”

Over the last 50 years, theoretical physicists have come up with hypotheses about what dark matter could be, and experimentalists like Perez have tested these hypotheses. They’ve smashed normal matter together in particle accelerators in an attempt to produce dark matter. They’ve buried vats of liquid xenon in former mine shafts and waited for dark matter to get stuck in it. This year, they plan to use telescopes to look for evidence that collisions between dark matter and visible matter are heating the solar system. So far, none of the hypotheses have panned out.

Currently, some theorists think that dark matter particles may create a type of exotic sub-atomic particle called antideuterons when they collide — something particles of normal matter can’t do. Detectors on the GAPS balloon are optimized to detect antideuterons, and thereby try to uncover the properties of dark matter.

To prepare for the GAPS mission, Roach and his collaborators spend their time testing the detectors to make sure they’ll pick up any antideuterons they come across. These particles will cause silicon discs within the detectors to produce x-rays, so Roach mimics this by exposing the detectors to an x-ray-producing chemical called Cadmium 109, then seeing if they respond properly.

Scientists have been kicking around the idea that dark matter interactions produce antideuterons for decades, and they’ve looked for antideuterons with balloons bearing less-sensitive detectors and with the space station. But despite these high-tech search tools, Perez said that when it comes to dark matter, “we know a bunch of things it’s not, but we don’t know what it is.”

Some scientists think it’s time to give up the search. Astronomer James Schombert from the University of Oregon suspects that after decades of failure, the quest has only continued because there are “big egos and lifetimes of work on the line,” he wrote in an email. “It becomes increasingly difficult to watch the physics community move the goalposts on dark matter detection with no framework to support their continued construction of new detectors (like GAPS).”

But theoretical physicist Hai-bo Yu from the University of California, Riverside says that most scientists disagree with Schombert. “There’s no question for me that dark matter should exist,” he said. “Dark matter is one assumption you make that explains almost everything related to [celestial] structure formation.” Not only does dark matter explain why galaxies spin as fast as they do, it also explains why galaxies bend passing light more than would otherwise be predicted, and why there are clumps in a layer of energy left over from the big bang. Without dark matter, scientists would have to use many different theories to explain all these observations.

“Truth be told, there have been times that I’ve been convinced by the argument that it doesn’t [exist],” Roach said, adding that it’s possible scientists simply don’t understand how gravity works. But he agrees with Yu that dark matter seems like the simplest, and therefore most likely, way to explain many of the universe’s inconsistencies at once.

Sometime over the next year, the team plans to assemble the detectors into a 3.5-ton mega detector called a spectrometer that will hang from the balloon. From there, it’s off to Antarctica, where they’ll hope the winds are in their favor. Roach is already excited about the possibility that one of the detectors he’s spent months testing will register an antideuteron.

“If we see even a few of them,” he said, “that will be revolutionary.”