MIT physicists on the mystery of dark matter in a Harvard talk

Scientists know dark matter exists because while we can’t see it, we can see the effects of what it’s doing in the world, much like a ghost bumping through a haunted house. And we’re not sure what it is, but some think it might just be a WIMP.

These are some of the insights that emerged from a Harvard Science Book Talk Monday, which included an online conversation between Peter Fisher, Thomas A. Frank Professor of Physics at MIT, who is in the process of publishing a book called “What Is Dark Matter ?” wrote. and Melissa Franklin, Mallinckrodt Professor of Physics at Harvard.

Fisher opened the event, sponsored by the Harvard Division of Science, Harvard Library and Harvard Book Store, with a brief answer to the question posed by the title: “The answer is that we don’t know.” He offered several possibilities, explaining that his subject could be a particle, a heavy particle, “tiny black holes from the beginning of the universe, or it could be something we haven’t even thought of.”

To shed some light on the subject, Fisher told the history of particle physics, from the invention of quantum mechanics in the 1930s to the development of the Standard Model theory in the 1990s, the three out of four known fundamental forces (electromagnetic, weak, and strong interactions, but none gravity).

Parallel to the development of this science, astronomers studying the universe were making discoveries about the movement of stars away from Earth – evidence that the universe is expanding. Astronomers realized that this movement was happening faster than forces like the gravity of individual stars could explain. “They studied the way galaxies moved relative to each other and the way stars moved within galaxies. And the only way to explain how everything moves at the largest scales in the universe was to introduce matter that we couldn’t see,” he said.

The answer, which required all of these disciplines, was that “there were actually two kinds of matter that we could not see.” These were dark energy and dark matter. “Dark matter causes particles or stars in galaxies to move faster than one would expect based on the mass in those galaxies,” he said.

To illustrate, Fisher shared a slide of the Andromeda Galaxy with its lush swirl of stars radiating out in a flat disk. “It looks a lot like the Milky Way at its center,” he said, pointing out the “glowing region right in the middle of it: a large black hole about a million times the size of our sun.” There’s a lot of matter being pulled into the dense central region, and you can see that there’s this beautiful spiral-armed pancake shape that contains about a trillion stars.

“What’s particularly interesting is that you can see that the disc part has a sharp end. And that edge is really only explainable if you hypothesize that there’s a substance called dark matter that exerts a gravitational pull that creates this shape.”

Andromeda is not unique. In fact, Fisher explained, Hubble Space Telescope images from space show a remarkable agreement. “Very detailed measurements have been taken of literally thousands of galaxies, and they all have the same characteristics,” he said. “A careful study of all these different types of galaxies always comes to the same conclusion, which is that the stars are moving too fast to be explained by the amount of light coming from that galaxy,” he said. “That must indicate the presence of dark matter in the galaxy.”

Franklin, paraphrasing Fisher’s book, likened the search to a ghost hunt. “If you have spirits in your house moving things, you cannot see, hear, or feel them. So what you want to do is use the movements to figure out what exactly is going on.”

What dark matter is, however, is much less clear. One theory is that it is a new type of particle, a weakly interacting massive particle (or WIMP). If that theory is correct, Fisher said, dark matter is probably everywhere — but “here on Earth, it’s difficult to find dark matter because there’s so much normal matter. You have to look at galaxies as a whole and think about it” to get a big enough scale to study dark matter.

Another theory holds that dark matter is primordial black holes, dating back to the very beginnings of the universe. If that’s the case, Fisher noted, these “tiny” chunks of matter “could just go right through Earth. They don’t take up much matter. You can go through pretty much anything and nobody really notices.”

The ongoing search, Fisher warned, will require continued advances in technology, but also caution and a thorough understanding of how our tools work. To illustrate what can go wrong, he described the nation’s Distant Early Warning Line, a system of radar stations along the Arctic Circle created as a defense against a possible Soviet missile attack during the Cold War. “These radar operators saw all sorts of things that took years to explain,” he said, leading to theories about UFOs that are still floating around. “Every time you build a new device, you see things you don’t expect.”

As the search for dark matter continues, such meticulous discipline is vital. However, despite the many questions that remain, we can be confident that dark matter exists because “all measurements are made repeatedly with very different types of telescopes,” he said. For example, the movement of stars has been observed with large optical telescopes and also with radio telescopes. “It’s not a guarantee, but it gives you the assurance that the same overall effect is being observed in two very different ways.”