Scientists know dark matter exists because even though we can’t see it, we can see the effects of what it’s doing in the world, much like a ghost bumping around a haunted house. And we don’t know 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 on Monday that featured an online conversation between Peter Fisher, physics professor Thomas A. Frank at MIT, who has just written a book called “What is dark matter?” 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 short answer to the question posed by the title: “The answer is 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 light on the subject, Fisher traced the history of particle physics, from the invention of quantum mechanics in the 1930s to the development of standard model theory in the 1990s, which explains three of the four known fundamental forces. (electromagnetic, weak and strong interactions, but not gravity).
Along with 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. This motion, the astronomers realized, was happening faster than forces such as the gravity of the component stars could account for. “They studied how galaxies moved relative to each other and how stars moved within galaxies. And the only way they could explain how everything moved in the larger scales of the universe was through the introduction of matter that we couldn’t see,” he said.
The answer, requiring all of these disciplines, was that “there were actually two kinds of matter that we couldn’t see.” These were dark energy and dark matter. “Dark matter causes particles or stars in galaxies to move faster than we would expect from the mass of those galaxies,” he said.
To illustrate, Fisher shared a slide of the Andromeda Galaxy, with its lush swirl of stars emanating from a flat disk. “It looks a lot like the Milky Way in the middle,” he said, pointing to the “bright region in the middle of that: a large black hole about a million times the mass of our sun.” There’s a lot of material being pulled into the dense central region, and you can see there’s this beautiful pancake shape with spiral arms,” containing about a trillion stars.
“What’s particularly interesting is that you can see there’s a pointed end to the disk part. And that edge is only really explainable if you hypothesize that there is a substance called matter. black which exerts a gravitational attraction which gives this shape.
Andromeda is not unique. In fact, Fisher explained, images of deep space provided by the Hubble Space Telescope reveal a striking consistency. “There have been very detailed measurements of literally thousands of galaxies, and they all share the same characteristics,” he said. “A careful study of all these different types of galaxies always leads to the same conclusion, which is that the stars are moving too fast to be explained by the amount of light coming from this galaxy,” he said. “This must indicate the presence of dark matter around the galaxy.”
Franklin, paraphrasing Fisher’s book, likened the search to ghost hunting. “If you have ghosts in your house that move objects, you can’t see them, hear them, or smell them. So what you want to do is figure out from the movements 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 hard to find dark matter because there’s so much normal matter around. You have to look and think about the galaxies as a whole” in order to obtain a large enough scale to study dark matter.
Another theory is that dark matter is made up of primordial black holes, dating back to the origins of the universe. If that’s the case, Fisher noted, these “tiny ‘chunks of matter’ “could just pass through the Earth. They don’t pick up a lot of matter. They can pass through just about anything and no one really notices. .
Ongoing research, 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 country’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 kinds of things that took years to explain,” he said, which gave rise to some UFO theories that still exist. “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 sure that dark matter exists because “all the measurements are made repeatedly using very different types of telescopes,” he said. The movement of stars, for example, has been observed with large optical telescopes and also radio telescopes. “It’s not a guarantee, but it does provide reassurance that the same overall effect is seen in two very different ways.”
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