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Here’s what you’ll learn when you read this story:
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Gravitational lensing is one of the few methods scientists have to search for elusive dark matter, which famously doesn’t interact with the electromagnetic force like normal matter.
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A new study using the elliptical galaxy B1938+666 as a gravitational lens reports the discovery of the lowest-mass dark matter clump ever.
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Although still incredibly massive—roughly one million solar masses—it’s still 100 times less massive than the previous record holder.
For nearly a century, finding evidence of dark matter has been top priority for astronomers and particle physicists alike. The urgency makes sense because according to the Lambda cold dark matter (ΛCDM) theory—our best guess at the machinations of the Big Bang cosmology and universal structures—dark matter forms the theory’s very foundation (hence the name) and contains roughly 10 times more mass than normal matter throughout the universe.
There’s just one problem—dark matter doesn’t directly interact with electromagnetic force like normal matter. However, dark matter does interact gravitationally, primarily through a method known as gravitational lensing. Using this technique, an international team of scientists believe they’ve spotted the lowest mass “dark object” ever detected using gravitational lensing. Of course, by “lowest” we still mean incredibly massive by comparison—about one million solar masses.
The team reported the results in two companion studies in Nature Astronomy and Monthly Notices of the Royal Astronomical Society. “Hunting for dark objects that do not seem to emit any light is clearly challenging,” Max Planck Institute for Astrophysics’s Devon Powell, lead author of the Nature Astronomy study, said in a press statement. “Since we can’t see them directly, we instead use very distant galaxies as a backlight to look for their gravitational imprints.”
To achieve this, Powell and his team relied on a massive elliptical galaxy called B1938+666, which acts as a gravitational lens for even more distant galaxies. Discovered as part of the Jodrell Bank-VLA Astrometric Survey (JVAS) and the Cosmic Lens All Sky Survey (CLASS)—a dedicated search for gravitational lenses—in the 1990s, B1938+666 was also the first Einstein Ring ever discovered (though, as its name suggests, Einstein theorized this kind of ring decades earlier). Einstein rings are a kind of useful cosmic mirage as a massive galaxy in the foreground bends the light emitted from another galaxy in the background, forming a kind eerie halo around when perfectly aligned.
European Space Agency
In this new study, astronomers relied on multiple radio telescopes, including the Green Bank Telescope, the Very Long Baseline Array, and the European Very Long Baseline Interferometric Network to form what they called an “Earth-sized super-telescope.” Using this “super-telescope,” astronomers were able to detect a subtle wobble within the arc of radio waves that form the Einstein ring, likely caused by an unexplained gravitational disturbance.
“From the first high-resolution image, we immediately observed a narrowing in the gravitational arc, which is the tell-tale sign that we were onto something.” University of Groningen’s John McKean, lead author of the Monthly Notices study, said in a press statement. “Only another small clump of mass between us and the distant radio galaxy could cause this.”
The team concluded the mass of the “dark object” must be around one million times that of the Sun (which is, amazingly, 100 times smaller than the previous record holder) and is located some 10 billion light-years from Earth. ΛCDM suggests that even our own Milky Way should be filled with these dark matter clumps, but theoretical suggestions and empirical evidence are two very different things—and it’s not a very easy chasm to bridge.
“Given the sensitivity of our data, we were expecting to find at least one dark object, so our discovery is consistent with the so-called ‘cold dark matter theory’ on which much of our understanding of how galaxies form is based,” Powell said in a press statement. “Having found one, the question now is whether we can find more and whether their number will still agree with the models.”
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