Despite a wealth of indirect evidence that dark matter — a mysterious form of matter that dominates galaxies and star clusters — exists, astronomers have yet to observe it directly.
But the search isn’t over yet.hypothesis of a nature dark matter Yes, some of them may be self-interacting, which means slight interactions between individual particles. If this is true, there will be many subtle observational clues about the existence of this subclass of dark matter.
Some of these hints were recently outlined in a paper submitted for publication in the journal Modern Physical Review and published to a preprint database arXiv (opens in new tab).
related: Here’s how giant galaxy clusters reveal the secrets of dark matter
Strong gravitational lensing
Strong lensing effects occur when a lucky coincidence is observed. For example, when astronomers look at a distant galaxy cluster, they can also see some light from more distant galaxies passing through the cluster.The mass of the galaxy cluster (usually 10^14 or 10^15 times solar mass) is so large that it bends and distorts the structure of space around it.This distorts the background image galaxytransforming them from familiar pinwheels and oval structures into long, wobbly snakes and other interesting shapes.
Astronomers can reconstruct these distorted images and use that reconstruction to determine how much mass is in a cluster and where it is concentrated. In general, self-interacting dark matter has different “clumps” than regular non-interacting dark matter. Non-interacting dark matter will continue to accumulate to incredibly high densities in the cores of galaxy clusters because nothing else can stop it. But when dark matter interacts with itself, this slows down the process of core building and smooths things out inside the cluster.
Detailed observations (as recently reported by James Webb Space Telescope) mass distribution inside galaxy clusters may provide clues to the existence of dark matter.
Weak gravitational lensing
the opposite of strong Gravitational lensing, weak lenses do not require huge obstacles.Instead, as light from many distant galaxies travels through the universe, the accumulated gravity All galaxies and other objects that light passes through on its journey alter it in tiny ways. For example, galaxies in one particular orientation may appear rounder or fatter than galaxies in other orientations.
Strong gravitational lensing requires lucky alignment, so we don’t have many clusters to work with. But even though weak gravitational lensing has a much smaller effect, we have more data to work with.Astronomers are very excited about this Launch of the Nancy Grace Roman Space Telescopewhich will provide a detailed weak-lensing map of the nearby universe and possibly tell us whether dark matter is self-interacting.
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In the 1970s, astronomers Vera Rubinobservation of movement Star The interior of galaxies provided the first major evidence for the existence of dark matter. In short, galaxies are spinning too fast. If we add up all the mass in the galaxy based on what we can see, there simply isn’t enough gravity to support a star with such a fast orbit. So there must be more mass we can’t see: dark matter.
Also, since self-interacting dark matter aggregates differently than non-interacting matter, this can alter the rotation curves of galaxies (plots of the velocities of stars in different orbits).
During their billion-year lifespan, matter is continuously poured down from every galaxy around them. In other words, every galaxy is swimming in a sea of matter. Such matter can include conventional matter and dark matter. When dark matter interacts with itself, this causes the dark matter portion of the galaxy to lag slightly behind normal matter (because normal matter can swim through all surrounding matter with no problem).
This causes the galaxy to have two slightly offset cores: one composed of ordinary matter and one composed of dark matter. This offset triggers tidal disruption across the galaxy and may even cause the Milky Way’s disk to bend. Detailed observations of galaxies in the future may reveal distortions in the disk that only self-interacting dark matter can explain.
When massive galaxy clusters merge, astronomers can look at the remnants to see what’s inside. For example, the famous Bullet Cluster shows what happens when two clusters merge: stars and dark matter (measured by gravitational lensing) pass through each other untouched, while all the loose gas in the clusters smashes into each other at the center of the collision.
The fact that dark matter is at the periphery of the system tells us that dark matter doesn’t interact with itself very often. Otherwise, it wraps around the center next to the gas. The Bullet Cluster and other similar clusters allow astronomers to limit the strength of dark matter’s interaction with itself. More observations will lead to more precise confinement, and possibly even positive evidence for self-interacting dark matter, if this provides a better fit for the observations.
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