Despite plenty of circumstantial evidence for the existence of dark matter – the mysterious form of matter that dominates galaxies and clusters – astronomers have yet to directly observe it.
But the search is not over yet. One hypothesis about the nature of dark matter is that some of them can be self-reactive, which means that individual particles interact little with each other. If this were true, there would be a host of careful observational evidence for the existence of this subclass of dark matter.
A few of these hints were recently outlined in a paper submitted for publication in Reviews of Modern Physics and published in the preprint database. arXiv (Opens in a new tab).
Related: Here’s how massive galaxy clusters reveal the secrets of dark matter
strong gravitational lens
The strobe lens occurs when there is a lucky coincidence of the observations. When astronomers look at a distant galaxy cluster, for example, they can also see some light from distant galaxies passing through the cluster. The mass of the galactic group (usually 10^14 or 10^15 times sun block) are so large that they bend and distort the texture of the space around them. This distorts background images galaxiestransforming them from familiar fan-shaped and oval structures into long, curvy snakes and other fun shapes.
Astronomers can reconstruct those distorted images and use this reconstruction to determine how much mass is in the mass and where it collects. Self-reactive dark matter usually has a different “clump” than regular, non-interacting dark matter. Uninteracted dark matter will continue to accumulate to incredibly high densities in the cores of galaxy clusters, because there is nothing else there to stop it. But when dark matter interacts with itself, it slows down the nucleus-building process and smooths things out in the cluster’s interior.
Detailed notes (like the one I gave recently James Webb Space Telescope) for the distribution of mass within galaxy clusters may provide evidence for the presence of dark matter.
double gravitational lens
In contrast to the strong gravity lensA weak lens does not require massive occlusion. Instead, as the light from many distant galaxies made its way through the universe, the galaxies piled up gravity Of all the galaxies and other objects that light passes near in its journey, it changes it in small ways. For example, galaxies in one direction may appear slightly more rounded or fatter than galaxies in other directions.
A strong gravitational lens requires lucky alignment, so we don’t have a lot of kits to work with. But although weak gravitational lenses produce a much smaller effect, we have plenty of data to use. Astronomers are very excited about Roman space telescope Nancy Grace launchedwhich will provide detailed weak lens maps of the nearby universe and may tell us whether dark matter is interacting with itself.
(Opens in a new tab)
In the 1970s, an astronomer Vera Robinmovement notes stars Inside galaxies provided the first significant evidence for the existence of dark matter. In short, galaxies rotate very quickly. If we sum up all the mass in a galaxy based on what we can see, there isn’t enough gravitational pull to hold stars with such fast orbits. Therefore, there must be more mass that we cannot see: dark matter.
Again, because self-reactive dark matter clumps differently than non-interacting matter, this can change the spin curves (the velocities charts of stars in different orbits) for galaxies.
Throughout its billions of years of life, it has constantly rained material on every galaxy of its surroundings. In other words, every galaxy is swimming in an ocean of things. These matter can include both normal matter and dark matter. When dark matter interacts with itself, this causes the dark matter portion of the galaxy to be pulled slightly behind normal matter (because normal matter can swim through all surrounding objects without a problem).
This can cause galaxies to have two slightly balanced cores: one made of normal matter and one made of dark matter. This displacement causes tidal disturbances throughout the galaxy, which can cause a disc Galaxy to warp. Future detailed observations of galaxies may reveal a twist in the disk that can only be explained by self-interacting dark matter.
When giant galaxy clusters merge, astronomers can look at the debris to understand what’s inside. For example, the famous Bullet Cluster shows what happened when two groups merged: stars and dark matter (measured through a gravitational lens) passed through each other untouched, while all the bulk gas in the groups smashed into each other at the center of the collision.
The fact that dark matter is present at the fringes of the system tells us that dark matter does not interact with itself much; Otherwise it would have tangled in the center next to the gas. The cluster of bullets and other clusters like it allow astronomers to place limits on how strong dark matter can interact with itself. More observations will lead to more precise bounds and possibly positive evidence for dark matter self-interaction, if that provides a better fit for the observations.
Learn more by listening to the Ask an Astronaut podcast, available at Itunes (Opens in a new tab)And the askaspaceman.com (Opens in a new tab). Ask your own question on Twitter using #AskASpaceman or by following Paul Tweet embed (Opens in a new tab)And the facebook.com/PaulMattSutter (Opens in a new tab).
Follow us on Twitter Tweet embed (Opens in a new tab) or on Facebook (Opens in a new tab).
“Infuriatingly humble alcohol fanatic. Unapologetic beer practitioner. Analyst.”