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Scientists can see how dark matter is distributed based on how its gravity affects light, but when astronomers compared recent data from the Hubble Space Telescope and the Very Large Telescope to current models, something didn’t add up. Does this mean our current assumptions about dark matter physics are wrong?

Hosted by: Hank Green

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Sources:
Sources:
https://arxiv.org/pdf/2002.02102.pdf
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.111105
https://www.eurekalert.org/pub_releases/2020-09/nsfc-hos091020.php
https://www.eurekalert.org/pub_releases/2020-09/aaft-msd090820.php
https://www.eurekalert.org/pub_releases/2020-09/eic-nhd091020.php
https://www.spacetelescope.org/static/archives/releases/science_papers/heic2016/heic2016a.pdf
https://news.ucr.edu/articles/2020/09/09/physicists-explain-mysterious-dark-matter-deficiency-galaxy-pair

Image Sources:
https://solarsystem.nasa.gov/resources/2524/newly-processed-views-of-venus-from-mariner-10/
https://www.eurekalert.org/multimedia/pub/242669.php?from=476757
https://svs.gsfc.nasa.gov/11894
https://svs.gsfc.nasa.gov/20244
https://commons.wikimedia.org/wiki/File:NGC_1052_SDSS.jpg
https://svs.gsfc.nasa.gov/11609
Hello there!

As a heads-up, we are super interested and excited about Venus news! But we are not doing our news this week on Venus because we want to spend some time with it and make sure that we're bringing some good stuff to the table.

So, get ready for that next week! But now, for this week's other, also very-exciting space news... {♫Intro♫}. We've known for a long time that a huge fraction of the matter in the universe is dark matter.

But despite knowing about it for decades, we still know next to nothing about what it actually is. We don't know what it's made of, and can't see it because it doesn't interact with light, but we can see the effects of its gravity. And by studying those effects in some distant galaxy clusters, the authors of a paper published last week in the journal Science revealed that this mysterious stuff may behave even more weirdly than we had imagined.

The team focused their observations on several galaxy clusters, which make great places to study dark matter because they're the most massive structures in the universe, so they contain lots of it. And scientists can see how this dark matter is distributed based on how its gravity affects light. See, like any other object with mass, dark matter distorts the fabric of spacetime.

And as a result, when light passes by, it bends, almost as if it were passing through a physical lens. So, if there's a blob of dark matter in between us and some visible object, like a galaxy, that galaxy will look warped and magnified. In other words, it gets gravitationally lensed.

In their study, the researchers used data from the Hubble Space Telescope and the Very. Large Telescope in Chile to identify all the different lensed objects they could see. By determining how strongly they'd been lensed, astronomers were able to map out where dark matter was, and how much was there.

And they found lenses of different sizes, reflecting the fact that dark matter in a galaxy cluster exists on different scales:. On one level, dark matter holds together the hundreds or thousands of galaxies in a cluster. But within that cluster, each individual galaxy also contains dark matter.

So you can see distorted light on both large and small scales. But something was weird about some of the smaller-scale lenses: The effects were much stronger than predicted. In fact, on small scales, researchers found 10 times more strongly lensed objects than they expected.

They based their predictions on simulations, which is another common way that astronomers study dark matter. Basically, they plug what they think they know about the universe into a model and watch how things like galaxies and clusters evolve over time. To a certain extent, the model matched real life: It produced smaller-scale lensing effects nestled within the larger distortions, a lot like the ones in the telescope images.

But in the center of the cluster, the simulated lensing effects were much weaker than in real life. That implied that the massive galaxies at the center of clusters have much more dark matter than models predict. Which most likely means one of two things.

Either something's wrong with our models on galactic scales, or there's something about dark matter itself that we don't yet understand. We don't know yet which is the case, but last week, a paper published in the journal. Physical Review Letters backs up the idea that we don't understand dark matter as well as we think.

See, as much as we still don't know about dark matter, one thing astronomers were pretty sure about was that it only interacts with things through its gravity. In other words, it can't collide with other stuff, including other dark matter. [N-G-C-1-0-5-2] But recent observations of two satellite galaxies orbiting the massive elliptical NGC 1052 suggest that might not be true. These two galaxies don't appear to have nearly as much dark matter as models predict, given the amount of light they emit.

There's still some uncertainty in the measurements, but we're talking around 300 times less dark matter than predicted. So scientists thought that NGC 1052 might have stolen that dark matter from them, a lot like how certain dense stars sometimes siphon off regular matter from their stellar companions. But they didn't know exactly how.

So the authors of the study used computer simulations to see exactly how this heist might have played out. In one simulation, they used a traditional model, which predicts that dark matter doesn't interact with anything. But the end result didn't resemble the true system that closely.

On the other hand, a newer model allowing dark matter to interact with itself did a much better job at replicating reality. The large elliptical galaxy was able to strip matter away from its satellites, because dark matter collisions within them caused some particles to fly outward, making their halos fluffier—in the words of one of the authors. That fluffiness made it easier for the main galaxy's gravity to pull the dark matter off its satellites.

Self-interacting dark matter isn't the only possible explanation for what's going on, though. By tweaking certain values in the traditional simulation of dark matter, the team was also able to reproduce the low concentrations of dark matter… only then its map of regular matter didn't match real-world observations as well. So while these results can't tell us anything for sure about how dark matter behaves, they at least throw a plausible new hypothesis into the ring.

Both of these dark matter studies are fairly small in scale, so it'll take more research before scientists can draw any strong conclusions. But findings like these hint that the truth about dark matter might be even stranger than anyone expected. Thanks for watching this episode of SciShow Space!

And a special thanks to our President of Space, SR Foxley, along with all of our patrons, for making episodes like this possible. If you'd like to join the amazing community of people helping us make science education free on the internet, you can find out more at patreon.com/SciShow. {♫Outro♫}.