- 3492 - DARK MATTER - hot and cold theories? If Dark Matter is 75% of all the matter in our galaxy, we ought to learn what it is! One of the first theories was that it was “cold dark matter” , which are invisible particles having relatively slow speed.
--------------------- 3492 - DARK MATTER - hot and cold theories?
- The alternative theory of faster-moving “hot dark matter“. The possibility that it is made of particles like neutrinos was quickly ruled it out. The theory of cold dark matter became astrophysicists’ “standard model” for two decades.
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- In 2022 astronomers completed the first part of a computer simulation of the dark-matter universe; it’s dubbed the ‘Simulations Beyond the Local Universe” project, or “SIBELIUS“.
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- This “dark-matter simulation” with galaxies modeled in it, is providing a detailed, three-dimensional picture of what our galaxy and our corner of the universe likely looks like, if the standard view of cold dark matter is right.
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- This is the first attempt to simulate our patch of the universe including the Coma cluster and the Virgo cluster. Those kinds of cosmic landmarks, which lie tens of millions of light years from Earth might matter for understanding the assembly and evolution of our own galaxy over billions of years.
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- Astronomers have thoroughly mapped out our local region, spotting dozens of small and faint “satellite” galaxies, like the “Large Magellanic Cloud“, which orbit the Milky Way similar to the way the moon orbits the Earth.
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- For decades they’ve also charted galaxy clusters and other objects beyond the neighborhood. SIBELIUS is more complex, because it builds on these impressive observations of our cosmic neighborhood and it actually tries to reproduce, to some extent, that local geography.
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- The SIBELIUS simulation is meant to resemble a 3D space that’s 3.3 billion light-years on a side. By design, in this virtual cosmos, we’re the center of the universe. The Milky Way resides in the middle, along with the neighboring Andromeda galaxy.
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- SIBELIUS is something called a “constrained realization,” meaning that simulations of these and other local galaxies must closely match what’s known about them in the real universe.
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- By mapping them in a broader context, the team wants to see whether this region is representative of the entire universe, or rather atypical. Atypical might mean that there are many more, or fewer, galaxies in the surrounding environs than the expected average.
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- Most physicists believe that huge yet hidden webs of dark matter hold galactic structures together. In some spots in the SIBELIUS box, there’s a little more dark matter than in others. Dark matter starts clumping together, and then those clumps grow. The model how galaxies build up and grow within those clumps, and then compare what happens in this simulation to what’s known about the real world.
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- We want to know how dark matter and other galaxies beyond the Milky Way’s borders shaped its past. How rare are certain attributes of the Milky Way, and how much of that is related to the larger-scale environment?
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- Astronomers have naturally focused their telescopes on the part of the universe closest to us, since those stars and galaxies can be examined in the greatest detail. But astrophysicists have sometimes struggled to square the population of our own galactic neighborhood with these dark-matter theories.
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- Earlier models predicted more neighboring galaxies than have actually been spotted in the real universe, an issue dubbed the “missing satellites” problem. Big clumps of dark matter should have enough gravitational pull to bring in the gas that builds up into stars and, later, galaxies.
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- Another problem is that some simulations end up producing big, orbiting dark matter clumps, which look like the ones that should host satellite galaxies, but , they don’t seem to have any real-universe counterparts. This is called the “too-big-to-fail” problem, since huge blobs of dark matter are thought to be too massive to fail to form galaxies within them.
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- A third challenge comes from the fact that the satellite galaxies swirling around the Milky Way and Andromeda seem to be orbiting in a plane, rather than spread out all around, something dark-matter physicists hadn’t predicted.
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- Astronomers using nearby supernova explosions and other local phenomena to measure how fast the universe is currently expanding get different answers than those probing the early universe. If dark-matter models are right, there has to be a way to resolve the troubling and persistent discrepancy between past and current observations.
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- Simulations like SIBELIUS might help. It might turn out that where a galaxy lives on the cosmic web of dark matter really does make a difference for measurements of the universe’s expansion rate.
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- What if the Milky Way lies sort of in a “hole” in the web. It’s more like a rural area between dark matter metropolises? If our part of the universe isn’t actually representative, then our local measurements of how fast the universe is blowing outward might be a little biased.
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- The Milky Way might happen to be situated in a fairly dense region of dark matter or in a sparse one. How typical or unusual is our local volume? How rare is the distribution of matter that we see around us? Are we on a mountain or are we in a valley?
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- When comparing galaxies observed with telescopes to what’s seen in simulations, it’s necessary to compare apples to apples. A similar kind of simulation, called “CLONE“, focused on galaxies in the Virgo cluster. You can not compare one type of cluster with another one if they don’t share the same history or the same environment.
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- Time on supercomputers took a single chance to run their full simulation took millions of hours of computing time on thousands of computer cores. Based on their simulation’s results, they find that the Milky Way’s neighborhood indeed seems atypical:
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--------------------- We live in a cosmic region with fewer than average galaxies, but there are also more big galaxy clusters than on average.
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- If we're in a sparse part of the universe, that might explain why local measurements of the expansion rate are different than one would expect based on measurements of the faraway universe.
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- If our galaxy is in the middle of an atypical neighborhood, that might explain why the satellites are in an unusual configuration, maybe they were pulled into the Milky Way's orbit in a particular way. If the Milky Way’s neighborhood is indeed unusual, it means the cold dark-matter theory will survive these challenges.
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- The jury’s still out. And there’s plenty of room for improvement with the SIBELIUS simulation. It would be an even better resource if their galaxy formation model incorporated fluid dynamics to follow the gas clouds that form new stars and make galaxies grow.
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- That way, galaxies would emerge more naturally within dark matter clumps, which could prove helpful for investigating the more subtle dark-matter problems.
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- How to get our galaxy and its neighbors replicated on a computer? A programming challenge for sure!
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March 7, 2022 DARK MATTER - hot and cold theories? 3492
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