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Astronomers have naturally centered their telescopes on the a part of the universe closest to us, since these stars and galaxies will be examined within the best element. However astrophysicists have typically struggled to sq. the inhabitants of our personal galactic neighborhood with dark-matter theories. For instance, earlier fashions predicted extra neighboring galaxies than have really been noticed in the actual universe, a problem dubbed the “lacking satellites” drawback.
Huge clumps of darkish matter ought to have sufficient gravitational pull to herald the fuel that builds up into stars and, later, galaxies. However one other drawback is that some simulations find yourself producing huge, orbiting darkish matter clumps, which appear to be those that ought to host satellite tv for pc galaxies—however they don’t appear to have any real-universe counterparts. That is referred to as the “too-big-to-fail” drawback, since big blobs of darkish matter are regarded as too large to fail to type galaxies inside them.
A 3rd problem comes from the truth that the satellite tv for pc galaxies swirling across the Milky Method and Andromeda appear to be orbiting in a airplane, reasonably than unfold out throughout—one thing dark-matter physicists hadn’t predicted.
There are additionally cosmological issues that Frenk and his colleagues need to deal with. Astronomers utilizing close by supernova explosions and different native phenomena to measure how briskly the universe is at the moment increasing get completely different solutions than these probing the early universe. If dark-matter fashions are proper, there must be a technique to resolve the troubling and chronic discrepancy between previous and present observations.
However simulations like SIBELIUS may assist. It’d end up that the place a galaxy lives on the cosmic net of darkish matter actually does make a distinction for measurements of the universe’s enlargement fee. What if the Milky Method lies form of in a “gap” within the net—if it’s extra like a rural space between darkish matter metropolises? If our a part of the universe isn’t really consultant, then our native measurements of how briskly the universe is blowing outward is perhaps a little bit biased.
The Milky Method may occur to be located in a reasonably dense area of darkish matter or in a sparse one, says Priyamvada Natarajan, a Yale College astrophysicist and dark-matter knowledgeable. “What’s cool about this simulation is that they’ll deal with: How typical or uncommon is our native quantity? How uncommon is the distribution of matter that we see round us? Are we on a mountain or are we in a valley?” she says.
When evaluating galaxies noticed with telescopes to what’s seen in simulations, it’s crucial to check apples to apples, says Jenny Sorce, an astrophysicist on the Institut d’Astrophysique Spatiale in Orsay, France, who helped design an analogous sort of simulation, referred to as CLONE, centered on galaxies within the Virgo cluster. “It’s not like you possibly can examine one sort of cluster with one other one in the event that they don’t share the identical historical past or the identical surroundings,” she says.
Frenk and his group did loads of preliminary checks with their very own computer systems at low decision. However time on supercomputers, like on telescopes, is restricted. They solely had a single probability to run their full simulation, which took thousands and thousands of hours of computing time on hundreds of laptop cores. However based mostly on their simulation’s outcomes, they discover that the Milky Method’s neighborhood certainly appears atypical: We stay in a cosmic area with fewer than common galaxies, however there are additionally extra huge galaxy clusters than on common. It’s like dwelling in a low-elevation metropolis, like Los Angeles, that nonetheless has mountain ranges within the distance.
If the Milky Method is certainly an oddball, it’d assist clarify some darkish matter mysteries, Frenk and Boylan-Kolchin speculate. If we’re in a sparse a part of the universe, which may clarify why native measurements of the enlargement fee are completely different than one would count on based mostly on measurements of the faraway universe.
And if our galaxy is in the midst of an atypical neighborhood, which may clarify why the satellites are in an uncommon configuration—possibly they have been pulled into the Milky Method’s orbit in a selected method.
In different phrases, if the Milky Method’s neighborhood is certainly uncommon, it means the chilly dark-matter concept will survive these challenges—for now.
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