A galaxy that’s supposedly devoid of all darkish matter may truly be stuffed with it.
And that, paradoxically, may imply darkish matter doesn't exist.
What?! Again in March, a crew of astrophysicists printed a paper within the journal Nature claiming to have discovered a galaxy missing darkish matter — a hypothetical type of invisible matter that appears to exert gravitational pull with out interacting with mild.
Scientists have advised the existence of this weird matter to elucidate a just-as-bizarre phenomenon: Based mostly on the sunshine astronomers can see with their telescopes, the universe acts like there may be way more mass, and subsequently way more gravitational power, than Albert Einstein's theories predict primarily based on what we are able to see.
The Nature paper was huge information. It might have been the primary such object ever discovered within the universe. Furthermore, it appeared to deal a loss of life blow to an alternate concept explaining the weirdness of the universe: In that concept, gravity itself simply acts just a little in a different way than we (and Einstein) thought. If gravity is just a little totally different than Einstein's equations counsel, then it needs to be totally different all over the place within the universe — together with on this seemingly dark-matter-free galaxy — not simply in some locations.
Now, a second crew of physicists has posted a paper to the preprint journal arXiv claiming that this galaxy acts as if it has darkish matter in any case. If true, that will imply that the main various to darkish matter may nonetheless be true.
Nonetheless, the brand new arXiv paper hasn't but been peer reviewed, and the authors of the March Nature paper don't agree with the second paper's conclusions.
Darkish matter or one thing else?
The unique paper from March primarily based its beautiful declare of a dark-matter-free galaxy on the way in which clusters of stars moved by means of the skinny, diffuse galaxy referred to as NGC1052–DF2: They appeared to maneuver at precisely the pace Einstein's equations of basic relativity would predict primarily based on the seen matter (so, slower than they might if the galaxy held darkish matter).
This new paper on arXiv advised in any other case: First, the authors identified that NGC1052–DF2 was already found method again in 1976 and has beforehand been referred to by three totally different names: KKSG04, PGC3097693 and [KKS2000]04.
Then, utilizing these names after which discovering all of the accessible knowledge on the galaxy, the researchers argued that the researchers from the March paper merely mismeasured the gap between that galaxy and Earth. This implies the galaxy might be a lot nearer to us than the unique researchers thought.
Astronomers calculate the mass of galaxies primarily based on the objects' brightness and distance. If the galaxy examined within the paper is nearer to Earth than beforehand thought, then its dimness means it's additionally a lot much less huge than researchers thought. And on the newly calculated, lighter mass, all the opposite options of the galaxy make much more sense, the researchers within the new paper stated. Its globular clusters aren't transferring slowly as a result of they're in some unusual darkish matter-desert; as a substitute, they're transferring on the common pace for a really light-weight galaxy, the arXiv authors stated. [The 18 Biggest Unsolved Mysteries in Physics]
Nonetheless, Pieter van Dokkum, an astrophysicist at Yale College and the primary writer of the March Nature paper, stated the brand new arXiv evaluation is flawed.
"The important thing distance measurement comes from the Hubble Area Telescope knowledge, and it appears to be like like they misinterpreted it. The argument is a bit technical, however basically, they mistook pixel variations for particular person stars. If NGC1052-DF2 had been as shut as they declare, then the galaxy can be resolved right into a sea of particular person stars," but it surely's not, Van Dokkum instructed Micronis in an electronic mail.
Nonetheless, Ignacio Trujillo, an astrophysicist on the Instituto de Astrofisica de Canarias and the primary writer of the brand new arXiv paper, stated he and his colleagues stand by their estimates. Many particular person stars might be seen in pictures of NGC1052-DF2 when the distinction is modified in pictures, Trujillo instructed Micronis in an electronic mail.
As well as, "our paper not solely makes use of a standards for estimating the gap to the galaxy, however as much as 5 totally different (and utterly impartial) strategies, and all of them converge to the identical quantity," he stated.
The gist of the (darkish) matter
There's one thing deeper happening, although, than a dispute concerning the motion in a small, faraway galaxy: a struggle concerning the existence of darkish matter itself. Researchers who argued that NGC1052–DF2 (or DF2 for brief) doesn't include darkish matter had been seen as providing proof that darkish matter actually does exist within the universe — bear in mind, the choice concept wants gravity to behave in a different way all over the place.
Since a minimum of the 1980s, a gaggle of astrophysicists has argued that the entire concept of darkish matter is defective science, as Micronis reported in March. The darkish matter proposal is basically an untestable concept, these scientists argue, that may be speculated about ceaselessly however by no means disproven. And whereas researchers have seen some oblique proof of darkish matter along with its pull on distant stars and galaxies, they’ve by no means captured a single fragment of the stuff or in any other case straight demonstrated its existence, skeptics have argued.
Most dark-matter skeptics have an interest within the various concept, wherein gravity is just a little off-kilter. Referred to as "Modified Newtonian Dynamics" (MOND), the speculation suggests a set of tweaks to the legal guidelines of physics at galactic scales that remove the necessity for darkish matter. If the true legal guidelines of gravity are only a bit totally different than what you'd at the moment discover in physics textbooks, these scientists argue, maybe darkish matter isn't obligatory to elucidate the conduct of the universe in any case.
When the paper on NGC1052-DF2 got here out in March, it was introduced as MOND's loss of life knell. Discovering one galaxy with out darkish matter doesn't disprove the existence of darkish matter, identical to a desert doesn't disprove the existence of lakes. However MOND doesn't have that luxurious; if the modified bodily legal guidelines are actual, they must be constant all through the universe. In different phrases, if one galaxy follows non-MOND legal guidelines of physics with out the affect of darkish matter, then MOND can't be true.
For that purpose, DF2 "appears to be like on the floor to be one other nail within the coffin of MOND," Kathryn Zurek, an astrophysicist on the Lawrence Berkeley Nationwide Laboratory in California, who didn’t work on both paper, wrote in an electronic mail to Micronis in March.
That was how most physicists reacted.
However not everybody.
Stacy McGaugh, an astrophysicist at Case Western Reserve College in Ohio, who has labored on each darkish matter and MOND however didn’t work on both current paper, was skeptical.
In a follow-up interview in April, McGaugh instructed Micronis that the DF2 claims didn't account for the "exterior subject impact" — a MOND tweak that will permit a big close by galaxy to affect DF2's conduct — suggesting that MOND hadn't suffered the blow some had advised.
He additionally stated the DF2 claims had been primarily based on some questionable statistics: The researchers measured the pace of simply 10 objects within the galaxy as a way to calculate DF2's total velocity, excluding one object that behaved very in a different way from the others. McGaugh stated that excluding anomalies is typically cheap however that on this case, his calculations confirmed that together with that very totally different object modified the paper's conclusions dramatically. With the article omitted, the galaxy is inconsistent with MOND. With the article included within the calculations, the entire galaxy appears to be like much more regular, McGaugh stated.
Now, with this new arXiv paper, which nonetheless hasn't been reviewed by peer scientists, there are new arguments in opposition to a dark-matter-free galaxy — and the very existence of darkish matter — however these arguments have spurred further arguments. The darkish matter saga continues.
Initially printed on Micronis.