Far out cosmology: Any explanation for dark matter? (Introduction)

by David Turell , Monday, September 05, 2016, 01:06 (2790 days ago) @ David Turell

Gravitational evidence says its there, but what is it? We don't know what energy particles are involved:-http://www.scientificamerican.com/article/physics-confronts-its-heart-of-darkness/-"The latest, most sensitive searches for the particles thought to make up dark matter—the invisible stuff that may comprise 85 percent of the mass in the cosmos—have found nothing. Called WIMPs (weakly interacting massive particles), these subatomic shrinking violets may simply be better at hiding than physicists thought when they first predicted them more than 30 years ago. Alternatively, they may not exist, which would mean that something is woefully amiss in the underpinnings of how we try to make sense of the universe.-***-"Whatever dark matter is, it is not accounted for in the Standard Model of particle physics, a thoroughly-tested “theory of almost everything” forged in the 1970s that explains all known particles and all known forces other than gravity. Find the identity of dark matter and you illuminate a new path forward to a deeper understanding of the universe—at least, that is what physicists hope.-***-"One of the latest null results in the search for WIMPs came from the Large Underground Xenon (LUX) experiment, a third of a ton of liquid xenon held at a frosty -100 degrees Celsius inside a giant water-filled tank buried one and a half kilometers beneath the Black Hills of South Dakota. There, shielded from most sources of contaminating noise, researchers have spent more than a year's worth of time looking for flashes of light emanating from WIMPs striking xenon nuclei. On July 21 they announced they had seen none.-***-"We are now more in the dark about dark matter than we were five years ago,” he says. So far, Kolb says, most theorists have responded by “letting a thousand WIMPs bloom,” creating ever-more baroque and exotic theories to explain how WIMPs have managed to dodge all our detectors.-***-"Gaitskell and other WIMP hunters are betting that bigger detectors will yield better results, and have plans for a new generation of experiments with dramatically larger sizes and sensitivities. “I started looking 28 years ago using a 10-gram detector,” Gaitskell says. “Today we're using a detector that is a third of a ton of liquid xenon. And within the next 10 to 15 years we will look with detectors that are 100 tons.”-***-"Many supersymmetry theories predict the lightest superpartner would be a stable, neutral, weakly interacting particle—that is, a WIMP. This is the phantom particle the LHC has been seeking—and failing to find—in its latest collisions. “It's remarkable how these two entirely separate lines of evidence converge to tell you these particles can exist and give you the right kind and amount of dark matter,” says Neal Weiner, a dark matter theorist at New York University. “That's the WIMP miracle.”-***-"Feng and many others propose that WIMPs are part of a much more complicated picture: An entirely new hidden realm of the universe filled with multiple varieties of dark particles interacting with one another through a suite of dark forces, perhaps exchanging dark charges via bursts of dark light. Because they offer theorists many more variables to play with, such “dark sector” models can be reconciled to fit into the ever-tighter straitjacket of facts placed on dark matter by new data.-***-“'Now that we have lost the guidance from the WIMP miracle, the space of available models is huge. It's a playground where we don't know what the right choices are—we now need more hints from nature about where to go next.”-***-"Last year a team of researchers won a Nobel Prize for discovering that ghostly, weakly interacting particles called neutrinos come in three “flavors” and possess mass. The three neutrino varieties are not massive enough to account for dark matter, but by virtue of having mass at all they open the possibility for the existence of a fourth—a massive, so-called “sterile neutrino.”-***
"For axions to explain dark matter, they would need to occupy a relatively narrow range of masses and be far lighter than WIMPs, potentially making them even harder to detect. “If we don't find the WIMP, theorists will just switch their bets to axions,” -***-“'The desire is for dark matter to not only exist but also to solve other outstanding problems of the Standard Model,” says Jesse Thaler, a physicist at Massachusetts Institute of Technology. “Not every new discovery can be a revelation like the Higgs, where afterward theories suddenly fit together much better. Sometimes new particles just make you say, ‘Who ordered that?' Do we live in a universe where each discovery leads to deeper, more fundamental insights or do we live in one where some parts have rhyme and reason but others don't? Dark matter offers either possibility.'”-Comment: We are the same point we were in the 1950's, not knowing exactly what might turn up. The Higgs that appeared was not as heavy as expected, but fits. New knowledge is over the horizon again.