There is no exotic dark matter, according to the most sensitive search to date. The ramifications for cosmology are enormous.
A lot of time, money, and effort has gone into looking for something that doesn’t exist. Cosmologists need cold dark matter to make their theories of the big bang work. They need something that doesn’t shine, doesn’t interact with normal matter, but has gravity. And they need lots of it: according to current theory, it outclasses normal matter ten to one. Astrophysicists have pondered what it might be: exotic entities like “weakly interacting massive particles” (WIMPS), axions, and other made-up names for Mysterious Unknown Stuff.
Now, the most sensitive detector has turned up nothing. Space.com reports, “Dark Matter Still a Mystery: Most Sensitive Search Yet Comes Up Empty.”
The incredibly sensitive LUX dark-matter detector, buried under a mile of rock, has come up empty on its 20-month search for dark matter — further narrowing down the possible characteristics of the strange substance.
Researchers presented the results today (July 21) at the 11th Identification of Dark Matter Conference (IDM2016) in Sheffield, U.K., which gathers together researchers seeking to understand dark matter, the mysterious material that appears to make up more than four-fifths of the universe’s mass, but which scientists have not observed directly.
Nature is apparently not cooperative with current theory. The null result comes even after collaborative efforts “pushed the sensitivity of the instrument to a final performance level that is four times better than the original project goals.” If WIMPs existed, scientists should have seen a flash from detectors monitoring 1/3 of a ton of liquid xenon surrounded by 72,000 gallons of purified water deep in a South Dakota gold mine.
“It would have been marvelous if the improved sensitivity had also delivered a clear dark-matter signal,” Gaitskell said. “However, what we have observed is consistent with background alone.”
Another lambasted Nature’s rudeness for being so uncooperative:
“Though a positive signal would have been welcome, nature was not so kind!” Cham Ghag, a physicist at University College London and collaborator on LUX, said in another statement. “Nonetheless, a null result is significant as it changes the landscape of the field by constraining models for what dark matter could be beyond anything that existed previously.”
But does a non-detection of ghosts “constrain models” for what they could be? Maybe it’s time for a major change to theories that demand something that isn’t there. But no; the search will go “Onward,” Space.com says. A next-generation LUX detector may achieve 70 times the sensitivity of the current LUX.
Dark energy: The other major “mysterious unknown stuff” of the universe is dark energy. Science Daily says the Royal Society is accelerating the search for it, but existing tests produce conflicting results. Philosophers of science take note: “Making up 95% of our universe, these substances have profound effects on the birth and lives of galaxies and stars and yet almost nothing is known about their physical nature.”
Radio bursts: What is the “most perplexing mystery in astronomy”? The answer, according to Nature, is the source of ultra-powerful radio bursts. No theorists predicted these, but they may be common. One of these bursts can emit the energy of 500 million suns in just 5 milliseconds, suggesting that they come from very compact objects. About 20 have been observed so far.
Whatever these objects are, recent observations suggest that they are common, with one flashing in the sky as often as every 10 seconds. Yet they still defy explanation. Theorists have proposed sources such as evaporating black holes, colliding neutron stars and enormous magnetic eruptions. But even the best model fails to account for all the observations, says Edo Berger, an astronomer at Harvard University in Cambridge, Massachusetts, who describes the situation as “a lot of swirling confusion”.
The power of these objects must be unbelievable if they come from cosmological distances. Astronomers hope that some day they might be used as probes of dark matter and dark energy.
There’s still a lot to learn about astronomy. In general, scientists cannot appeal to unobservable reality forever. The historical examples of phlogiston and caloric show that wrong paradigms can prevail among top experts for decades or centuries. Sooner or later, though, scientists need to consider the possibility they have been engaged in a snipe hunt, and need to find reality elsewhere.