A slow, patient survey of the skies, extended from work originally done in the 1990s, has a team from Lawrence Livermore National Laboratory recording the brightness of the stars for years at a time, hoping to see one-in-a million events that might help explain why so much mass seems to be missing from the universe.
“We’re just looking for a needle in a haystack,” said Nathan Golovich, one of the scientists looking for Dark Matter, as the missing mass is often called.
The project’s approach to the problem of missing matter is to look for and analyze telltale brightening and then dimming of stars that can occur in the rare event that a black hole passes between a star and Earth.
No light escapes from a black hole, so it would not be visible directly, but its extraordinarily powerful gravitational field would bend and distort light traveling toward Earth.
“The probability (of a black hole in the right place) is very small, one in a million or less, so we need to observe millions of stars for tens of years,” Golovitch said last week, speaking to a meeting of LLNL’s Retirees Association.
The research is an extension of LLNL efforts carried out in the 1990s under the leadership of Charles Alcock, now director of the Harvard-Smithsonian Center for Astrophysics.
That search was not focused specifically on black holes but on any large chunks of matter, perhaps the remnants of burned out stars, that might drift unseen at the outer edges of the Milky Way and explain at least part of the missing matter.
The idea that matter is missing from the visible universe was suggested as early as the 1880s by the British polymath scientist, Lord Kelvin.
Half a century later, another extraordinary European, Swiss-born Fritz Zwicky, realized that galaxies were spinning faster than could be explained by their observable mass. They should literally fly apart absent some additional matter.
Working at CalTech, he calculated that there must be 400 times more mass in the galaxies than could be seen. He called it “dunkle materie” – Dark Matter, in German.
In recounting the history of the search for Dark Matter, Golovitch told the retirees group that his personal scientific hero was Vera Rubin, the pioneering Carnegie Institution astronomer who brought scientific credibility to the Dark Matter search in the 1970s.
Her work challenged astronomical orthodoxy, but spectroscopic technology had created better measurements than were available to Zwicky. Rubin’s arguments could not be dismissed for long. It is now understood that Dark Matter constitutes most of the Universe -- about 85 percent, according to current estimates.
Many – including Golovich – believe that Rubin deserved a Nobel prize because of the fundamental nature of her work. It is too late to do so now, however. She died two years ago; Nobels are not given posthumously.
Black holes are far from the only possible explanation for Dark Matter. Competing proposals include exotic but never-seen particles like the axion and the whimsically named WIMPs, or weakly interacting massive particles. More recently, theorists have proposed SIMPs, for strongly interacting massive particles.
Some have suggested subtle changes to Newton’s laws of motion, a proposal known as MOND for modified Newtonian dynamics.
WIMPS were in fact the topic of Golovitch’s PhD thesis at the University of California at Davis.
He is now skeptical that they will turn out to be the explanation for Dark Matter. As he pointed out last week, an extremely sensitive detection system in South Dakota, called LUX, for large underground xenon experiment, has failed to find any.
MACHOs Meet WIMPs
LLNL has pursued several of the approaches to the Dark Matter question.
The approach Golovich described last week has a history, a 1990s effort to find large, dark clumps of ordinary matter drifting unseen at the edge of the galaxy.
In humorous counterpoint to the WIMPs, these objects were called MACHOs, for massive compact halo objects. Unlike WIMPs, which might not exist at all, MACHOs were made of ordinary matter. The LLNL problem was to find enough of them to be able to estimate whether they might solve the missing matter puzzle.
A decade-long search found 17 MACHOs by scanning the Large Magellanic Cloud, a satellite galaxy of the Milky Way that is visible from the Southern Hemisphere.
Finding MACHOs at this rate did not represent a complete answer to the Dark Matter puzzle but might account for at least some of it, perhaps 20 percent, according to some estimates.
The method of looking for MACHOs, called gravitational lensing or microlensing, was suggested at Princeton in 1986.
The idea was that an object the size of Jupiter or larger might very occasionally drift exactly between a distant star and Earth, gravitationally focusing the star’s light so that, from Earth, it brightens for a few days and then dims when the dark object moves on.
It was estimated that the chance of finding such a line-up was perhaps one in two million. To make the search practical, Alcock’s team developed automated means of scanning millions of stars per night.
The same concept is used by the research team that Golovich is part of, but the focus now is on black holes. If enough can be shown to exist, they might help explain the missing mass.
Until recently, two sizes of black holes had been identified. One, the end product of a star’s life, is around 10 times the mass of our sun; that is, 10 solar masses.
The other, called a supermassive black hole, is hundreds of thousands to millions of times more massive than our sun. Supermassive black holes are generally not so hard to find, being identified by the motion of stars near them.
Black holes of an intermediate mass, around 30 to 100 solar masses, had not been observed until recently, although noted LLNL physicist George Chapline offered a theoretical explanation of how they might evolve from the early universe.
Theory and Observation
Black holes of about 30 solar masses were discovered suddenly and dramatically three years ago as part of a massive experiment that proved the existence of gravitational waves. The research, described last January in a Rae Dorough Speakers Series talk at Livermore’s Bankhead Theater, promptly won the Nobel prize for the MIT and CalTech scientists who led it.
It has added excitement to the LLNL MACHO study by establishing a new class of black holes to search for.
The search is taking place using mountaintop telescopes in Chile. A remote viewing room at LLNL with high-quality communication allows Golovich and colleagues to observe without the expense and inconvenience of international travel.
Golovich has also spent the past year reviewing the observations of the Alcock studies, applying today’s astronomical understanding and LLNL’s powerful computational resources to reinterpret old findings and screen for possible new sources of Dark Matter.
He expects the coming years to yield a more comprehensive judgment about the role played by black holes in Dark Matter, particularly after LLNL’s observation platform is linked to the powerful new telescope called LSST, the Large Synoptic Survey Telescope.
LSST is due to begin operations in 2021. In the decade after that, Golovich believes, “if black holes exist, we will detect them….” Alternatively, he said, the telescope could “rule them out to uninteresting levels,” meaning the likelihood of finding them is too small to be worth pursuing.