On September 13, 1993, a young postdoctoral fellow approached Professor Charles Alcock in the Lawrence Livermore National Laboratory in California, clutching an image. A star in the Large Magellanic Cloud, 160,000 light years away, was glowing with an unusual intensity. Alcock turned to the telescope with his research team, transfixed as the distant star continued to shine seven times as bright as usual for three weeks. They knew they were witnessing the invisible being made visible.
This seemingly insignificant event, occurring to one in every two million stars or so, was rare proof of previously undetectable ‘dark matter’ in the far reaches of the universe.
Astronomers once thought that dark matter—objects of large mass that drift about the universe radiating little or no light—accounted for the majority of mass in the universe. Alcock’s work proved that dark matter existed (present as massive astrophysical compact halo objects, or MACHOs), and also confirmed that another previously unknown form of matter made up the bulk of the universe. It is now known that ‘dark energy’, plus dark matter and ordinary matter, complete the cosmic balance sheet. Stars, planets (and people) account for just five per cent.
Charles Alcock sits before me, one of the University of Auckland’s newest Distinguished Alumni. He wears a bushy moustache and a dark jacket concealing suspenders. He speaks eloquently and with measured enthusiasm of his work, and growing up in New Zealand.
Alcock spent his teens in Auckland, he and his friends tinkering with motorcycles and cars. “I think just the experience of working with mechanical and electrical systems, where you really had to make it work or else you couldn’t get somewhere, was very helpful.
“I think that helped enormously, the expectation that you can take something apart and put it back together again and it will work.”
Alcock calls himself a “latecomer” to astronomy. Growing up a “science nerd”, he developed a passion for physics that steadily grew during study at Westlake Boys High School, then at the University of Auckland, where he took every physics course available during his four-year bachelor of science degree. But by 1972, Alcock was at a crossroads and unsure where to head next, until he read about Jocelyn Bell’s discovery of radio pulsars.
Bell, along with her PhD thesis supervisor at Cambridge University in England, Antony Hewish, had detected a series of pulses using a radio telescope constructed of chicken wire and wooden posts in a paddock. A flurry of papers followed the monumental discovery.
“I just thought this was just the most magical thing I had ever read,” says Alcock. “I knew what I wanted to do.”
In early 1993, Alcock and a colleague at the Lawrence Livermore National Library in California engineered a system that allowed astrophysicists to routinely record images of 20 million stars every night; until then astronomers had been able to measure ‘only’ 1000 stars at once.
After just eight months came that moment the team had been waiting for—the first proof that dark matter existed. As a star passed behind the dark matter, the MACHO’s gravity bent and magnified its light, making the star brighten intensely. Without the bright backdrop, the dark matter would have been invisible.
“We just felt that we knew we could do this,” says Alcock. “Now everyone else could see it too.”
Over the course of the 11-year American-Australian collaboration, more than 17 events were recorded while the project team monitored the Large Magellanic Cloud, a satellite galaxy orbiting the Milky Way. The team systematically, painstakingly recorded and analysed more than 25,000 images of stars collected each night from a telescope-camera combination in Australia.
“Everyone in the team was completely confident that we were going to find this,” says Alcock. “The feeling for all of us was that this was the most important thing we’d ever done. And that kept us going.”
But while the discovery proved the existence of MACHOs, it could not account for all the unknown mass of the universe.
The search for unidentified matter in the universe remains Alcock’s all-consuming focus as director of the Harvard-Smithsonian Centre for Astrophysics in Cambridge, Massachusetts. “Astronomy is a demanding career; it takes a lot of time. And I can’t think of anything else more interesting.”
Alcock finds the presence of an unknown in science fundamentally unsatisfactory. For one thing, the mathematical fate of the universe hangs in the balance. “It’s important scientifically, because
it’s the universe, it’s the biggest thing we know of. To have most of it unaccounted for is just unacceptable—intellectually, even metaphorically.
“We don’t know how to describe the world we live in until we know what it’s made out of.”