Astrophysicists find Evidence of Dark Matter, the "Missing Matter" of the Universe

September 24, 1993

By Lynn Yarris,

One of the prime candidates for dark matter, the mysterious "missing mass" of the universe, may have been spotted. Scientists at the Center for Particle Astrophysics (CPA) have reported the first evidence for the existence of MACHOs -- MAssive Compact Halo Objects -- stars too small to shine that are concentrated in the halo of galaxies.

Working with a specially equipped telescope on Mount Stromlo, near Canberra, Australia, the scientists detected a sudden, brief brightening of what is usually a dim star in the Large Magellanic Cloud galaxy. This brightening was attributed to a phenomenon known as "microlensing," in which the gravity of a massive invisible object acts like a magnifying lens while it crosses in between a visible star and a telescope on earth.

Ninety percent or more of the matter that makes up the universe is considered to be "dark," meaning it does not radiate any light, even though observations of its gravitational effects confirm its existence. There are two major schools of thought as to the nature of dark matter. One contends that dark matter primarily consists of exotic non-baryonic particles, relics from the early universe known as WIMPs (Weakly Interacting Massive Particles). The other argument is for ordinary baryonic matter (protons, electrons, and neutrons) MACHOs, such as brown dwarfs and planets the size of Jupiter, which are too small to be seen directly.

Bernard Sadoulet, director of the CPA and a staff scientist with LBL's Physics Division, announced the first potential finding of a MACHO last Monday (Sept. 20) in Gran Sasso, Italy, at the Gran Sasso Conference on Underground Particle Physics. The finding was also announced that same day by CPA's David Bennett, an astrophysicist with Lawrence Livermore National Laboratory (LLNL), in Capri, Italy, at the International Conference on the Cosmic Microwave Background Radiation. A team of scientists from the French national laboratory at Saclay announced similar but independent observations at the same two conferences.

"It will take more statistical information to confirm that this object exists," said Sadoulet, in a phone call from Italy. "We will also need to find more of them around other stars, but even with confirmation, there may be other forms of dark matter still to be found."

The finding announced by Sadoulet and Bennett was accomplished by a team of American and Australian scientists led by LLNL physicist Charles Alcock. The other Americans, all of whom are affiliated with the CPA, included LBL physicist Saul Perlmutter, University of California researchers Carl Akerloff, Kim Griest, Stuart Marshall, Mark Pratt, Will Sutherland, and Christopher Stubbs, and LLNL researchers Robyn Allsman, Timothy Axelrod, Kem Cook, and Hye-Sook Park. The Australian researchers were with the Mount Stromlo and the Siding Springs Observatories.

What made the observations of the CPA collaboration possible was the addition of a new, automated CCD (charge-coupled device) camera to the 1.27 meter (50-inch) "Great Melbourne Telescope" on Mount Stromlo. Extremely sensitive to the measurement of collected light, this digital camera is capable of imaging 20 to 30 million stars each night. For the past year, it has been focused on about 3 million stars in the galaxy nearest our own Milky Way, the Large Magellanic Cloud. Approximately 160,000 light years away, the Large Magellanic Cloud is visible only from the southern hemisphere.

The invisible object detected by the CPA scientists and their Australian collaborators is at least as massive as Jupiter. The brightening that it caused -- a seven-fold increase over the visible star's normal brightness -- was observed over a 34 day period in both red and blue-green light. The visible star has given no indication of pulsing or any other stellar activity that could account for the observed increase in brightness, the researchers said.

"We're extremely excited," team leader Alcock told reporters after the finding was made public, "but we keep reminding ourselves that this is only one event, so we can say that our experiment is working. Now, if we can confirm the discovery of more objects like this one, we may be able to say we have found clear evidence for at least some of the dark matter that makes up the universe's missing mass."

The CPA was established by the National Science Foundation expressly to study dark matter. Located on the UC Berkeley campus, it has close ties with research teams at LBL and LLNL as well as with other UC campuses. Captions for MACHO story photos:

Scientists with the Center for Particle Astrophysics used the 1.27 meter Great Melbourne Telescope which is equipped with the two most powerful digital cameras in the world to detect what could be the first MAssively Compact Halo Object ever found.

The Large Magellanic Cloud is our nearest-neighbor galaxy and the site of an observed brightening of a normally dim star that could shed light on dark matter.