COLUMN ONE : The Dark Matter Mystery : Scientists say planets, stars and gases do not account for all the gravity in the universe. Something else must be out there. But finding it is not easy when researchers cannot even prove it exists.
Daniel P. Snowden-Ifft is looking for something, but he’s not sure what it is.
Charles Alcock knows what he’s looking for, but isn’t sure he knows how to find it.
Neither scientist knows if the object of his search actually exists.
Snowden-Ifft and Alcock are among hundreds of researchers around the world who are trying to detect “dark matter,” a scientific catchall devised to explain an enormous apparent excess of gravity in the universe.
The search for dark matter is one of the great detective stories in space science, and not just because invisible material is intrinsically intriguing. Dark matter is of profound interest to researchers because the motions of stars and interstellar gas indicate that it may make up 99.9% of everything in the universe.
“We’re not only not the center of the universe,” said Princeton University astrophysicist P. J. E. (Jim) Peebles, “we are not even made of what most of the rest of the universe is made of.”
Scientists believe dark matter exists because the universe’s entire stock of visible matter--planets and stars, dust and gas--is too small to account for the gravity needed to make the universe work. To make gas clump into stars, stars gather into galaxies and galaxies whirl the way they do, there must be some vast, invisible source of gravity, scientists conclude.
They call this gravity source dark matter because they cannot see it with telescopes or other instruments.
No one knows what it is. Some theorists suggest that it is composed of ordinary atoms. Others say it must be an entirely new form of matter. Some contend it moves at nearly the speed of light, others that it hardly moves at all. It has alternately been said to consist of specks too small to see and blobs the size of Jupiter.
“When it comes to dark matter,” sighed UC Santa Barbara astrophysicist David O. Caldwell, “the only thing that we are convinced of at the moment is that it’s there.”
Actually, scientists cannot even agree on that. Mordechai Milgrom of the Weizmann Institute in Israel suggests that dark matter need not exist at all, that the effects attributed to it may be explained by a breakdown in gravity over great distances. Gravity, he argues, is known to break down at short distances, as between an atom’s nucleus and its electrons.
Experiments have not proven Milgrom right, and dark matter doubters are a small minority of astrophysicists. Maddeningly, however, experimenters have not proven him wrong, either, because no one has directly detected dark matter.
“It’s very humbling,” said Michael S. Turner, an astrophysics professor at the University of Chicago. “The origin, composition, energy and mass of the most common matter in the entire universe is unknown.”
But if no one knows what dark matter is, how could anyone build a detector to find it?
If it consists of regular gases or minerals that do not reflect enough light to be easily seen, astronomers believe that exhaustive observations of the night sky would let them see it directly or detect its effect on objects that they can see. Recent developments in robotic telescopes, electronic imaging and computer analysis make such observations possible.
But many scientists suspect that dark matter--or at least a big share of it--is something other than the protons, neutrons and electrons in atoms.
“It may well be that we live in a boring universe of ordinary matter,” Turner recently told a gathering of astronomers in Berkeley, “but there is increasing evidence that most of the dark matter--and so most of everything--is something else.”
But what?
In search of an answer to the mystery, astrophysicists are turning from the vastness of outer space to the inner workings of atoms.
Particle physicists use high-energy accelerators to crack open protons and other particles inside atoms to see how they work and what holds them together. In the process, they simulate on a very small scale the extreme heat and pressure that existed a few billionths of a second after the Big Bang created the universe.
These experiments have turned up all manner of curious matter, such as virtually weightless neutrinos and energetic muons. But theorists think these particles are only some of the fundamental ingredients of matter.
Mathematical models suggest that the rest of the recipe consists of particles still to be discovered, such as ghostlike “super-symmetric” neutralinos. Physicists are searching for these new particles by using powerful particle accelerators in Illinois and Switzerland.
But their discoveries may have to wait for the superconducting supercollider under construction in Texas or, if it fails to survive budget cuts, some other accelerator capable of smashing particles with far more energy than is now possible.
Meanwhile, astrophysicists believe that the Big Bang probably created the same variety of particles found in these simulations--and that only a part of this primordial soup coagulated into ordinary matter.
Because physical laws state that matter, like energy, cannot be destroyed, oceans of undiscovered particles must still exist in some form. This, they believe, could account for the apparent excess of gravity in the universe.
Assuming these hypothetical particles do constitute a part of the answer to the dark matter riddle, researchers predict the nature of such particles then design experiments to detect them.
Bernard Sadoulet of UC Berkeley plans to freeze a matchbox-sized block of a very pure crystal, germanium, to minus 459 degrees Fahrenheit--less than one degree from absolute zero--and wait for a tiny but discrete increase in heat or energy.
Because the experiment is buried underground to shield it from cosmic rays and is built to block out background radiation, Sadoulet believes that heat could be generated only if a neutralino collides with the nucleus of a germanium atom and rattles the Tinkertoy-like lattice of the crystal.
Some of his colleagues at the Berkeley-based Center for Particle Astrophysics--run jointly by UC campuses in Berkeley, Santa Barbara and San Diego as well as the Lawrence Livermore National Laboratory--are preparing a different experiment to look for another dark matter candidate, axions.
The mathematical models that predict the existence of axions also indicate that they would decay in a strong magnetic field--and emit telling flashes of microwave radiation. Scientists hope to cause an axion to decay and look for the telltale microwaves. To do this, they plan to create a very powerful magnetic field surrounded by microwave sensors in a laboratory.
Researchers are going to great lengths--and depths--in their search for the elusive dark matter. In Japan, scientists eager to measure neutrinos plan to bury 33,000 tons of water at the bottom of a zinc mine. Others, also seeking to avoid the confounding influence of cosmic rays, are conducting experiments in an Italian highway tunnel, a hollowed-out Russian mountain and a South Dakota gold mine.
The Center for Particle Astrophysics in Berkeley, the focus of dark matter study in the United States, receives $2.5 million a year from the National Science Foundation for such research; the center, in turn, sponsors the research of dozens of scientists who are pursuing almost as many different theories.
At stake, some scientists believe, is a Nobel Prize.
The wide-open hunt is exemplified by Bay Area researchers Snowden-Ifft and Alcock, and by the targets of their experiments--possible dark matter objects that scientists call WIMPs and MACHOs.
Snowden-Ifft, a postdoctoral fellow at UC Berkeley, runs a sparse operation in his laboratory’s back yard, working only with his faculty adviser and using simple materials. He is looking for WIMPs--weakly interactive massive particles.
Alcock, a Lawrence Livermore astrophysicist, works with 16 colleagues on a computer analysis of millions of astronomical images from the other side of the globe. He is looking for a type of dark matter called MACHOs--massive compact halo objects.
Snowden-Ifft dreamed up his admittedly quixotic experiment while reading an article on dark matter as a diversion from a tedious experiment in his primary field, physics.
“I had no idea even what dark matter was,” he said. “I was just looking for something to read.” He said he still is not sure what WIMPs are, but he believes he knows how to find their footprints.
Every second, the theory goes, billions of WIMPs can zip through an object--right through its atoms--without leaving a trace. On rare occasions, WIMPs should slam directly into an atom’s nucleus. Such collisions should send the nucleus skittering, leaving a telltale trail of destruction that can be seen.
Snowden-Ifft read that mica, a smooth and layered mineral, should be particularly good for preserving these events. And because a piece of mica can be many millions of years old, it has had ample opportunity to record many WIMP “recoils,” no matter how rare. To find these microscopic tracks, he and UC Berkeley physics professor P. Buford Price slice open samples and use an extremely powerful microscope to search for fantastically small ruts in the mica.
Snowden-Ifft acknowledged that several common phenomena, such as cosmic rays and background radiation, may leave similar footprints. But he hopes that WIMP trails would be distinctive enough for him to shed light on how WIMPs work--or to discount their existence.
While Snowden-Ifft searches for disturbances among atoms, Alcock is going to the opposite extreme. He is looking for great blobs of dark matter in intergalactic space.
Using the 125-year-old Great Melbourne Telescope in Canberra, Australia, Alcock and his colleagues are cataloguing the brightness of more than 40 million stars. They are looking for a star to suddenly and inexplicably brighten. This, they believe, would indicate that a MACHO is passing between that star and Earth.
MACHOs, if they exist, would probably be made of the same light gases as stars but would not burn with thermonuclear fusion. Dark, cold and slow moving, MACHOs would emit no radiation--not even X-rays or radio waves--that could be detected on Earth. But these “dead stars” would have enough gravity to slightly bend nearby light rays, as the lens in a magnifying glass bends light to make objects appear larger.
To someone on Earth looking through a telescope, the star would appear to brighten briefly when a MACHO passed by. By studying the star’s light spectrum, Alcock said, he could distinguish a MACHO from other phenomena.
Even though Alcock and others suspect that there may be as many as 1,000 trillion MACHOs huddled around Earth’s home galaxy, the Milky Way, they are devilishly hard to spot. Alcock said light from only one star in 2 million is likely to be amplified by a MACHO at any given time.
That is why his robotic telescope surveys 20 million stars each night, a feat possible only with technology developed for the Star Wars ballistic missile defense program. Light from each star is measured by an electronic device similar to the light-sensitive component of a home video camera. Computers cross-check the measurements to find any stars that brighten.
“The data stream off this is enormous,” he said. A single sky image generates enough data to fill up the memory in a Macintosh computer, he said, and the telescope records 10 images an hour.
He is not certain this process will find any MACHOs, but Alcock said he cannot imagine a better way to look for them.
“We’ve already made more measurements of star brightnesses than in the previous history of astronomy,” Alcock said. He declined to say if he and his colleagues had found any MACHO candidates. Such an announcement would have to wait at least two years, he said, until they have collected a sufficient quantity of data to support their assertions.
But, he offered, “we’re learning an enormous amount about (stars).”
Shedding Light on Dark Matter
What is the universe made of? Only a small fraction consists of stars, planets, dust and gas. The rest -- so-called dark matter -- cannot be seen and is known only by the gravity it exerts. No one is sure, but theorists have some ideas on why it exists and what it is.
Astronomers believe dark matter exists because of the odd way galaxies spin. Here’s why:
A. In the solar system, planets closer to the sun revolve faster than more distant siblings, obediently following Newton’s law of gravity.
B. In a spiral galaxy, stars on the outermost tips of its wispy arms revolve around the core as quickly as stars near the center. This violates Newton’s law, unless the whole galaxy is enveloped in some unseen matter exerting at least 10 times the gravity expected from the stars alone.
MACHOS: * What they are: If they exist, these “massive compact halo objects” would be gas blobs not massive enough to trigger the thermonuclear fire needed to turn them into stars. They would not reflect enough light to be seen, but would possess enough gravity to bend light from distant stars. * Possible search methods: Astronomers would use a robotic telescope to look for unusually bright stars, because the lens-like effect of a MACHO would make stars seem brighter when it passes in front.
AXIONS: * What they are: Very light hypothetical particles. None has been detected experimentally, but mathematical models suggest they were produced along with all known matter by the Big Bang, which scientists generally believe created the universe about 15 billion years ago. * Possible search methods: Axions should decay and emit microwave photons (packets of electromagnetic energy) when subjected to a powerful magnetic field. Scientists plan to place microwave detectors close to a box in which they have created a powerful magnetic field. Then they would slowly increase and decrease the strength of the field, hoping to detect the predicted microwave burst. WIMPS: * What they are: Relatively heavy hypothetical “weakly interactive massive particles.” None has been detected experimentally, but mathematical models suggest they were produced by the Big Bang. * Possible search methods: With their larger mass, WIMPs could shake up any atomic nuclei they happened to bump into. This effect might be easiest to see in a germanium crystal, where atoms line up in an orderly lattice; a rattled atom would resonate with others, producing heat that scientists could detect. To protect the test from cosmic rays, the germanium would be concealed underground.
