Science / Medicine : Dark Matter

The Big Bang created galaxies and may also have started other, more exotic forms of matter--so much of it that the stars we see probably exist in a sea of invisible stuff that makes up as much as 99% of everything. This “hidden” matter has been largely unrecognized by astronomers and physicists and remains a mystery of nature.

From the Big Bang explosion of creation 20 billion years ago came the matter and the forces that have given rise to galaxies, stars and, on Earth, life. That much science understands reasonably well.

But nature may also have created other, more exotic forms of matter in the Big Bang--so much of it that the galaxies we see shining in the skies are like froth on a sea of invisible stuff that makes up as much as 99% of everything.

Until recently, this “hidden” or “dark matter” was largely unrecognized by astronomers and physicists. It cannot be seen with any telescope; its existence has been detected only from its gravitational influence in shaping galaxies.

But where is this dark matter? What is it? And why does nature behave this way? The questions have galvanized astronomers and physicists in this decade to join their observational and theoretical skills in a cosmic whodunit to discover the universe’s unaccounted-for matter.

Is it in conventional matter, such as galaxies or stars and planets that are too dim to have been seen yet? More likely, physicists say, it will be found in a spate of exotic particles, such as axions, photinos, and neutrinos, that cosmological theories predict pervade the universe.

Fundamentally different from atoms that make up ordinary matter such as the Earth, these infinitesimal particles do not absorb or radiate light. They are predicted to have a mass so tiny that in the span of a second, billions of them pass through every square inch of ordinary matter--including people--without hitting any part of an atom.

Yet available by the billions of billions throughout intergalactic space, these strange particles that physicists call WIMPs--the acronym for weakly interacting, yet massive particles--may account for virtually all the universe’s matter.

“The fact is, we don’t know what the universe is made of,” said Michael Turner, a physicist at the University of Chicago. “There is dark matter in the universe, no question about it. The clues are all there that there might be these exotic particles.”

Proof that the universe is dominated by WIMPs could revolutionize physics, giving important answers as well about how galaxies formed in the early universe and about whether the universe will expand forever or slow and, perhaps, eventually collapse.

A WIMP-dominated world also means, said Claude Canizares, a physicist at the Massachusetts Institute of Technology, “the basic stuff we’re made of is a kind of minor pollutant in the universe.”

Astronomers had thought they knew what matter looked like: rotating galaxies, made of billions of stars and strewn with some gas. With telescopes, they see the radiation the galaxies emit as visible and infrared light, radio waves and X-rays.

In the past decade, as astronomers’ observations improved, they became convinced that a curious anomaly of galactic motion had profound implications. The outer arms of spiral galaxies were rotating as fast as the galaxies’ more central portions.

Such orbital rotation would require a substantial mass in the galaxies, but the emitted light waves suggest no such mass. In fact, as much as 10 times the matter in stars would be required to create the strong gravitational forces detected, but astronomers cannot find the matter.

“No one doubts the observations,” said Vera Rubin, astronomer at the Carnegie Institution in Washington who herself has studied more than 150 galaxies. “The results were so clean.”

Embedding the shining galaxies in a sphere of invisible matter would explain the high rotational velocities. Without this extra mass, the galaxies would collapse into bar shapes, Jeremiah P. Ostriker, a Princeton University physicist, has calculated.

Studies of galaxies of irregular shape and of galaxy clusters also find a much stronger gravitational force than can be explained by stars’ masses.

Astronomers have imagined the dark matter as very dim stars, so-called brown dwarf stars about the size of the gaseous planet Jupiter. Enough dim stars surrounding the galaxy could have a combined mass about 10 times the shining stars.

Others suggest that the unaccounted-for gravitational force could arise from black holes. These intense vortices of gravity, left where a massive star collapsed, are invisible because not even light can escape their gravity. But astronomers as yet have no evidence for either black holes or Jupiter-like surrounding galaxies, Canizares said.

But a final dark matter clue, the inflation theory, says the universe contains still much more matter--100 times the matter seen in shining stars.

The inflation theory, proposed in 1981 by Alan Guth, an MIT physicist, is a scientifically compelling description of the universe’s rapid expansion at creation. It predicts the universe contains enough matter to slow its expansion almost to a halt and that such a feat requires 100 times the matter visible in stars.

The problem remains: A mere 1% of inflation’s prediction is seen in ordinary stars. And physicists can calculate that the amount of ordinary atomic matter made at creation likely equals no more than 10% of inflation’s prediction, Canizares said.

“I think we’re forced to concede that between 90% and 99.9% of the universe is made of something we don’t know about,” Canizares said. “That’s a pretty sobering thought.”

Theorists have tried to remedy the matter shortfall in many provocative ways, from suggesting a dark matter of infinitesimal particles to postulating a bizarre “shadow world” of stars and galaxies like ordinary stars and galaxies and existing alongside them, except that the shadow stars are invisible. Some have even questioned our understanding of how gravity works across galactic distances.

The search is focusing on the infinitesimal, massive particles--the WIMPs. WIMPs are predicted by theories about basic constituents of matter. They seem compatible with astronomical observations, and they may have properties scientists can search for in experiments.

Two WIMPs, axions and so-called supersymmetric particles, were perhaps created a fraction of a second into the Big Bang. Supersymmetric theory predicts each known matter particle, such as a particle of light, called a photon, has a supersymmetric partner, the photino. Yet supersymmetric partners only interact through gravity.

Another WIMP candidate, the neutrino, is known to exist, but numerous experiments to determine its mass have given uncertain results.

If WIMPs have even a tiny mass--perhaps in the case of axions several thousand times smaller than a proton in an atomic nucleus--yet fill all of space, the particles could make up 99% of all matter.

“These things are on the edge of our ability to conceive of them--they’re like ghosts--and they’re on the edge of our ability to really measure them,” said Robert Kirshner, a Harvard University astronomer. “Yet, there is this paradoxical idea that they could dominate the mass of the universe.”

In a sort of cosmic archeology, physicists are undertaking difficult experiments to find WIMPs, based on the premise that the particles occasionally interact with atoms, said Lawrence Krauss, theoretical physicist at Yale University.

“Not only is it (dark matter) out there, it’s in this room,” Krauss said. “You don’t have to look in the heavens; you can look in the basement.”

If physicists measure the energy transfer in a fleeting interaction between a WIMP and atomic matter, they might determine a WIMP mass, Krauss said. Then they might calculate whether a universe filled with the particles would ever stop expanding.

They also could test theories that the shining galaxies lit up in regions where WIMPs, coalescing under the force of gravity, were densest.

Collaborators at Brookhaven National Laboratory are already testing whether an intense magnetic field can induce an axion to convert into radiation as a very weak but measurable microwave signal.

Led by Adrian Melissinos, a University of Rochester physicist, the collaborators have surrounded eight copper cylinders with a strong magnetic field in hopes that some of the billions of axions in every square inch of the cavities will turn into microwaves. A computer is scanning 5 million microwave frequencies for an axion signal.

Melissinos’ group is designing another experiment to “create” axions by sending a laser past a strong magnetic field.

Thomas J. Weiler, a visiting physicist at the University of Hawaii, and Thomas Kephart, a Vanderbilt University physicist, have suggested that some axions may decay to ultraviolet light, leaving a halo around galaxies that would be visible with a telescope above Earth’s atmosphere.

UC Berkeley physicist Bernard Sadoulet plans to search for supersymmetric particles within small blocks of pure silicon or boron. He plans to supercool the material to within 1/10th of a degree of absolute zero (-459 F), a state in which an object has no heat. A particle might hit an atomic nucleus, causing it to vibrate and resonate enough heat to measure.

Stanford University physicist Blas Cabrera wants to determine a mass for neutrinos using a similar experiment with supercooled material covered with a superconducting material, such as supercooled aluminum. A vibration from a neutrino interaction would interrupt the current through the superconductor. The experiment might detect supersymmetric particles as well, he said.

The experiments are a year or more from operating, their designers said, because of difficulties in developing materials and sensors for such subtle effects.

The planned superconducting super collider, which would be the world’s most powerful particle accelerator, could drive protons so close to the speed of light that their collisions could produce supersymmetric particles, theorists speculate.

And the experiments may not identify the dark matter. It may have some other bizarre form, cautioned Weiler, adding, “Maybe God is just as perverse and humorous as physicists.”

HUNTING FOR HIDDEN MATTER AMONG THE STARS Astronomers’ observations have led to the suggestion tat spiral galaxies may be embedded in a sphere of invisible matter. This dark matter, having substantial mass, would explain abnormalties in galactic motion.

Astronomers thought they knew what matter looked like: rotating galaxies made of billions of stars and strewn with some gas. But in the last 10 years they noticed that the outer arms of spiral galaxies were rotating as fast as the galaxies’ central portions--a phenomenon that theoretically would require as much as 10 times the matter as seen. Thus began the search for this “hidden” or “dark” matter.

One theory suggests that dark matter comes in the form of brown dwarfs, stars of about the same size and composition of Jupiter that cannot be seen at great distances; stars that are too small to ignite by nuclear fusion. Enough brown dwarfs in a galaxy could account for the missing matter.

Dark matter would not be anything like our sun, which has an easily documented life span and is the basis for one solar mass. From birth through phases such as a red giant and white dwarf, it is mass as we know it.

Others say the unaccounted-for gravitational force could come from black holes, resulting from a massive star collapse that creates a center of gravity so intense that not even light escapes it. Such stars of 30 to 50 solar masses may live a relatively short life and burn themselves out in dramatic fashion before becoming black holes.