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September 02, 2004 Subscribe Now and Save
August 30, 2004
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Scaled-Up Darkness
Could a single dark matter particle be light-years wide?
By George Musser

Science Image: galaxy
SMALL GALAXIES such as NGC 3109 are rarer and less compacted than they would be if matter clumped freely, perhaps because colossal particles that might be the universe's "missing mass" resist clumping.
In 1996 Discover magazine ran an April Fools' story about giant particles called "bigons" that could be responsible for all sorts of inexplicable phenomena. Now, in a case of life imitating art, some physicists are proposing that the universe's mysterious dark matter consists of great big particles, light-years or more across. Amid the jostling of these titanic particles, ordinary matter ekes out its existence like shrews scurrying about the feet of the dinosaurs.

This idea arose to explain a puzzling fact about dark matter: although it clumps on the vastest scales, creating bodies such as galaxy clusters, it seems to resist clumping on smaller scales. Astronomers see far fewer small galaxies and subgalactic gas clouds than a simple extrapolation from clusters would imply. Accordingly, many have suggested that the particles that make up dark matter interact with one another like molecules in a gas, generating a pressure that counterbalances the force of gravity.

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The big-particle hypothesis takes another approach. Instead of adding a new property to the dark particles, it exploits the inherent tendency of any quantum particle to resist confinement. If you squeeze one, you reduce the uncertainty of its position but increase the uncertainty of its momentum. In effect, squeezing increases the particle's velocity, generating a pressure that counteracts the force you apply. Quantum claustrophobia becomes important over distances comparable to the particle's equivalent wavelength. Fighting gravitational clumping would take a wavelength of a few dozen light-years.

What type of particle could have such astronomical dimensions? As it happens, physicists predict plenty of energy fields whose corresponding particles could fit the bill--namely, so-called scalar fields. Such fields pop up both in the Standard Model of particle physics and in string theory. Although experimenters have yet to identify any, theorists are sure they're out there.

Cosmologists already ascribe cosmic inflation, and perhaps the dark energy (distinct from dark matter) that is now causing cosmic acceleration, to scalar fields. In these contexts, the fields work because they are the simplest generalization of Einstein's cosmological constant. If a scalar field changes slowly, it resembles a constant, both in its fixed magnitude and in its lack of directionality; relativity theory predicts it will produce a gravitational repulsion. But if the field changes or oscillates quickly enough, it produces a gravitational attraction, just like ordinary or dark matter. Physicists posited bodies composed of scalar particles as long ago as the 1960s, and the idea was revived in the late 1980s, but it only really started to take hold four years ago.

Two leaders of the subject are Tonatiuh Matos Chassin of the Center for Research and Advanced Studies in Mexico City and Luis Ureña López of the University of Guanajuato. At a workshop at the Central University of Las Villas (UCLV) in Cuba in June, they described how scalar particles can reproduce the internal structure of galaxies: when the particles clump on galactic scales, they overlap to form a Bose-Einstein condensate--a giant version of the cold atom piles that experimenters have created over the past decade. The condensate has a mass and density profile matching those of real galaxies.

That inflation, dark energy and dark matter can all be laid at the doorstep of scalar fields suggests that they might be connected. Israel Quiros of UCLV argued at the workshop that the same field could account for both inflation and dark energy. Other physicists have worked on linking the two dark entities. "As my senior colleagues used to say, 'You only get to invoke the tooth fairy once,'" says Robert Scherrer of Vanderbilt University. "Right now we have to invoke the tooth fairy twice: we need to postulate a yet to be discovered particle as dark matter and an unknown source for dark energy. My model manages to explain both with a single field."

But all these models suffer from a nagging problem. Because the wavelength of a particle is inversely proportional to its mass, the astronomical size corresponds to an almost absurdly small mass, about 10-23 electron volts (compared with the proton's mass of 109 electron volts). That requires the laws of physics to possess a hitherto unsuspected symmetry. "Such symmetries are possible, although they appear somewhat contrived," says physicist Sean Carroll of the University of Chicago. Moreover, the main motivation for big particles--their resistance to clumping--has become less compelling now that cosmologists have found that more prosaic processes, such as star formation, can do the trick. Still, as physicists cast about for some explanation of the mysteries of dark matter, it is inevitable that some pretty big ideas will float around.


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