Could a single dark matter particle be light-years
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.
MENDEX/UNIVERSITY OF CALIFORNIA, BERKELEY/KECK
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
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.
|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
|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
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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