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lmost 60 years ago pioneering cos-mologist Fritz Zwicky made the shocking claim that much of the

mass of the universe is òmissing.ó With-in the past decade, improved observa-tions have transformed Zwicky?s asser-tion into the accepted wisdom. The rate at which galaxies rotate and the man-ner in which they sail about in clusters and superclusters indicate that as much as 99 percent of the cosmos consists of an invisible component,known as dark matter. Theorists have identiTed three major possible dark components of the universe: MACHOs, WIMPs and the more prosaic neutrinos. A bevy of new exper-iments using computerized telescopes,particle accelerators and neutrino de-tectors may Tnally pin down the true nature of this mysterious matter. As Kim Griest of the University of Califor-nia at Berkeley puts it: òDark matter?s time has come.ó

One of the most fruitful places to search for dark matter is in the outer halos of galaxies. Studies of disk galax-ies show that their outer regions rotate much faster than would be expected from just the visible stars and gas they contain. Large amounts of unseen mat-ter must be present to create an extra gravitational tug.

Many astronomers have speculated that the mass in the outer parts of galax-

ies may be hidden in such nonluminous bodies as free-ranging planets, burned-out stars, brown dwarfs (starlike ob-jects too small to shine) and black holes.Griest has whimsically coined the term òMACHOsó?massive compact halo ob-jects?for this class of dark matter can-didates. Charles Alcock of Lawrence Liv-ermore National Laboratory, working with Griest and several others, has re-cently embarked on an ambitious search for MACHOs, as have groups of French and Polish astronomers.

MACHOs cannot be perceived direct-ly, but if one were to pass between the earth and a more distant star, its grav-ity would slightly bend and amplify the star?s light. Alcock and his collabora-tors are monitoring three million stars in the Magellanic Clouds for telltale signs of previously unperceived cos-mic vagrants. The rate at which a star?s brightness changes would reveal the in-tervening object?s mass. A Jupiter-mass body would cause a star to brighten and dim over the course of a few days,whereas events associated with black holes could last well over a year.

Alcock?s MACHO investigation will run for four years, but positive results could show up much sooner. òIf we Tnd MACHOs, we will have solved the ob-servationally secure dark matter prob-lem,ó Griest says. If, on the other hand,the various surveys come up empty-handed, astronomers will be forced to consider some of the more bizarre ex-planations for dark matter.

In fact, most cosmologists have al-ready come to believe that unfamiliar

MACHO s or WIMP s?

Astronomers stalk the invisible cosmic majority

LAWRENCE KRAUSS of Yale University has abandoned the telescope in favor of the

particle accelerator to search for the missing mass in the universe.

S CIENTIFIC A MERICAN January 1993

J A S O N G O L T Z

mans had 13 sexes, and a person could mate with anyone but one?s own sex,there would be no more lonely nights.òIt would be gorgeous beyond belief?the Trst person you meet you could mate with,ó Hurst says.

But, Hurst notes, the slime mold pays an intracellular price for its bliss.To curb cytoplasmic con?ict, the slime molds have a rigid hierarchy that sets out which sexes can inherit the mito-chondria of others. Moreover, this elab-orate bookkeeping system is subject to cheating. In his own journeys through the literature, Hurst found documenta-tion of a renegade mitochondrion that refused to respect the pecking order.Avoiding this complexity is probably why evolution favors two sexes, the Oxford researchers argue. Indeed, the slime molds may Tnd themselves hard-pressed in centuries to come as sexual chaos reigns. òMultiple sex types might be expected to collapse to binary types,óthey write.

Although amused, not everyone is convinced of this explanation. òIt?s what I would call advocacy science,ó com-ments Brian Charlesworth, a researcher in population genetics at the University of Chicago, who has proposed a diYer-ent theory for how cytoplasmic genes are inherited. òYou try to make a mod-el and then Tnd something later that supports it.ó Charlesworth also charges that Hurst and Hamilton fail to explain adequately in their paper the origins of sexual dimorphism: why the male and female sex cells take on diYerent sizes and shapes, something that Hurst as-serts is a distinct evolutionary issue.Charlesworth also raises questions about whether Hurst and Hamilton?s several dozen citations constitute a broad enough inspection of the litera-ture to make sweeping claims about the evolution of separate sexes. òNeither of these guys works in experimental ge-netics, which is involved with these phe-nomena,ó Charlesworth remarks. òThey are very much armchair theorists. It is di?cult to evaluate these data without hands-on experience.ó

Hurst acknowledges the need for ex-periments to conTrm his ideas. He is working with Rolf Hoekstra, a biologist at the Agricultural University of Wagen-ingen in the Netherlands, who supplied the evolutionary model used by Hurst and Hamilton in making their predic-tions. Hoekstra is trying to determine whether a highly inbred species of fun-gus, Aspergillus nidulans, whose cyto-plasmic genes would be identical from cell to cell, has any need for separate sexes. That work may provide a clue as to why opposites attract?or pluses and minuses, if you prefer.?Gary Stix

28 S CIENTIFIC A MERICAN January 1993

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