The discovery by Zwicky that visible matter accounts for only a tiny fraction of all the mass in the universe may turn out to have been one of the most profound new insights produced by scientific exploration during the 20th century.

For cosmologist Rocky Kolb, size does matter.

To him the idea that most of the matter in the universe is made of particles called WIMPs is somehow a source of embarrassment. True, WIMPs are the most likely members of the supersymmetry side of the subatomic family tree to be found in space. They may very well account for much of the mysterious dark matter in the universe.

But maybe not. Some computer simulations suggest that a universe full of WIMPs would not produce the right number of small “dwarf” galaxies that surround big galaxies like the Milky Way. Other

nagging inconsistencies suggest that the universe could not be built by WIMPs alone. As a result, the identity of the dark matter remains mysterious, prompting Kolb and others to propose a zooful of novel species of matter to populate the cosmos.

Peruse the astrophysics literature and you’ll find more candidates for dark matter’s identity than remakes of Godzilla films. Proposals include large bodies like black holes, brown dwarfs, red dwarfs, and white dwarfs; massive quantities of small particles, like WIMPs, axions, or strange quark nuggets; and more exotic speculations, like mirror matter or cosmic Q-balls.

And then there’s Rocky Kolb’s favorite candidate. He calls them WIMPZILLAS. He can get away with what seems an outlandish suggestion for a good reason—nobody else really has the slightest idea of what the dark matter really is.

It’s been that way since the 1930s, when Fritz Zwicky, a cantankerous Caltech astronomer, noted some strange behavior in a group of galaxies known as the Coma Cluster. Those galaxies moved across the sky with a speed that simply couldn’t be explained if they made up all the matter in the cluster. In 1933 Zwicky reported that the galactic motions implied that the Coma Cluster contained a lot of matter that astronomers couldn’t see. “If this . . . is confirmed,” he wrote, “we would arrive at the astonishing conclusion that dark matter is present with a much greater density than luminous matter.”¹

Later, observations of other clusters confirmed the discrepancy— more mass appeared to be present than the amount that visible galaxies could account for. A further dark-matter mystery arose in 1939, when Horace Babcock measured how fast the outer region of the Andromeda galaxy was spinning. He found that stars on Andromeda’s outer edges appeared to revolve around the galaxy much more rapidly than they should, based on simple applications of Newton’s law of gravity.

Farther-out stars should be revolving more slowly, just as Pluto, the farthest planet from the sun, orbits at a much more leisurely pace

than the innermost planet, Mercury. It turned out that not only Andromeda, but other galaxies as well, rotated just as fast on their outer edges as they did much closer to their centers. Apparently, the mass of those galaxies increases with distance from the center, while their brightness does not. The only plausible explanations are that something is wrong with the law of gravity (which few physicists think is likely) or that the visible part of a galaxy is embedded in a vast massive halo of unseen (that is, dark) matter.

At first, nobody seemed to make the connection between this missing matter around galaxies and the missing matter in the Coma Cluster. Astronomers in general did not worry about dark matter much at all until the 1970s. But then further studies by the astronomer Vera Rubin and colleagues found more and more galaxies with high outer rotation rates. Observations of other clusters confirmed Zwicky’s suspicions as well. By the 1980s it was well-established that 90 percent or so of the mass of a typical galaxy is unseen and that massive amounts of dark matter lurk both in galactic halos and in the vast spaces between galaxies as well.

In a way, it’s pretty amazing that after all this time, astronomers cannot say what this dark matter is made of. It’s one of the greatest mysteries in the history of science, or perhaps in the history of anything. Imagine living in a house and having no clue to what it is made of. Or realizing that the inside of your body is something other than skin, but not having any idea what. You’d want to know. Astronomers and physicists desperately want to know what the universe is made of, too. And here’s a prediction: when scientists finally do find out what the dark matter is, it will be something that somebody has already predicted. A prediscovery.

No realm of physics and cosmology provides a more fertile field for prediscovery than the dark-matter mystery. In fact, one candidate for contributing to the dark matter is itself one of science’s greatest prediscoveries, the ghostlike particle known as the neutrino.

In the form of radioactivity known as beta decay, an atomic nucleus shoots out an electron. (Electrons emitted in this way are therefore called beta particles.) Careful measurements show that the electrons that fly away do not always possess the same amount of energy. Even if you account for the energy of the motion of the atomic nucleus they come from, these electrons can still exhibit a range of energies. But the total amount of energy in a process is supposed to remain constant, as the law of conservation of energy requires.

When radioactivity was discovered, at the end of the nineteenth century, the law of energy conservation was only a few decades old. Some scientists suspected that perhaps that law had been repealed by radioactivity. Or maybe it was just an approximate law that was unenforceable on the atomic scale. But plenty of experimental evidence argued otherwise. Finally an alternate solution was proposed by one of the most critical thinkers of his era, the Austrian physicist Wolfgang Pauli.

Pauli, who was born in 1900 and died in 1958, remains one of the legendary figures of physics lore. His signature scientific achievement was the Pauli exclusion principle—the limit on packing particles together that helped Dirac prediscover antimatter. Pauli was also famous for the “Pauli effect,” based on his experimental ineptitude. When a lab apparatus would blow up for some unknown reason, physicists suspected that Pauli must have been passing through town at that moment. (On one occasion, while riding on a streetcar, Pauli and some colleagues witnessed a crash between two other street-cars. Pauli turned to his friends and said, “Pauli effect!”)² Everyone was thankful that Pauli was a theorist.

More seriously, Pauli was known as the sharpest critic of new ideas among the leaders of European physics in the first half of the twentieth century. Nobody was quicker than Pauli to spot a flaw in someone else’s equations. “No form of approval could be more pre-

cious to physicists . . . than Pauli’s benevolent nodding,” the physicist Léon Rosenfeld once wrote.³

Much of the time, though, Pauli’s response to a presentation was not so benevolent. He was likely to blurt out something like “but this is all wrong!” Chen Ning Yang recalls delivering a seminar where Pauli interjected such virulent criticism that Yang decided to sit down and stop in mid-presentation.⁴

One joke circulated about a dream of Pauli’s in which he had died and gone to heaven, where he met face-to-face with God. Pauli seized the opportunity to ask a favorite question among physicists— why a certain combination of physical quantities produced a number almost exactly equal to 137.

“Why 137?” Pauli asked.

“It’s all here in these equations,” God responded, handing over a sheet of paper.

Pauli looked it over for a few seconds and then said, “But this is all wrong!”

Just as surely, Pauli knew that something was wrong with the theory of beta decay. As other mysteries of quantum physics began to clear up toward the end of the 1920s, the beta particle problem became even more perplexing. Abandoning the principle of energy conservation, radical as it seemed, was actually considered a serious option by some physicists. Pauli proposed a perhaps more palatable but equally bold solution—the existence of an entirely new particle unlike anything previously known to physics. It was nothing like any ordinary bit of matter, but rather something ghostlike, a sort of stealth particle that adjusted the speed of beta particles by siphoning off the missing energy itself.

Pauli articulated his new idea in a letter sent to a scientific meeting in Tübingen in 1930. (He should have been at the meeting but elected to remain in Zurich so he could go to a dance instead.) He called the proposed particle a neutron, since it should carry no electrical charge.

“Dear Radioactive Ladies and Gentlemen,” Pauli wrote in his letter, dated December 4. “I have considered . . . a way out for saving the . . . conservation of energy.” Atomic nuclei must contain a neutral particle (a neutron) that would be very light, perhaps about the same mass as electrons, he explained. “The continuous beta spectrum would then be understandable,” he continued, “assuming that in the beta decay together with the electron, in all cases, also a neutron is emitted, in such a way that the sum of the energy of the neutron and of the electron remains constant.”⁵

Neutron turned out to be a bad choice of names, because of confusion with the particle we now call the neutron, discovered in 1932 by James Chadwick in England. But Chadwick’s neutron was a massive particle, the size of a proton, nothing like the mysterious lightweight particle that Pauli had in mind.

By 1934, the Italian genius Enrico Fermi solved the nomenclature conflict in a paper working out the math behind Pauli’s idea. Fermi changed the name of Pauli’s neutron to neutrino, Italian for “little neutral one.”

Shortly thereafter, the physicists Hans Bethe and Rudolf Peierls analyzed the theory to calculate how much neutrinos would interact with ordinary matter. The answer: not much. An average neutrino could zip through a wall made of lead trillions of miles thick with no problem. And that raised a serious question about how you could know whether neutrinos really existed; after all, interaction with matter of some sort would seem to be the only way a neutrino could actually be detected. Bethe and Peierls suggested that you’d have a chance of catching one in liquid hydrogen, the catch being that your liquid hydrogen tank would need to be a thousand light-years wide. (At 65 mph, it would take 10 billion years to drive that far.) Bethe and Peierls concluded that “there is no practically possible way of observing the neutrino.”⁶

Pauli himself did not have very high hopes that anyone would

ever prove the existence of his particle. “I have done a terrible thing,” he said. “I have postulated a particle that cannot be detected.”⁷

But that didn’t stop Fred Reines from trying.

In the early 1950s, Reines was a young physicist at the Los Alamos laboratory in New Mexico, searching for a significant project. Finally he decided to try to detect the neutrino. Working at a weapons lab, he knew that nuclear bombs produced an enormously intense blast of neutrinos, just what you’d need to have a chance of nabbing one. After all, Bethe and Peierls had made their prediction long before such a prolific source of neutrinos was available. Reines enlisted the help of Clyde Cowan, another physicist, and they began a collaboration to show that it was possible to do the impossible.

Of course, there were some problems with setting up a detector next to an atomic bomb. Repeating the measurements would be difficult, for example. But it turned out that nuclear reactors also produce a good enough supply of neutrinos, and setting up a detector near a reactor seemed a lot easier. So Reines and Cowan altered their strategy, opting for reactors over bombs. By the mid-1950s they had succeeded, detecting the unmistakable signal of a neutrino striking a proton. After a conclusive experiment in 1956, they telegraphed Pauli that they had found the proof of his neutrino’s reality.

“Everything comes to him who knows how to wait,” Pauli wrote back.

Reines also knew how to wait. In 1995, almost 40 years after the experiment, he won a Nobel Prize for detecting the neutrino. (Cowan had died in 1974.)

Yet long before Reines trapped a neutrino, its existence had been taken for granted by most physicists—there simply was no other way to explain beta decay. Later it would turn out that the neutrino would have other uses—perhaps, for example, explaining dark matter.

Before the 1980s, the standard view of particle physics held neutrinos to be massless. But here and there hints began to appear that maybe the neutrino had a little bit of mass after all. If so, neutrinos might make up a major portion of the dark matter in space. Even a tiny mass would add up, considering the numerous neutrinos speeding through the cosmos. (At any moment, thousands of neutrinos are zipping through your body.)

By the end of the 1980s, though, most astrophysicists concluded that neutrinos could not be the dark matter, for their speed would have worked against the need to build galaxies in the universe’s youth. At most, it seemed neutrinos could possess only a tiny amount of mass and therefore would zip through space at very nearly the speed of light. In astrophysical terms, neutrinos would be “hot” particles—that is, particles that move very rapidly. (Slowpoke particles are considered “cold.”) Hot dark matter did not appear to be the right ingredient to explain the galactic superstructures the universe had cooked up.

Before the 1980s, astronomers knew only that they couldn’t see some of the matter out in space, and had no clue about whether the mystery matter was hot or cold. But midway through the 1980s, new observations revealed that the universe was a more complicated place than anyone had previously realized. Galaxies (or small clusters of galaxies) were not, as astronomers had generally assumed, scattered randomly through the cosmos. Instead the universe turned out to be an architectural marvel, a network of bubbles and walls stretching across all of visible space.

The bubble story popped into astronomical consciousness in 1986, when astronomers Margaret Geller, John Huchra, and Valérie de Lapparent reported their first efforts at mapping the locations of about a thousand galaxies in a slice of sky visible in the Northern Hemisphere. The astronomy world was astounded. Their map re-

vealed a universe of richer structure than previously believed possible, showing that galaxies are not scattered throughout space but congregated along the surfaces of imaginary spheres, like giant bubbles. (The “Lawrence Welk universe,” one headline writer called it.) Later Geller and Huchra found that some clusters of galaxies seem to be arranged over several bubble surfaces to form a long “Great Wall” extending 100 million light-years across the sky—bigger than the solar system to the degree that the Great Wall of China is bigger than a bacterium. Subsequently a similar structure was found in the southern sky.

Around the same time, other astronomers reported that some galaxy clusters seem to be streaming rapidly toward a massive “Great Attractor,” a mysterious unseen but unusually dense region of the universe, further suggesting structure in the universe on very large scales.

The discovery of such fantastic structures presented a new challenge to astronomers trying to explain how galaxies formed in the early universe. You’d think that galaxies should be arranged at random. And you’d think so because when the universe was young, matter was spread uniformly through space, with no large lumps. Lumps from back then would show up today as cold or hot spots in the cool glow of radiation left over from the birth of the universe.

This “cosmic microwave background” radiation was generated less than half a million years after the big bang, so its features provide astronomy’s equivalent of a fossil from the universe’s youth. It represents a snapshot of space at an early epoch, reflecting the distribution of matter before galaxies existed. And that radiation looks very smooth: its temperature is almost exactly the same no matter what direction in the sky it comes from. Therefore the universe back then must have been filled with a smooth sea of matter, and thus galaxies should have formed at random. Just by chance, a few bits of matter would have bumped into each other to form a lump a little denser than the matter around it. A slight density advantage would

then be magnified by the action of gravity, as one lump would draw more matter in. Giant galaxies would grow from those tiny matter seeds.

But galaxies did not form at random. Galaxies formed in clusters, and clusters of clusters, separated by the giant bubble-like “voids” in which relatively few galaxies are found. The seeds of matter in space must have been planted in a complicated way.

It wasn’t until the 1990s, though, that astronomers detected signs of those seeds in the cosmic microwave background. Starting with the famous satellite COBE (for Cosmic Background Explorer), various measurements have shown subtle patterns of temperature differences, reflecting tiny lumps of matter from the early days. The trouble is, the lumps were too small to have grown, in the time available, into the giant structures visible today, if those lumps were made only of ordinary matter. Ordinary matter (primarily protons and neutrons) could not coagulate rapidly enough. Some other form of matter must have been present—a form that could have coagulated earlier than ordinary matter, but without disturbing the microwave radiation.

Naturally enough, astronomers suspect that the dark matter they can’t see today might be the mystery matter that existed back then. If so, the dark matter cannot be ordinary (baryonic) matter.

Neutrinos are not baryons, but the dark matter can’t all be neutrinos, either. Experiments showing that neutrinos have a small mass indicate that it is not enough to account for all the matter that is missing. All the neutrinos added together might weigh as much as the visible matter (basically, stars) in space, but that’s only about a tenth as much as all the matter out there. Besides, neutrinos make hot dark matter. Hot dark matter would require more time to cook up large-scale clustering than the age of the universe provides.

All these developments have led many astronomers and physicists to believe in the existence of “cold dark matter,” so named in the early 1980s by the cosmologist J. Richard Bond. Cold dark matter consists of slower-moving particles that seem to offer just the right

ingredient to make the galaxy clustering work out the way it is supposed to.

It turned out that WIMPs—the SUSY partner particles from Chapter 3—would be perfect cold dark matter candidates. They’d be heavy (otherwise they would already have been discovered), ranging from 50 to 100 or even 1,000 times the mass of a proton. Therefore they’d move rather slowly. And they would be weakly interacting. That would give them just the right combination of properties to help make the seeds that grew into galactic superclusters.

On the other hand, maybe the dark matter is something even stranger. SUSY-WIMPs may exist, but they might not tell the whole story. Plenty of other potential prediscoveries have been postulated to make up some, if not all, of the dark matter. All of these hypothetical matters are pretty strange. But one of the strangest is also one of my favorites, the curious characters known as cosmic Q-balls.

Q-balls have nothing to do with cotton swabs, billiards, or villains on Star Trek: The Next Generation. Q-balls are lumps of super matter that may have formed when tiny superparticles coagulated in the hot dense phase of the early universe. If they really exist, it’s possible that some Q-balls lurk in the shadows of galactic halos even today, making up at least some of the dark matter.

Q-balls offer more than a possible solution to the dark-matter mystery. If real, they could alter the history of the universe, provide power beyond the dreams of the Energizer bunny and produce weapons dwarfing the destructive force of mere atomic explosives. A Q-ball-bomb could outbang the first atomic bombs as much as those bombs outblasted a stick of dynamite.

So far as I know, Q-balls have never received any significant attention in newspapers, apart from a column I wrote about them in

1997. (The British science magazine New Scientist did contain an indepth report on them in May 2000). But they do show up from time to time in talks at scientific meetings.⁸ I encountered them first in several papers appearing on the Internet, such as one by Alexander Kusenko, at the time a physicist at the CERN laboratory near Geneva. When the universe was very young and hot, immediately after the big bang, Q-balls could have been produced in huge quantities, Kusenko noted in his paper.⁹

It’s by no means a sure thing, but it’s plausible, assuming the validity of supersymmetry. When the universe was young and hot, squarks and sleptons (the SUSY partners of quarks and leptons) could have coagulated into balls. Some such balls would have been unstable and broken apart, or they might simply have evaporated away without doing any damage. But some might have survived long enough to inject extra ingredients into the primordial soup of matter and energy. Q-balls therefore might have affected early-universe events, such as the creation of different chemical elements, in ways that standard accounts of the universe’s history haven’t considered.

In another paper Kusenko and a colleague calculated that some Q-balls might even survive for billions of years and are perhaps still floating through space today. “The relic Q-balls can . . . survive until present and contribute to the dark matter in the universe,” wrote Kusenko and Mikhail Shaposhnikov.¹⁰

A lone Q-ball floating through interstellar space would be hard to see, they acknowledged. But the gravity of stars and planets might lure Q-balls to stellar or planetary centers. “It is conceivable that the deep interior of small planets might become accessible for exploration in the future and reveal storages of primordial Q-balls,” the physicists said.

Well, a voyage to a small planet, or to the center of the Earth, would probably take too long to satisfy most physicists. But there are other prospects for Q-ball prospecting. More powerful atom smash

ers might someday be able to produce those superpartner squarks and sleptons, possibly allowing the study of Q-balls in the laboratory. If so, Q-balls might provide a new avenue for probing higher energies, as the interior of a Q-ball would provide information on energy levels far greater than an affordable atom smasher can achieve. And there might be a big bonus for society. Just as a Q-ball bomb would give 100 H-bombs’ worth of bang, a Q-ball power plant would provide a practically inexhaustible supply of ordinary energy.

The best part is the Q-balls wouldn’t be used up—they would just be catalysts. The fuel would be protons, abundantly available from the hydrogen in water. Shoot a beam of protons into a Q-ball, and its internal superparticles would rip each proton’s quarks apart, releasing all the energy that had been holding those quarks together. You could use the energy released to boil water (the way most ordinary electric power plants do to drive steam turbines) or figure out some other scheme to tap the Q-ball energy output, Kusenko, Shaposhnikov, and Gia Dvali wrote in another Q-ball paper. “There are several processes that can yield large amounts of energy once a Q-ball is assembled and placed in a beam of protons,” they wrote.¹¹

Of course, the promise of cheap, inexhaustible energy has been heard before. And apart from the scientific uncertainties, the technological challenges of coping with Q-balls would argue against buying stock in any such venture at this time (although California may want to look into the possibility of Q-ball power). As the CERN scientists put it, “Technical and engineering aspects of such process, which may or may not be possible to realize in practice, lie outside the scope of our investigation.”

In any event, if Q-balls exist, they would surely be one of the strangest forms of strange matter in the universe. But they would not win the title of strangest name. I award that honor to Rocky Kolb’s invention, the WIMPZILLAS.

Rocky Kolb is my favorite cosmologist, because he’s the best there is at capturing the drama and substance inside science and communicating it to people on the outside. I met him in 1991, at a physics meeting in Vancouver. Although I’d heard him talk once or twice before, it was Vancouver where I realized what a spectacular spokesman for science he was.

Let’s face it, Carl Sagan is dead. Science needs people who can speak. Getting the message of science out to the public is even tougher than figuring out what the dark matter is. And very few scientists can communicate the way Rocky can. While most physicists don’t know the difference between a sound bite and a snake bite, Rocky is as quotable as Will Rogers or Mark Twain.

At the Vancouver meeting, Rocky was one of three speakers in an evening session for the general public. Hundreds of visitors packed an auditorium on the University of British Columbia campus to hear what physicists were up to these days. The first two speakers treated the audience to slides showing diagrams and equations and pictures of big atom smashers. Rocky, last on the agenda, took the audience on a tour of the universe.

He explained how the young universe, just after it exploded into existence, was a dense, hot, primordial “soup” of tiny particles: protons, neutrons, leptons. “Generically, they’re known in the primordial soup as croutons,” Rocky said. His next slide showed a can of Campbell’s soup, labeled “Primordial.” On his chart showing the history of the universe, he listed important events, including the formation of atoms, formation of galaxies, birth of the solar system, and “Cubs win World Series.” He showed a before-and-after slide of Supernova 1987A, a small star (indicated by an arrow on the left-hand side) that exploded into a bright spot filling most of the right-hand side.

This supernova was very important for helping scientists learn how to tell which stars will explode, Rocky said. “The stars that explode are the ones with arrows pointing toward them.”

He used only one equation in the whole talk. His humor was mixed with a lot of solid science, presented in a clear way to provide people with a real sense of what science does and doesn’t know about the universe and what scientists are doing to find out more.

Rocky himself is always doing something to try to find out more about the universe. He’s after the big picture, the whole story of how the universe came into being and evolved into the cosmos that today’s telescopes reveal to inquiring minds like his. But along the way he likes to have fun, and a big part of the fun is coming up with names like WIMPZILLAS.

It is a great name. What better way to convey the notion of a particle vastly more massive than a WIMP, a monstrous WIMP, a particle heavier than a million billion ordinary subatomic particles? And the best thing is there’s an outside chance that WIMPZILLAS might really exist.

Rocky’s first paper proposing the WIMPZILLA idea was hard to miss because of its catchy title: WIMPZILLAS!¹² I had given it a brief mention in one of my columns, but somehow never got around to the whole story. So I was pleased to hear the way Rocky put it all in historical context during a talk at the Texas symposium in Austin in December 2000.

In a way, the WIMPZILLA story goes back to the beginning of the universe. If WIMPZILLAS are around today, they would have been created way back then, in the opening moments of the universe’s existence. Their manner of birth was not imagined, however, until the 1930s, when the Austrian physicist Erwin Schrödinger put his mind to what space had been doing back at the beginning.

As most everybody now knows—although it wasn’t so clear to everybody back then—space, after the beginning, was expanding.

The big bang set the universe in motion. For some reason Schrödinger was worried about this.

Back in 1939, Rocky said, you’d think that Schrödinger would have had other things to worry about. But he was concerned that the expansion of space offered a way to make matter. Thanks to quantum mechanics—which Schrödinger had played an important role in inventing—the vacuum is not a calm and quiet place on the subatomic scale. Because quantum physics allows it, particles can pop into existence out of thin space all the time. But they appear in pairs: a particle is always created along with its antimatter counterpart. That way there is no danger of so many new particles coming into existence that the universe is overwhelmed by them. Soon after their appearance, the matter and antimatter particles bump into each other and disappear in a flash, returning the energy they had borrowed from the vacuum to fuel their ephemeral existence.

Everybody seemed happy enough with this picture, but Schrödinger saw something to be concerned about. In the early universe, the universe was rapidly expanding. A particle and antiparticle, popping into existence right next to each other, might not recombine soon enough, and the expansion of space could pull them away from each other. If that happened, they would not annihilate, and the population of particles in the universe would increase by two. And in fact, Rocky says, that may very well be the process that provided the universe with the original particles it needed to make seeds for galaxies.

In truth, it’s a little more complicated. “It’s really the changing gravitational field that’s responsible for the particle creation,” Rocky explained to me. Particles get created around black holes in a similar way—as you move through space to get closer to a black hole’s outer boundary, the strength of gravity changes sharply. In the first instants of the universe, the gravitational field changes rapidly in time. In either case, particles popping into existence because of quantum

fluctuations are no longer able to annihilate. “If you have a rapid change in the gravitational field, like you would around a black hole, where the gravitational field changes rapidly in space, or in the early universe . . . where there’s a rapid change in the expansion rate, then particle creation is more effective,” Rocky said.

Specifically, the expansion rate would change very rapidly if the popular theory of inflation is right. Inflation is the name given to a very brief but extremely rapid puff of expansion that supposedly occurred for something like a tiny fraction of a trillionth of a second. Putting the brakes on inflation—to return the universe to a more leisurely expansion rate—required a quick and dramatic slowing of the expansion rate, a good time for producing particles.

Inflation itself requires the existence of a field, called the inflaton, to provide energy for driving the rapid expansion. A particle made from an inflaton field would be very massive. If that mass scale has some fundamental significance, then maybe other particles of similar mass should exist as well—perhaps the ones that Rocky calls WIMPZILLAS.

“Generally when you look at nature there’s not just one particle of a certain mass, but it’s a scale; there are many particles of that mass,” Rocky said. “So if there is an inflaton that has this certain mass, if it’s a fundamental mass scale, then there would be other particles of that mass. So far in our experience of nature, if you find one, you’ll find another.”¹³

The realization of the WIMPZILLA possibility was serendipitous, Rocky recalled. “It’s something we stumbled upon,” he told me when I visited him at Fermilab in May 2001.

Rocky, his student Dan Chung, and a postdoc, Tony Riotto, had been discussing ways that dark matter might have been made during the early universe. Maybe there was some connection to the inflaton, the field responsible for inflation, they decided. If some matter field interacted strongly with the inflaton, they convinced themselves,

then you could show how the very massive dark matter particles could have been produced. Chung worked on a computer program to simulate the early universe and see if the rough calculations held up.

The results were surprising. Sure enough, you could make heavy dark matter particles this way. But the computer said you’d get the heavy particles even if the dark matter field didn’t interact with the inflaton at all.

“We kept finding dark matter,” Rocky recalled. “And we scratched our head for a day or a couple of days and said what the hell is this?” Maybe something was wrong with the computer program, they wondered. “Then we realized that it was in fact the gravitational production,” Rocky said. The computer code contained the equations for the changing gravitational field. The math knew about the particle-production possibility that Schrödinger had identified in 1939.

About the same time, other researchers proposed similar ideas. So Rocky pursued the WIMPZILLA possibility more seriously. It turned out that if the young universe gave birth to WIMPZILLAS, they might have disintegrated into other particles by now. But it’s possible that they disintegrate slowly—on a time scale comparable to the age of the universe. If so, enough of them may still be around to account for the dark matter.

He didn’t stop there. In his relentless devotion to astronomical and lexicographical exploration, he also considered the possibility that WIMPZILLAS would actually interact strongly with each other. And if so they might better be known as SIMPZILLAS, short for strongly interacting WIMPZILLAS.

Whether WIMPZILLAS or SIMPZILLAS will solve the dark-matter mystery remains to be seen, of course. Plain old WIMPs may turn out to be all that astronomy needs. But that’s not so obvious. WIMPs would be plain old cold dark matter, and for most of the past 20 years cold dark matter has been the favorite way to explain how galaxies formed and coagulated. But by the end of the century, a number of computer simulations began to cast doubt on whether the

missing matter really is all cold and dark. Cold dark matter’s problems were a hot topic at the Texas symposium in December 2000.

Some physicists there suggested that the mystery mass is colder than cold, so cold that it becomes “fuzzy dark matter.” Some argued that the dark matter is really lukewarm. Others suspected the need for yet another entirely new idea, generally described by the ugly acronym SIDM, for self-interacting dark matter.

“There’s lots of crazy solutions out there,” Ben Moore of Durham University in England said at the Texas symposium.

Moore reported there on what he called “the dark matter crisis,” instigated by computer simulations of cold dark matter theory that did not produce pictures of the universe that look like the one we live in. Of course, the accuracy of the pictures depended on how closely you looked at them. On the grand scale of galactic clusters, the cold dark matter scenario looked pretty good. The simulations show that a universe full of cold dark matter should in fact produce huge clusters of galaxies, as astronomers observe. But the simulations don’t show the right picture on smaller scales. Full-sized galaxies, like the Milky Way, should be surrounded by perhaps 1,000 smaller, dwarf “satellite” galaxies, the simulations show. But real-life surveys of the space around the Milky Way reveal only about 10 or so such satellites.

Other astrophysicists at the meeting pointed out further discrepancies. The density of matter observed in the cores of the dwarfs is less than the theory predicts, for example. And while the theory says the cores of the dwarfs should be lopsided, they actually appear to be pretty round.

All these observations seemed to say that WIMPs—generally regarded as the most likely component of cold dark matter—did not possess the proper properties to build the universe. And the culprit

property seemed to be that WIMP particles did not interact very much with other particles or with themselves. (Remember, the WI part of WIMP stands for “weakly interacting.”) Another way of saying this is that these particles don’t bump into each other very much.

Princeton University physicist Paul Steinhardt says to think in terms of a pool table. If you spread a few very small spheres over the table—marbles, say—they can move around without much chance of colliding. But replace the marbles with billiard balls, and collisions become much more common. Assuming the problems with standard cold dark matter remain unsolved otherwise, Steinhardt said, it may be that the solution would require cold dark matter particles that collide with each other a lot.¹⁴

“The simplest explanation is that the dark matter is interacting with itself,” he said at the 2000 Texas symposium.

Such “collisional” dark matter particles would interact often enough to deter the formation of dwarf galaxies around big ones, explaining why the Milky Way has only a few satellites. But the problem with collisional dark matter particles is that nobody knows what such a particle would be. Astronomers may need to be searching for something that hasn’t even shown up yet in anybody’s theory. “Perhaps . . . we should be looking for a different kind of particle altogether,” Steinhardt said.

Rocky Kolb, of course, recommends WIMPZILLAS. “If you want strong interactions, then the WIMPZILLA scenario is very promising,” he told me. (In that case the particles would be called SIMPZILLAS, as they would be strongly interacting.) On the other hand, Rocky and others have investigated the prospect that the simulation discrepancies might be solved with the one dark matter particle already known to exist, our friend the neutrino. Except for this purpose it would have to be a species of neutrino with more mass than scientists usually expect a neutrino to have. Extra mass would slow the neutrinos down, making them “warm” rather than very fast and “hot.” Some “warm” dark matter might be the right ingredient

needed to make the mix of galaxies, clusters, and satellites come out just right. (But it’s not so clear that warm dark matter exists, and there remains some question whether it would solve all the universe’s problems even if it did exist.)

Some of our other old friends might get involved in solving the dark matter puzzle as well. Vic Teplitz and Rabindra Mohapatra say that mirror-matter particles playing the role of cold dark matter would produce the observed density in the cores of dwarf galaxies. You never know.

I could go on and on describing possible strange candidate dark-matter particles. For example, some physicists suggest that the cosmic dark matter is actually a solid. This is very hard to picture, but it could be that the dark matter is distributed and evolves in such a way that the ordinary rules for describing solids would apply. Solid dark matter would have some pretty strange properties, of course, such as allowing people and planets and stars to move around through it.

An even more intriguing idea to me, perhaps because of its soft-and-cuddly sounding name, is known as “fuzzy dark matter.” I believe the first use of this label came in a paper published in 2000 by three physicists then at the Institute for Advanced Study: Wayne Hu, Rennan Barkana, and Andrei Gruzinov. They calculated that extremely lightweight particles—it would take 10 million trillion trillion of them to weigh as much as an atom—might explain the structure of the dwarf galaxies and their scarcity. In the coldness of space, such minuscule particles would spread out in the form of waves, as dictated by the requirements of quantum theory. The waves of individual particles would overlap and merge, generating a fuzzy substance known as a Bose-Einstein condensate. Calculations show that fewer dwarfs might form from such a condensate, and the ones that did would be less dense at the core, as observations indicate.

Fuzzy dark matter particles sound similar to the axion, a particle proposed years ago by physicists Roberto Peccei, Helen Quinn, Frank Wilczek, and Steven Weinberg. (Wilczek named the particle

after a brand of laundry detergent.) Axions, which solve a problem in the theory describing how the atomic nucleus holds itself together, would be very lightweight (though not as light as the fuzzy particles) but otherwise would behave much like WIMPs. And of course if axions exist, space might also be populated by axinos—their SUSY partners—another ingredient to consider in the strange mix of matters that might make up the universe.

On the other hand, the problems with cold dark matter may be resolved as observations (and theories) improve. The dark matter picture is always changing in some way or another. One year, everything seems fine with cold dark matter; the next, somebody has identified some observation that doesn’t seem to fit the cold dark matter scenario. I can remember a talk from the early 1990s when one speaker displayed a cartoon with a tombstone proclaiming CDMRIP. It came back to life.

In any event, the dark matter problem has proved to be both profound and difficult. The search for its identity goes beyond mere idle curiosity. Not only does it make up the bulk of the matter in the universe, but its properties determine why the universe is such an architectural masterpiece. Yet despite dark matter’s importance, and all the attention paid to it for decades, its identity remains a mystery.

I suspect that the answer to the dark matter question remains elusive because theorists don’t yet have as good a grasp on cosmology as they sometimes seem to believe. Clues to the answer, I think, may come with a better understanding of how the universe itself came to be—an understanding that began in the twentieth century with one of the most fantastic of prediscoveries: the expansion of the universe.