13 Things That Don't Make Sense (3 page)

The same principle is at work in the motions of the planets. The Earth, in its position close to the Sun, moves much faster
in its orbit than Neptune, which is farther out. The reason is simple: it’s about balancing forces. The gravitational pull
of the Sun is stronger at Earth’s radial distance out from the Sun than at Neptune’s. Something with Earth’s mass has to be
moving relatively fast to maintain its orbit. For Neptune to hold its orbit, with less pull from the distant Sun, it goes
slower to keep in equilibrium. If it moved at the same speed as Earth, it would fly off and out of our solar system.

Any orbiting system ought to follow this rule: balancing a gravitational pull and the centrifugal forces means that, the farther
something is from whatever is holding it in orbit, the slower it will move. And, in 1933, that is exactly what a Swiss astronomer
called Fritz Zwicky didn’t see.

As construction began on the Golden Gate Bridge and a forty-three-year-old Adolf Hitler was appointed chancellor of Germany,
Zwicky noticed something odd about the Coma cluster of galaxies. Roughly speaking, stars emit a certain amount of light per
kilo, so, looking at the amount of light coming out of the Coma cluster, Zwicky could estimate how much stuff it contained.
Zwicky’s problem was that the stars on the edges of the galaxies were moving far too fast to be constrained by the gravitational
pull of that amount of material. According to his calculations, the only explanation was that there was about four hundred
times more mass in the Coma cluster than could be accounted for by the cluster’s visible matter.

It should have been enough to launch the dark matter hunt, but it wasn’t—for the worst of scientific reasons. Comb the Internet
for references to Zwicky, and you’ll find
brilliant
next to
maverick
,
genius
next to
insufferable
. Like Slipher, he doesn’t figure large in the astronomy textbooks, despite his many important discoveries. He was the first
to see that galaxies form clusters. He coined the term
supernova
. He was certainly one of a kind. He built a ski ramp next to the Mount Wilson Observatory in the San Gabriel Mountains of
California, for example; in the winter Zwicky would haul his skis to work so he could keep his ski-jumping skills honed. But
it was his interpersonal skills that needed most attention. He was a prickly, difficult man, convinced of his own genius,
and convinced that he never got the recognition he deserved. He had a tendency to refer to all his colleagues as “spherical
bastards”: bastards whichever way you looked at them. Small wonder, then, that his colleagues turned a blind eye to his discovery
of the Coma cluster’s missing mass.

But he was right. Something about the mass of galaxies just doesn’t add up—unless, that is, the universe is heavily sprinkled
with dark matter. In 1939, at the dedication of the McDonald Observatory in Texas, the Dutch astronomer Jan Oort added to
the evidence. Oort gave a lecture in which he showed the distribution of the mass in a certain elliptical galaxy had to be
very different from the distribution of the light. He published the data three years later, making this very point clear in
the abstract. Again, in a classic Kuhnian response, no one reacted. This spectacular ability to ignore such anomalous results
continued for decades until, for some reason, people finally listened to Vera Rubin.

Rubin, who is now in her late seventies, made her first big mark on cosmology at the age of twenty-two. The New Year’s Eve,
1950, edition of the
Washington Post
reported on a talk she gave at the American Astronomical Society, hailing her achievements under the headline “Young Mother
Figures Center of Creation by Star Motions.” The accompanying piece described how Rubin’s work was “so daring … that most
astronomers think her theories are not yet possible.” But her most daring work, the fight to get dark matter taken seriously,
was still to come.

Not that she even took herself seriously to start with. The story, she says, is a lesson in how dumb a scientist can be. In
1962 Rubin was teaching at Georgetown University in Washington, D.C. Most of her students were from the U.S. Naval Observatory
down the road, and they were very good astronomers, she recalls. Together they were able to map out the
rotation curve
of a galaxy. This is a graph that shows how the velocity of the stars changes as you move out from the center of the galaxy.
As with that weighted string twirling around your head, the velocities should fall as you get farther out. For Rubin and her
naval researchers, though, they didn’t; once they got away from the center, the curve was flat. They presented the results
in a series of three papers, and Rubin made nothing of it.

Three years later, in 1965, she took a job at the Carnegie Institution of Washington. After a year in the cutthroat business
of looking for quasars, the most distant objects known, she wanted to do something a little less competitive, something she
could make her own. She decided to look at the outside of galaxies because no one had studied them—everyone concentrated on
the centers. Not only had Rubin completely forgotten about her work with the Naval Observatory students, she also didn’t believe
her own results as she was gathering them. She measured the speeds by looking at how the motion had changed the spectrum of
light coming from a star. Rubin was gathering about four spectra each night, gradually going farther and farther out from
the center of the galaxy. Even though she developed the spectra as she went along, and they all looked the same, the penny
didn’t drop.

“You always thought the next point would fall,” she says. “And it just didn’t.”

Eventually, though, she got it. By 1970 Rubin had mapped out the rotation curve for Andromeda; the star velocities remained
the same however far out she looked. With the velocities of the stars remaining high at the edge, centrifugal forces should
be throwing Andromeda’s outer stars off into deep space. By rights, Andromeda should be falling apart. Unless, that is, it
is surrounded by a halo of dark matter.

NO
one knows what the dark matter actually is. When the Cambridge professor Malcolm Longair wrote his cosmology primer
Our Evolving Universe
, he listed some of the things it might turn out to be. At the top of the list were things like interstellar planets and low
mass stars. Toward the bottom of the list were house bricks and copies of the
Astrophysical Journal
. This last candidate seems most appropriate; if it were discovered to be the answer, it would add a pleasing irony to the
dark matter story. The
Astrophysical Journal
is where, in 1970, Rubin published her results and brought dark matter in from the cold.

Not that you’d necessarily get that from the paper. The title seems innocuous: “Rotation of the Andromeda Nebula from a Spectroscopic
Survey of Emission Regions.” The abstract, the summary of the paper, seems to say nothing controversial. The conclusions of
the paper are similarly disappointing. It presents the data—measurements of the rotation speeds of the stars in Andromeda—and
says nothing more. The graph from page 12 is still on the wall of Rubin’s office at the Carnegie Institution’s Department
of Terrestrial Magnetism in Washington, D.C., however. And today it remains just as relevant, and just as mysterious, as it
was on publication.

The idea of a clutch of invisible matter holding on to Andromeda’s outer stars didn’t catch on straightaway, but at least
this time it wasn’t ignored. First, astronomers justified the blind eye they had turned for thirty-seven years. They started
constructing their own rotation curves, for example, by coming up with exotic explanations for how the mass might be distributed
through the galaxies. None of these efforts ever convinced Rubin, she says; somehow, a couple of the points were always so
far off the curve—and ignored—as to make the ideas laughable.

By the 1980s astronomers had given up trying to fudge the data. Something about the gravitation of galaxies didn’t fit, and
the best explanation was the existence of some matter that didn’t shine like the stars, or reflect light, or give off detectable
radiation, or behave in any way that would make its presence known—except by its gravitational pull. The quest was now on
to find out what this strange stuff was.

The first meeting on the subject of the new dark matter was held at Harvard University in 1980. Rubin then confidently proclaimed
to the audience that we would know what dark matter was in just a decade. That deadline came and went, and we were none the
wiser. In 1990, at a meeting in Washington, D.C., Martin Rees, the English astronomer royal, told an audience that the mystery
would be solved within ten years. Then, in 1999, one year away from the deadline he had imposed in Washington, Rees gave an
extension, declaring, “[I am] optimistic that if I were writing in five years’ time, I would be able to report what the dark
matter is.”

His optimism was misplaced. We still don’t know what the dark matter is. A series of exotica have been suggested, everything
from black holes to as-yet-undiscovered particles with extraordinary properties. Nothing that fits the bill has yet been discovered.
And it’s not for want of looking.

SEARCHING
for dark matter is not for the fainthearted; the stuff has eluded detection for thirty years for good reason. Nevertheless,
scientists do have some ideas of how to look. Physicists have models for what kind of particles might have been created in
the big bang that could still be hanging around in the cosmos to act as dark matter. Their best guess is something called
weakly interacting massive particles,
or
WIMPs
. If this is right, there’s no shortage of dark matter to hunt for. According to the particle physicists, the Earth is drifting
through a mist of dark matter right now; something like a billion WIMPs are washing through your head every second.

Among the WIMPs, there is one outstanding candidate: the neutralino. It is stable enough to still be filling the cosmos 13
billion years after the big bang. It would be suitably difficult to see or feel; it doesn’t interact via the strong force
that holds nuclei together, and it ignores and is ignored by electromagnetic fields. Crucially, it has enough mass—about one
hundred times the mass of a proton—to have the necessary effect on galaxies. The only drawback is that no one knows whether
the neutralino really exists.

If you want to find experimental evidence for dark matter, you have to get it to interact with something. Our best chance
of that comes with atoms that have large nuclei. The dark matter hunters use large arrays of silicon or germanium crystals,
or huge vats of liquid xenon. The hope is that one of the WIMPs will make a direct hit on one of these fat atomic nuclei.
If that happens, the nucleus should recoil a little bit (in the case of the crystals) or send out an electrical signal (from
the liquid xenon). There are a couple of complications, though.

First, the nuclei vibrate naturally anyway, so physicists need to hold them still in order to avoid a false detection in the
apparatus. The crystal arrays, for instance, have to be cooled down to a fraction of a degree above absolute zero, the temperature
where everything stops moving. Cooling the detectors this much is cumbersome and difficult. And then there’s the second complication:
cosmic rays.

Earth is continually bombarded by high-speed particles from space, and these cosmic rays produce exactly the same signature
as WIMPs in a WIMP detector. So the searches have to take place deep underground, beyond the reach of the rays. It is a complication
that makes the dark matter hunters the inhabitants of some of the most inaccessible laboratories on Earth. An Italian group
have put their detector under a mountain. The neutralino search in the United Kingdom takes place 1,100 meters underground,
in a potash mine whose tunnels reach out under the seafloor. U.S. researchers have set up a dark matter hunt seven hundred
meters underground, in an abandoned iron mine in northern Minnesota.

When you understand the working conditions, you know these people must be serious. And yet, so far, they have found precisely
nothing. The searches have been going on for more than a decade; indeed, many of the researchers have dedicated more than
two decades of their lives to the quest for dark matter. Upgrades are making the equipment more sensitive all the time, but
we still have no defensible idea of what is causing that strange pull in the heavens.

It seems somehow impossible that, when this stuff makes up a quarter of the universe, we don’t know yet what it is. But we
should perhaps take comfort in the fact that we at least noticed it was missing. If we hadn’t, it’s hard to imagine how wrong
we’d have got things when, in 1997, it became apparent that another bit of the universe was also absent without leave. If
dark matter was a problem, the discovery of dark energy was a catastrophe.

IF
the universe is expanding, as Hubble showed it is, two questions spring immediately to mind. First, how fast is it expanding?
Second, will it keep expanding forever?