http://www.nytimes.com/2007/03/11/magazine/11dark.t.html?ex=1331269200&en=3bdcd27b4e5dea54&ei=5088&partner=rssnyt&emc=rss
Three days after learning that he won the 2006 Nobel Prize in Physics,
George Smoot was talking about the universe. Sitting across from him in
his office at the University of California, Berkeley, was Saul
Perlmutter, a fellow cosmologist and a probable future Nobelist in
Physics himself. Bearded, booming, eyes pinwheeling from adrenaline and
lack of sleep, Smoot leaned back in his chair. Perlmutter, onetime
acolyte, longtime colleague, now heir apparent, leaned forward in his.
“Time and time again,” Smoot shouted, “the universe has turned out to be
really simple.”
Perlmutter nodded eagerly. “It’s like, why are we able to understand the
universe at our level?”
“Right. Exactly. It’s a universe for beginners! ‘The Universe for
Dummies’!”
But as Smoot and Perlmutter know, it is also inarguably a universe for
Nobelists, and one that in the past decade has become exponentially more
complicated. Since the invention of the telescope four centuries ago,
astronomers have been able to figure out the workings of the universe
simply by observing the heavens and applying some math, and vice versa.
Take the discovery of moons, planets, stars and galaxies, apply Newton’s
laws and you have a universe that runs like clockwork. Take Einstein’s
modifications of Newton, apply the discovery of an expanding universe
and you get the big bang. “It’s a ridiculously simple, intentionally
cartoonish picture,” Perlmutter said. “We’re just incredibly lucky that
that first try has matched so well.”
But is our luck about to run out? Smoot’s and Perlmutter’s work is part
of a revolution that has forced their colleagues to confront a universe
wholly unlike any they have ever known, one that is made of only 4
percent of the kind of matter we have always assumed it to be — the
material that makes up you and me and this magazine and all the planets
and stars in our galaxy and in all 125 billion galaxies beyond. The rest
— 96 percent of the universe — is ... who knows?
“Dark,” cosmologists call it, in what could go down in history as the
ultimate semantic surrender. This is not “dark” as in distant or
invisible. This is “dark” as in unknown for now, and possibly forever.
If so, such a development would presumably not be without philosophical
consequences of the civilization-altering variety. Cosmologists often
refer to this possibility as “the ultimate Copernican revolution”: not
only are we not at the center of anything; we’re not even made of the
same stuff as most of the rest of everything. “We’re just a bit of
pollution,” Lawrence M. Krauss, a theorist at Case Western Reserve, said
not long ago at a public panel on cosmology in Chicago. “If you got rid
of us, and all the stars and all the galaxies and all the planets and
all the aliens and everybody, then the universe would be largely the
same. We’re completely irrelevant.”
All well and good. Science is full of homo sapiens-humbling insights.
But the trade-off for these lessons in insignificance has always been
that at least now we would have a deeper — simpler — understanding of
the universe. That the more we could observe, the more we would know.
But what about the less we could observe? What happens to new knowledge
then? It’s a question cosmologists have been asking themselves lately,
and it might well be a question we’ll all be asking ourselves soon,
because if they’re right, then the time has come to rethink a
fundamental assumption: When we look up at the night sky, we’re seeing
the universe.
Not so. Not even close.
In 1963, two scientists at Bell Labs in New Jersey discovered a
microwave signal that came from every direction of the heavens.
Theorists at nearby Princeton University soon realized that this signal
might be the echo from the beginning of the universe, as predicted by
the big-bang hypothesis. Take the idea of a cosmos born in a primordial
fireball and cooling down ever since, apply the discovery of a microwave
signal with a temperature that corresponded precisely to the one that
was predicted by theorists — 2.7 degrees above absolute zero — and you
have the universe as we know it. Not Newton’s universe, with its
stately, eternal procession of benign objects, but Einstein’s universe,
violent, evolving, full of births and deaths, with the grandest birth
and, maybe, death belonging to the cosmos itself.
But then, in the 1970s, astronomers began noticing something that didn’t
seem to fit with the laws of physics. They found that spiral galaxies
like our own Milky Way were spinning at such a rate that they should
have long ago wobbled out of control, shredding apart, shedding stars in
every direction. Yet clearly they had done no such thing. They were
living fast but not dying young. This seeming paradox led theorists to
wonder if a halo of a hypothetical something else might be cocooning
each galaxy, dwarfing each flat spiral disk of stars and gas at just the
right mass ratio to keep it gravitationally intact. Borrowing a term
from the astronomer Fritz Zwicky, who detected the same problem with the
motions of a whole cluster of galaxies back in the 1930s, decades before
anyone else took the situation seriously, astronomers called this
mystery mass “dark matter.”
So there was more to the universe than meets the eye. But how much more?
This was the question Saul Perlmutter’s team at Lawrence Berkeley
National Laboratory set out to answer in the late 1980s. Actually, they
wanted to settle an issue that had been nagging astronomers ever since
Edwin Hubble discovered in 1929 that the universe seems to be expanding.
Gravity, astronomers figured, would be slowing the expansion, and the
more matter the greater the gravitational effect. But was the amount of
matter in the universe enough to slow the expansion until it eventually
stopped, reversed course and collapsed in a backward big bang? Or was
the amount of matter not quite enough to do this, in which case the
universe would just go on expanding forever? Just how much was the
expansion of the universe slowing down?
The tool the team would be using was a specific type of exploding star,
or supernova, that reaches a roughly uniform brightness and so can serve
as what astronomers call a standard candle. By comparing how bright
supernovae appear and how much the expansion of the universe has shifted
their light, cosmologists sought to determine the rate of the expansion.
“I was trying to tell everybody that this is the measurement that
everybody should be doing,” Perlmutter says. “I was trying to convince
them that this is going to be the tool of the future.” Perlmutter talks
like a microcassette on fast-forward, and he possesses the kind of
psychological dexterity that allows him to walk into a room and
instantly inhabit each person’s point of view. He can be as persuasive
as any force of nature. “The next thing I know,” he says, “we’ve
convinced people, and now they’re competing with us!”
By 1997, Perlmutter’s Supernova Cosmology Project and a rival team had
amassed data from more than 50 supernovae between them — data that would
reveal yet another oddity in the cosmos. Perlmutter noticed that the
supernovae weren’t brighter than expected but dimmer. He wondered if he
had made a mistake in his observations. A few months later, Adam Riess,
a member of a rival international team, noticed the same general drift
in his math and wondered the same thing. “I’m a postdoc,” he told
himself. “I’m sure I’ve messed up in at least 10 different ways.” But
Perlmutter double-checked for intergalactic dust that might have skewed
his readings, and Riess cross-checked his math, calculation by
calculation, with his team leader, Brian Schmidt. Early in 1998, the two
teams announced that they had each independently reached the same
conclusion, and it was the opposite of what either of them expected. The
rate of the expansion of the universe was not slowing down. Instead, it
seemed to be speeding up.
That same year, Michael Turner, the prominent University of Chicago
theorist, delivered a paper in which he called this antigravitational
force “dark energy.” The purpose of calling it “dark,” he explained
recently, was to highlight the similarity to dark matter. The purpose of
“energy” was to make a distinction. “It really is very different from
dark matter,” Turner said. “It’s more energylike.”
More energylike how, exactly?
Turner raised his eyebrows. “I’m not embarrassed to say it’s the most
profound mystery in all of science.”
Extraordinary claims,” Carl Sagan once said, “require extraordinary
evidence.” Astronomers love that saying; they quote it all the time. In
this case the claim could have hardly been more extraordinary: a new
universe was dawning.
It wouldn’t be the first time. We once thought the night sky consisted
of the several thousand objects we could see with the naked eye. But the
invention of the telescope revealed that it didn’t, and that the farther
we saw, the more we saw: planets, stars, galaxies. After that we thought
the night sky consisted of only the objects the eye could see with the
assistance of telescopes that reached all the way back to the first
stars blinking to life. But the discovery of wavelengths beyond the
optical revealed that it didn’t, and that the more we saw in the radio
or infrared or X-ray parts of the electromagnetic spectrum, the more we
discovered: evidence for black holes, the big bang and the distances of
supernovae, for starters.
The difference with “dark,” however, is that it lies not only outside
the visible but also beyond the entire electromagnetic spectrum. By all
indications, it consists of data that our five senses can’t detect other
than indirectly. The motions of galaxies don’t make sense unless we
infer the existence of dark matter. The brightness of supernovae doesn’t
make sense unless we infer the existence of dark energy. It’s not that
inference can’t be a powerful tool: an apple falls to the ground, and we
infer gravity. But it can also be an incomplete tool: gravity is ... ?
Dark matter is ... ? In the three decades since most astronomers
decisively, if reluctantly, accepted the existence of dark matter,
observers have eliminated the obvious answer: that dark matter is made
of normal matter that is so far away or so dim that it can’t be seen
from earth. To account for the dark-matter deficit, this material would
have to be so massive and so numerous that we couldn’t possibly miss it.
Which leaves abnormal matter, or what physicists call nonbaryonic
matter, meaning that it doesn’t consist of the protons and neutrons of
“normal” matter. What’s more (or, perhaps more accurately, less), it
doesn’t interact at all with electricity or magnetism, which is why we
wouldn’t be able to see it, and it can rarely interact even with protons
and neutrons, which is why trillions of these particles might be passing
through you every second without your knowing it. Theorists have
narrowed the search for dark-matter particles to two hypothetical
candidates: the axion and the neutralino. But so far efforts to create
one of these ghostly particles in accelerators, which mimic the high
levels of energy in the first fraction of a second after the birth of
the universe, have come up empty. So have efforts to catch one in
ultrasensitive detectors, which number in the dozens around the world.
For now, dark-matter physicists are hanging their hopes on the Large
Hadron Collider, the latest-generation subatomic-particle accelerator,
which goes online later this year at the European Center for Nuclear
Research on the Franco-Swiss border. Many cosmologists think that the
L.H.C. has made the creation of a dark-matter particle — as George Smoot
said, holding up two fingers — “this close.” But one of the pioneer
astronomers investigating dark matter in the 1970s, Vera Rubin, says
that she has lived through plenty of this kind of optimism; she herself
predicted in 1980 that dark matter would be identified within a decade.
“I hope he’s right,” she says of Smoot’s assertion. “But I think it’s
more a wish than a belief.” As one particle physicist commented at a
“Dark Universe” symposium at the Space Telescope Science Institute in
Baltimore a few years ago, “If we fail to see anything in the L.H.C.,
then I’m off to do something else,” adding, “Unfortunately, I’ll be off
to do something else at the same time as hundreds of other physicists.”
Juan Collar might be among them. “I know I speak for a generation of
people who have been looking for dark-matter particles since they were
grad students,” he said one wintry afternoon in his University of
Chicago office. “I doubt how many of us will remain in the field if the
L.H.C. brings home bad news. I have been looking for dark-matter
particles for more than 15 years. I’m 42. So most of my colleagues, my
age, we are kind of going through a midlife crisis.” He laughed. “When
we get together and we drink enough beer, we start howling at the moon.”
Although many scientists say that the existence of the axion will be
proved or disproved within the next 10 years — as a result of work at
Lawrence Livermore National Laboratory — the detection of a neutralino
one way or the other is much less certain. A negative result from an
experiment might mean only that theorists haven’t thought hard enough or
that observers haven’t looked deep enough. “It could very well be that
Mother Nature has decided that the neutralino is way down there,” Collar
said, pointing not to a graph that he taped up in his office but to a
point below the sheet of paper itself, at the blank wall. “If that is
the case,” he went on to say, “we should retreat and worship Mother
Nature. These particles maybe exist, but we will not see them, our sons
will not see them and their sons won’t see them.”
The challenge with dark energy, as opposed to dark matter, is even more
difficult. Dark energy is whatever it is that’s making the expansion of
the universe accelerate, but, for instance, does it change over time and
space? If so, then cosmologists have a name for it: quintessence. Does
it not change? In that case, they’ll call it the cosmological constant,
a version of the mathematical fudge factor that Einstein originally
inserted into the equations for relativity to explain why the universe
had neither expanded nor contracted itself out of existence.
After the discovery of dark energy, Perlmutter concluded that the next
generation of dark-energy telescopes would have to include a space-based
observatory. But the search for financing for such an ambitious project
can require as much forbearance as the search for dark energy itself. “I
don’t think I’ve ever seen as much of Washington as I have in the last
few years,” he says, sighing. Even if his Supernova Acceleration Probe
didn’t now face competition from several other proposals for federal
financing (including, perhaps inevitably, one involving his old rival
Riess), delays have prevented it from being ready to launch until at
least the middle of the next decade. “Ten years from now,” says Josh
Frieman of the University of Chicago, “when we’re talking about spending
on the order of a billion dollars to put something up in space — which I
think we should do — you’re getting into that class where you’re
spending real money.”
Even some cosmologists have begun to express reservations. At a
conference at Durham University in England last summer, a “whither
cosmology?” panel featuring some of the field’s most prominent names
questioned the wisdom of concentrating so much money and manpower on one
problem. They pointed to what happened when the government-sponsored
Dark Energy Task Force solicited proposals for experiments a couple of
years ago. The task force was expecting a dozen, according to one
member. They got three dozen. Cosmology was choosing a “risky and not
very cost-effective way of moving forward,” one Durham panelist told me
later, summarizing the sentiment he heard there.
But even if somebody were to figure out whether or not dark energy
changes across time and space, astronomers still wouldn’t know what dark
energy itself is. “The term doesn’t mean anything,” said David Schlegel
of Lawrence Berkeley National Laboratory this past fall. “It might not
be dark. It might not be energy. The whole name is a placeholder. It’s a
placeholder for the description that there’s something funny that was
discovered eight years ago now that we don’t understand.” Not that
theorists haven’t been trying. “It’s just nonstop,” Perlmutter told me.
“There’s article after article after article.” He likes to begin public
talks with a PowerPoint illustration: papers on dark energy piling up,
one on top of the next, until the on-screen stack ascends into the
dozens. All the more reason not to put all of cosmology’s eggs into one
research basket, argued the Durham panelists. As one summarized the
situation, “We don’t even have a hypothesis to test.”
Michael Turner won’t hear of it. “This is one of these godsend
problems!” he says. “If you’re a scientist, you’d like to be around when
there’s a great problem to work on and solve. The solution is not
obvious, and you could imagine it being solved tomorrow, you could
imagine it taking another 10 years or you could imagine it taking
another 200 years.”
But you could also imagine it taking forever.
“Time to get serious.” The PowerPoint slide, teal letters popping off a
black background, stared back at a hotel ballroom full of cosmologists.
They gathered in Chicago last winter for a “New Views of the Universe”
conference, and Sean Carroll, then at the University of Chicago, had
taken it upon himself to give his theorist colleagues their marching
orders.
“There was a heyday for talking out all sorts of crazy ideas,” Carroll,
now at Caltech, recently explained. That heyday would have been the
heady, post-1998 period when Michael Turner might stand up at a
conference and turn to anyone voicing caution and say, “Can’t we be
exuberant for a while?” But now has come the metaphorical morning after,
and with it a sobering realization: Maybe the universe isn’t simple
enough for dummies like us humans. Maybe it’s not just our powers of
perception that aren’t up to the task but also our powers of conception.
Extraordinary claims like the dawn of a new universe might require
extraordinary evidence, but what if that evidence has to be literally
beyond the ordinary? Astronomers now realize that dark matter probably
involves matter that is nonbaryonic. And whatever it is that dark energy
involves, we know it’s not “normal,” either. In that case, maybe this
next round of evidence will have to be not only beyond anything we know
but also beyond anything we know how to know.
That possibility always gnaws at scientists — what Perlmutter calls
“that sense of tentativeness, that we have gotten so far based on so
little.” Cosmologists in particular have had to confront that
possibility throughout the birth of their science. “At various times in
the past 20 years it could have gotten to the point where there was no
opportunity for advance,” Frieman says. What if, for instance,
researchers couldn’t repeat the 1963 Bell Labs detection of the supposed
echo from the big bang? Smoot and John C. Mather of NASA (who shared the
Nobel in Physics with Smoot) designed the Cosmic Background Explorer
satellite telescope to do just that. COBE looked for extremely subtle
differences in temperature throughout all of space that carry the
imprint of the universe when it was less than a second old. And in 1992,
COBE found them: in effect, the quantum fluctuations that 13.7 billion
years later would coalesce into a universe that is 22 percent dark
matter, 74 percent dark energy and 4 percent the stuff of us.
And if the right ripples hadn’t shown up? As Frieman puts it: “You just
would have thrown up your hands and said, ‘My God, we’ve got to go back
to the drawing board!’ What’s remarkable to me is that so far that
hasn’t happpened.”
Yet in a way it has. In the observation-and-theory, call-and-response
system of investigating nature that scientists have refined over the
past 400 years, the dark side of the universe represents a disruption.
General relativity helped explain the observations of the expanding
universe, which led to the idea of the big bang, which anticipated the
observations of the cosmic-microwave background, which led to the
revival of Einstein’s cosmological constant, which anticipated the
observations of supernovae, which led to dark energy. And dark energy is
.... ?
The difficulty in answering that question has led some cosmologists to
ask an even deeper question: Does dark energy even exist? Or is it
perhaps an inference too far? Cosmologists have another saying they like
to cite: “You get to invoke the tooth fairy only once,” meaning dark
matter, “but now we have to invoke the tooth fairy twice,” meaning dark
energy.
One of the most compelling arguments that cosmologists have for the
existence of dark energy (whatever it is) is that unlike earlier
inferences that physicists eventually had to abandon — the ether that
19th-century physicists thought pervaded space, for instance — this
inference makes mathematical sense. Take Perlmutter’s and Riess’s
observations of supernovae, apply one cornerstone of 20th-century
physics, general relativity, and you have a universe that does indeed
consist of .26 matter, dark or otherwise, and .74 something that
accelerates the expansion. Yet in another way, dark energy doesn’t add
up. Take the observations of supernovae, apply the other cornerstone of
20th-century physics, quantum theory, and you get gibberish — you get an
answer 120 orders of magnitude larger than .74.
Which doesn’t mean that dark energy is the ether of our age. But it does
mean that its implications extend beyond cosmology to a problem Einstein
spent the last 30 years of his life trying to reconcile: how to unify
his new physics of the very large (general relativity) with the new
physics of the very small (quantum mechanics). What makes the two
incompatible — where the physics breaks down — is gravity.
In physics, gravity is the ur-inference. Even Newton admitted that he
was making it up as he went along. That a force of attraction might
exist between two distant objects, he once wrote in a letter, is “so
great an Absurdity that I believe no Man who has in philosophical
Matters a competent Faculty of thinking can ever fall into it.” Yet fall
into it we all do on a daily basis, and physicists are no exception. “I
don’t think we really understand what gravity is,” Vera Rubin says. “So
in some sense we’re doing an awful lot on something we don’t know much
about.”
It hasn’t escaped the notice of astronomers that both dark matter and
dark energy involve gravity. Early this year 50 physicists gathered for
a “Rethinking Gravity” conference at the University of Arizona to
discuss variations on general relativity. “So far, Einstein is coming
through with flying colors,” says Sean Carroll, who was one of the
gravity-defying participants. “He’s always smarter than you think he was.”
But he’s not necessarily inviolate. “We’ve never tested gravity across
the whole universe before,” Riess pointed out during a news conference
last year. “It may be that there’s not really dark energy, that that’s a
figment of our misperception about gravity, that gravity actually
changes the way it operates on long ranges.”
The only way out, cosmologists and particle physicists agree, would be a
“new physics” — a reconciliation of general relativity and quantum
mechanics. “Understanding dark energy,” Riess says, “seems to really
require understanding and using both of those theories at the same time.”
“It’s been so hard that we’re even willing to consider listening to
string theorists,” Perlmutter says, referring to work that posits
numerous dimensions beyond the traditional (one of time and three of
space). “They’re at least providing a language in which you can talk
about both things at the same time.”
According to quantum theory, particles can pop into and out of
existence. In that case, maybe the universe itself was born in one such
quantum pop. And if one universe can pop into existence, then why not
many universes? String theorists say that number could be 10 raised to
the power of 500. Those are 10-with-500-zeros universes, give or take.
In which case, our universe would just happen to be the one with an
energy density of .74, a condition suitable for the existence of
creatures that can contemplate their hyper-Copernican existence.
And this is just one of a number of theories that have been popping into
existence, quantum-particle-like, in the past few years: parallel
universes, intersecting universes or, in the case of Stephen Hawking and
Thomas Hertog just last summer, a superposition of universes. But what
evidence — extraordinary or otherwise — can anyone offer for such
claims? The challenge is to devise an experiment that would do for a new
physics what COBE did for the big bang. Predictions in string theory, as
in the 10-to-the-power-of-500-universes hypothesis, depend on the
existence of extra dimensions, a stipulation that just might put the
burden back on particle physics — specifically, the hope that evidence
of extra dimensions will emerge in the Large Hadron Collider, or perhaps
in its proposed successor, the International Linear Collider, which
might come online sometime around 2020, or maybe in the supercollider
after that, if the industrial nations of 2030 decide they can afford it.
“You want your mind to be boggled,” Perlmutter says. “That is a pleasure
in and of itself. And it’s more a pleasure if it’s boggled by something
that you can then demonstrate is really, really true.”
And if you can’t demonstrate that it’s really, really true?
“If the brilliant idea doesn’t come along,” Riess says, “then we will
say dark energy has exactly these properties, it acts exactly like this.
And then” — a shrug — “we will put it in a box.” And there it will
remain, residing perhaps not far from the box labeled “Dark Matter,” and
the two of them bookending the biggest box of them all, “Gravity,” to
await a future Newton or Einstein to open — or not.
Richard Panek is the author of “The Invisible Century: Einstein, Freud
and the Search for Hidden Universes.”
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