[tt] NYT: (Dark Matter) Out There

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Out There
http://select.nytimes.com/preview/2007/03/11/magazine/1154667870562.html

By RICHARD PANEK

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. Its like, why are we able to understand
the universe at our level?

Right. Exactly. Its 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 Newtons laws and you have
a universe that runs like clockwork. Take Einsteins modifications
of Newton, apply the discovery of an expanding universe and you get
the big bang. Its a ridiculously simple, intentionally cartoonish
picture, Perlmutter said. Were just incredibly lucky that that
first try has matched so well.

But is our luck about to run out? Smoots and Perlmutters 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; were not even made of the same stuff as most of the rest
of everything. Were 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. Were
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? Its a question cosmologists have
been asking themselves lately, and it might well be a question well
all be asking ourselves soon, because if theyre right, then the
time has come to rethink a fundamental assumption: When we look up
at the night sky, were 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 Newtons universe, with its stately, eternal procession
of benign objects, but Einsteins 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
didnt 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 Perlmutters 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 persons point of view. He can be as persuasive as any force of
nature. The next thing I know, he says, weve convinced people, and
now theyre competing with us!

By 1997, Perlmutters 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 werent 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. Im a postdoc, he told himself. Im sure Ive 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. Its more
energylike.

More energylike how, exactly?

Turner raised his eyebrows. Im not embarrassed to say its 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 wouldnt 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
didnt, 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 didnt, 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 cant
detect other than indirectly. The motions of galaxies dont make
sense unless we infer the existence of dark matter. The brightness
of supernovae doesnt make sense unless we infer the existence of
dark energy. Its not that inference cant 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 cant be
seen from earth. To account for the dark-matter deficit, this
material would have to be so massive and so numerous that we
couldnt possibly miss it.

Which leaves abnormal matter, or what physicists call nonbaryonic
matter, meaning that it doesnt consist of the protons and neutrons
of normal matter. Whats more (or, perhaps more accurately, less),
it doesnt interact at all with electricity or magnetism, which is
why we wouldnt 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 hes right,
she says of Smoots assertion. But I think its 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 Im
off to do something else, adding, Unfortunately, Ill 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. Im 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 havent
thought hard enough or that observers havent 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 wont see them.

The challenge with dark energy, as opposed to dark matter, is even
more difficult. Dark energy is whatever it is thats 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, theyll 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 dont think Ive ever seen as much of
Washington as I have in the last few years, he says, sighing. Even
if his Supernova Acceleration Probe didnt 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 were talking about spending on the
order of a billion dollars to put something up in space which I
think we should do youre getting into that class where youre
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 fields 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 wouldnt know what
dark energy itself is. The term doesnt 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. Its a placeholder for the description that theres
something funny that was discovered eight years ago now that we
dont understand. Not that theorists havent been trying. Its just
nonstop, Perlmutter told me. Theres 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 cosmologys eggs into one research
basket, argued the Durham panelists. As one summarized the
situation, We dont even have a hypothesis to test.

Michael Turner wont hear of it. This is one of these godsend
problems! he says. If youre a scientist, youd like to be around
when theres 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, Cant we
be exuberant for a while? But now has come the metaphorical morning
after, and with it a sobering realization: Maybe the universe isnt
simple enough for dummies like us humans. Maybe its not just our
powers of perception that arent 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 its 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 couldnt 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 hadnt shown up? As Frieman puts it: You
just would have thrown up your hands and said, My God, weve got to
go back to the drawing board! Whats remarkable to me is that so far
that hasnt 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 Einsteins
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 Perlmutters and Riesss
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
doesnt 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 doesnt 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 dont think we really understand what
gravity is, Vera Rubin says. So in some sense were doing an awful
lot on something we dont know much about.

It hasnt 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. Hes always smarter
than you think he was.

But hes not necessarily inviolate. Weve never tested gravity across
the whole universe before, Riess pointed out during a news
conference last year. It may be that theres not really dark energy,
that thats 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.

Its been so hard that were 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). Theyre 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 its more a pleasure if its boggled
by something that you can then demonstrate is really, really true.

And if you cant demonstrate that its really, really true?

If the brilliant idea doesnt 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.