NY Times: Dark matter & dark energy – remains unexplained


Richard Moore

Original source URL:

March 11, 2007

Out There

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.

³Time and time again,² Smoot shouted, ³the universe has turned out to be really 

Perlmutter nodded eagerly. ³It¹s like, why are we able to understand the 
universe at our level?²

³Right. Exactly. It¹s a universe for beginners! ŒThe Universe for Dummies¹!²

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 Newton¹s laws and you have a 
universe that runs like clockwork. Take Einstein¹s modifications of Newton, 
apply the discovery of an expanding universe and you get the big bang. ³It¹s a 
ridiculously simple, intentionally cartoonish picture,² Perlmutter said. ³We¹re 
just incredibly lucky that that first try has matched so well.²

But is our luck about to run out? Smoot¹s and Perlmutter¹s 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 

³Dark,² cosmologists call it, in what could go down in history as the ultimate 
semantic surrender. This is not ³dark² as in distant or invisible. This is 
³dark² 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 ³the ultimate Copernican revolution²: not only are we not at
the center of anything; we¹re not even made of the same stuff as most of the 
rest of everything. ³We¹re just a bit of pollution,² Lawrence M. Krauss, a 
theorist at Case Western Reserve, said not long ago at a public panel on 
cosmology in Chicago. ³If 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. We¹re completely irrelevant.²

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? It¹s a question cosmologists have 
been asking themselves lately, and it might well be a question we¹ll all be 
asking ourselves soon, because if they¹re right, then the time has come to 
rethink a fundamental assumption: When we look up at the night sky, we¹re 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 Newton¹s universe, with its 
stately, eternal procession of benign objects, but Einstein¹s 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 didn¹t 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 ³dark 

So there was more to the universe than meets the eye. But how much more? This 
was the question Saul Perlmutter¹s 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. ³I was trying to tell everybody 
that this is the measurement that everybody should be doing,² Perlmutter says. 
³I was trying to convince them that this is going to be the tool of the future.²
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 person¹s point of view. He can be as persuasive as any force of 
nature. ³The next thing I know,² he says, ³we¹ve convinced people, and now 
they¹re competing with us!²

By 1997, Perlmutter¹s 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 weren¹t 
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. 
³I¹m a postdoc,² he told himself. ³I¹m sure I¹ve messed up in at least 10 
different ways.² 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 ³dark energy.²
The purpose of calling it ³dark,² he explained recently, was to highlight the 
similarity to dark matter. The purpose of ³energy² was to make a distinction. 
³It really is very different from dark matter,² Turner said. ³It¹s more 

More energylike how, exactly?

Turner raised his eyebrows. ³I¹m not embarrassed to say it¹s the most profound 
mystery in all of science.²

Extraordinary claims,² Carl Sagan once said, ³require extraordinary evidence.² 
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 wouldn¹t 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 didn¹t, 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 didn¹t, 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 ³dark,² 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 can¹t detect other than indirectly. The
motions of galaxies don¹t make sense unless we infer the existence of dark 
matter. The brightness of supernovae doesn¹t make sense unless we infer the 
existence of dark energy. It¹s not that inference can¹t 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 can¹t be seen from earth. To account for the dark-matter 
deficit, this material would have to be so massive and so numerous that we 
couldn¹t possibly miss it.

Which leaves abnormal matter, or what physicists call nonbaryonic matter, 
meaning that it doesn¹t consist of the protons and neutrons of ³normal² matter. 
What¹s more (or, perhaps more accurately, less), it doesn¹t interact at all with
electricity or magnetism, which is why we wouldn¹t 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 ‹ ³this close.² 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. ³I hope he¹s right,² she says of Smoot¹s assertion. 
³But I think it¹s more a wish than a belief.² As one particle physicist 
commented at a ³Dark Universe² symposium at the Space Telescope Science 
Institute in Baltimore a few years ago, ³If we fail to see anything in the 
L.H.C., then I¹m off to do something else,² adding, ³Unfortunately, I¹ll be off 
to do something else at the same time as hundreds of other physicists.²

Juan Collar might be among them. ³I know I speak for a generation of people who 
have been looking for dark-matter particles since they were grad students,² he 
said one wintry afternoon in his University of Chicago office. ³I 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. I¹m 42. So most of my 
colleagues, my age, we are kind of going through a midlife crisis.² He laughed. 
³When we get together and we drink enough beer, we start howling at the moon.²

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 haven¹t thought hard enough or that observers haven¹t looked deep 
enough. ³It could very well be that Mother Nature has decided that the 
neutralino is way down there,² 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. ³If that is the case,² he went on to say, ³we 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 won¹t see them.²

The challenge with dark energy, as opposed to dark matter, is even more 
difficult. Dark energy is whatever it is that¹s 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, they¹ll 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. ³I don¹t think
I¹ve ever seen as much of Washington as I have in the last few years,² he says, 
sighing. Even if his Supernova Acceleration Probe didn¹t 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. ³Ten years 
from now,² says Josh Frieman of the University of Chicago, ³when we¹re talking 
about spending on the order of a billion dollars to put something up in space ‹ 
which I think we should do ‹ you¹re getting into that class where you¹re 
spending real money.²

Even some cosmologists have begun to express reservations. At a conference at 
Durham University in England last summer, a ³whither cosmology?² panel featuring
some of the field¹s 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 ³risky 
and not very cost-effective way of moving forward,² 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 wouldn¹t know what dark energy itself 
is. ³The term doesn¹t mean anything,² said David Schlegel of Lawrence Berkeley 
National Laboratory this past fall. ³It might not be dark. It might not be 
energy. The whole name is a placeholder. It¹s a placeholder for the description 
that there¹s something funny that was discovered eight years ago now that we 
don¹t understand.² Not that theorists haven¹t been trying. ³It¹s just nonstop,² 
Perlmutter told me. ³There¹s article after article after article.² 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 cosmology¹s eggs into one research basket,
argued the Durham panelists. As one summarized the situation, ³We don¹t even 
have a hypothesis to test.²

Michael Turner won¹t hear of it. ³This is one of these godsend problems!² he 
says. ³If you¹re a scientist, you¹d like to be around when there¹s 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.²

But you could also imagine it taking forever.

³Time to get serious.² 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 ³New Views of the Universe² conference, and Sean 
Carroll, then at the University of Chicago, had taken it upon himself to give 
his theorist colleagues their marching orders.

³There was a heyday for talking out all sorts of crazy ideas,² 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, ³Can¹t we be exuberant for a while?² But now has come 
the metaphorical morning after, and with it a sobering realization: Maybe the 
universe isn¹t simple enough for dummies like us humans. Maybe it¹s not just our
powers of perception that aren¹t 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 it¹s not 
³normal,² 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 

That possibility always gnaws at scientists ‹ what Perlmutter calls ³that sense 
of tentativeness, that we have gotten so far based on so little.² Cosmologists 
in particular have had to confront that possibility throughout the birth of 
their science. ³At various times in the past 20 years it could have gotten to 
the point where there was no opportunity for advance,² Frieman says. What if, 
for instance, researchers couldn¹t 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 hadn¹t shown up? As Frieman puts it: ³You just would 
have thrown up your hands and said, ŒMy God, we¹ve got to go back to the drawing
board!¹ What¹s remarkable to me is that so far that hasn¹t happpened.²

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 Einstein¹s 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: ³You get to invoke 
the tooth fairy only once,² meaning dark matter, ³but now we have to invoke the 
tooth fairy twice,² 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 
Perlmutter¹s and Riess¹s 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 doesn¹t 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 doesn¹t 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 ³so great an Absurdity that I 
believe no Man who has in philosophical Matters a competent Faculty of thinking 
can ever fall into it.² Yet fall into it we all do on a daily basis, and 
physicists are no exception. ³I don¹t think we really understand what gravity 
is,² Vera Rubin says. ³So in some sense we¹re doing an awful lot on something we
don¹t know much about.²

It hasn¹t escaped the notice of astronomers that both dark matter and dark 
energy involve gravity. Early this year 50 physicists gathered for a ³Rethinking
Gravity² conference at the University of Arizona to discuss variations on 
general relativity. ³So far, Einstein is coming through with flying colors,² 
says Sean Carroll, who was one of the gravity-defying participants. ³He¹s always
smarter than you think he was.²

But he¹s not necessarily inviolate. ³We¹ve never tested gravity across the whole
universe before,² Riess pointed out during a news conference last year. ³It may 
be that there¹s not really dark energy, that that¹s a figment of our 
misperception about gravity, that gravity actually changes the way it operates 
on long ranges.²

The only way out, cosmologists and particle physicists agree, would be a ³new 
physics² ‹ a reconciliation of general relativity and quantum mechanics. 
³Understanding dark energy,² Riess says, ³seems to really require understanding 
and using both of those theories at the same time.²

³It¹s been so hard that we¹re even willing to consider listening to string 
theorists,² Perlmutter says, referring to work that posits numerous dimensions 
beyond the traditional (one of time and three of space). ³They¹re at least 
providing a language in which you can talk about both things at the same time.²

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 

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.

³You want your mind to be boggled,² Perlmutter says. ³That is a pleasure in and 
of itself. And it¹s more a pleasure if it¹s boggled by something that you can 
then demonstrate is really, really true.²

And if you can¹t demonstrate that it¹s really, really true?

³If the brilliant idea doesn¹t come along,² Riess says, ³then we will say dark 
energy has exactly these properties, it acts exactly like this. And then² ‹ a 
shrug ‹ ³we will put it in a box.² And there it will remain, residing perhaps 
not far from the box labeled ³Dark Matter,² and the two of them bookending the 
biggest box of them all, ³Gravity,² to await a future Newton or Einstein to open
‹ or not.

Richard Panek is the author of ³The Invisible Century: Einstein, Freud and the 
Search for Hidden Universes.²

Copyright 2007 The New York Times Company

Escaping the Matrix website        http://escapingthematrix.org/
cyberjournal website               http://cyberjournal.org
Community Democracy Framework: http://cyberjournal.org/DemocracyFramework.html
subscribe cyberjournal list        mailto:•••@••.•••
Posting archives                   http://cyberjournal.org/show_archives/