[tt] NYT: After Years of Effort, Dark Energy Still Puzzles Scientists

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After Years of Effort, Dark Energy Still Puzzles Scientists
New York Times, 8.6.3
http://www.nytimes.com/2008/06/03/science/03dark.html

Dark, Perhaps Forever

By DENNIS OVERBYE

BALTIMORE -- Mario Livio tossed his car keys in the air.

They rose ever more slowly, paused, shining, at the top of their
arc, and then in accordance with everything our Galilean ape brains
have ever learned to expect, crashed back down into his hand.

That was the whole problem, explained Dr. Livio, a theorist at the
Space Telescope Science Institute here on the Johns Hopkins campus.

A decade ago, astronomers discovered that what is true for your car
keys is not true for the galaxies. Having been impelled apart by the
force of the Big Bang, the galaxies, in defiance of cosmic gravity,
are picking up speed on a dash toward eternity. If they were keys,
they would be shooting for the ceiling.

"That is how shocking this was," Dr. Livio said.

It is still shocking. Although cosmologists have adopted a cute
name, dark energy, for whatever is driving this apparently
antigravitational behavior on the part of the universe, nobody
claims to understand why it is happening, or its implications for
the future of the universe and of the life within it, despite
thousands of learned papers, scores of conferences and millions of
dollars' worth of telescope time. It has led some cosmologists to
the verge of abandoning their fondest dream: a theory that can
account for the universe and everything about it in a single breath.

"The discovery of dark energy has greatly changed how we think about
the laws of nature," said Edward Witten, a theorist at the Institute
for Advanced Study in Princeton, N.J.

This fall, NASA and the Department of Energy plan to invite
proposals for a $600 million satellite mission devoted to dark
energy. But some scientists fear that might not be enough. When
astronomers and physicists gathered at the Space Telescope Science
Institute recently to take stock of the revolution, their despair of
getting to the bottom of the dark energy mystery anytime soon, if
ever, was palpable, even as they anticipate a flood of new data from
the sky in coming years. When it came time for one physicist to
discuss new ideas about dark energy, he showed a blank screen.

The institute's director, Matt Mountain, said that dark energy had
given this generation of astronomers a rare opportunity, and he
admonished them to use it wisely.

"We are placing a large bet," Dr. Mountain said, "using our
credibility as collateral, that we as a community know what we are
doing."

But many stressed that it was going to be a long march with no clear
end in sight. Lawrence Krauss of Case Western Reserve University
told them, "In spite of the fact that you are liable to spend the
rest of your lives measuring stuff that won't tell us what we want
to know, you should keep doing it."

Scuffling in the Dark

Through myriad techniques and observations, cosmologists have
recently arrived, after decades of strife, at a robust but dark
consensus regarding a cosmos in which stars and galaxies, as well as
the humans who gawk at them, amount to barely more than a
disputatious froth. It was born 13.7 billion years ago in the Big
Bang. By weight it is 4 percent atoms and 22 percent so-called dark
matter of unknown identity -- perhaps elementary particles that will
be discovered at the Large Hadron Collider starting up outside
Geneva this year. That leaves 74 percent for the weight of whatever
began causing the cosmos to accelerate about five billion years ago.

As far as astronomers can tell, there is no relation between dark
matter, the particles, and dark energy other than the name, but you
never know. Some physicists are even willing to burn down their old
sainted Einstein and revise his theory of gravity, general
relativity, to make the cosmic discrepancies go away. There is in
fact a simple explanation for the dark energy, Dr. Witten pointed
out, one whose tangled history goes all the way back to Einstein,
but it is also the most troubling.

"Dark energy has the somewhat unusual property that it was
embarrassing before it was discovered," he said.

In 1917, Einstein invented a fudge factor known as the cosmological
constant, a sort of cosmic repulsion to balance gravity and keep the
universe in balance. He abandoned his constant when the universe was
discovered to be expanding, but quantum physics resurrected it by
showing that empty space should be foaming with energy that had the
properties of Einstein's constant.

Alas, all attempts to calculate the amount of this energy come up
with an unrealistically huge number, enough energy to blow away the
contents of the cosmos like leaves in a storm before stars or
galaxies could form. Nothing could live there.

Dr. Witten and other physicists used to think this conundrum "would
somehow go away." Something was missing in physicists' understanding
of physics, the logic went. The constant was really zero for deep
reasons that, when revealed, would lead physicists closer to an
understanding of what they call "the vacuum," that is to say, the
structure of reality.

"It seems now that the answer is not really zero," Dr. Witten said.

Requiem for a Dream

Einstein's constant is the most economical explanation for dark
energy, Dr. Witten said. The others, involving new force fields or
tinkering with Einstein's gravity, are hard to make work and raise
more questions than they answer. But if dark energy is the
cosmological constant, it is smaller than predicted by a shocking
factor of 10^60. No fundamental principles can explain why
Einstein's constant, or any physical parameter, could be so small
without being zero, Dr. Witten said. Zero can be a fundamental
number, he said, but not a 1 with 59 zeroes between it and the
decimal point.

As a result, he said, maybe physicists should give up trying to
explain that number and look instead for a theory that generates all
kinds of universes, a so-called multiverse.

That idea has been given mathematical form by string theory, which
portrays the constituents of nature as tiny wriggling strings, an
elegant idea that in principle explains all the forces of nature but
in practice leads to at least 10^500 potential universes.

This maze was an embarrassment for string theory. As Dr. Witten, one
of the leaders of the field, said, "I am tempted to say this was an
embarrassment of my youth."

"Who needs that mess?" he recalled thinking. "There is just one
world we live in."

Now, Dr. Witten allowed, dark energy might have transformed this
fecundity from a vice into a virtue, a way to generate universes
where you can find any cosmological constant you want. We just live
in one where life is possible, just as fish only live in water.

"This interpretation of string theory might be close to the truth,"
Dr. Witten said. But that truth comes at a cost.

"Before the discovery of the dark energy, quantum physicists tended
to assume that the `vacuum' we live in has some deep meaning that
reflects nature's deepest secrets," Dr. Witten said. But if ours is
only one of a zillion in a haystack, there is nothing special about
it, no secret to be found.

It could still turn out that dark energy is some as-yet-undiscovered
"fifth force," say, or the result of not understanding gravity. In
that case, Dr. Witten said, "All the old viewpoints would be
correct," and physicists could go back to dreaming of a final
theory.

"I'd be happy if that happened," he said. "Our reward would be to go
back to where we were, not understanding the cosmological constant."

The notion that there are a zillion universes, whose individual
properties are just a cosmic dice throw, is a story that has been
told before and "raises the blood pressure of many physicists
seriously," as Dr. Livio put it. But the idea has rarely been
mentioned by Dr. Witten, who is seen in the community as a symbol of
the old Einsteinian ideal.

Dr. Witten said he was just doing his duty to explain what dark
energy meant to physics.

"As for how I feel personally, I am not sure what to say," he said
in an e-mail message. "I wasn't terribly enthusiastic the first, or
even second, time I heard the proposal of a multiverse. But none of
us were consulted when the universe was created."

Astronomy of the Invisible

The trouble started in 1998 when two competing teams of astronomers,
one led by Saul Perlmutter of the Lawrence Berkeley National
Laboratory in California and the other by Brian Schmidt of the
Australian National University, discovered that the expansion of the
universe was inexplicably accelerating.

Both teams were using a kind of exploding star known as a Type 1a
supernova as standard candles -- objects whose distance can be
inferred from their apparent brightness and a few other tricks of
the trade -- to investigate the history and fate of the universe.
They found, on the basis of a few dozen of these stars, that the
more distant ones were dimmer than expected, meaning that they had
been carried farther away by the cosmic expansion than expected,
meaning that the universe was speeding up. The car keys were
streaking for the ceiling.

The groups quibble about who saw and said what first, but they have
shared in a cavalcade of awards and prizes, among them the $1
million Shaw Prize in 2006 and the $500,000 Gruber Cosmology Prize,
awarded last fall at Cambridge University in England, where Dr.
Perlmutter and Dr. Schmidt lectured jointly, trading sentences.

Since then myriad collaborations have joined in the hunt for these
exploding stars. In Baltimore, Dr. Perlmutter reported on a new
analysis of "the world's data set," more than 300 supernovas
observed by various groups, which he said would provide the tightest
constraints on the nature of dark energy "for at least the next 15
minutes."

Dr. Perlmutter's results, along with all the others that were
presented over the next four days, were consistent with Einstein's
cosmological constant, plus or minus 10 percent, but with just about
everything else the theorists can throw into the pot, as well.

Nor is there any solid evidence yet that dark energy is or is not
varying with time -- if it is not constant, it cannot be Einstein's
constant. Adam Riess of the Johns Hopkins space telescope institute,
a key member of Dr. Schmidt's team, said, "The biggest thing we
could learn is by ruling that out."

He added, "We have a suspect, but we're not ready to convict anyone
yet."

Dr. Perlmutter said, "The challenge is to make dramatic improvements
in the quality of the data," adding, "The next decade should be a
very fertile time."

Astronomers have developed a smorgasbord of other ways of tracking
the effect of dark energy. They have learned how to map the growth
of clusters of galaxies, by analyzing how their gravity distorts the
light from galaxies far behind them. Gravity makes the clusters
grow; dark energy holds them back.

"We can see dark matter, and in principle even invisible clusters,"
said Henk Hoekstra of the University of Victoria in Canada.

Another technique is to simply count the clusters at different times
in the cosmic past, the way one might count trees to gauge the
growth of a forest. Yet another method is to use sound waves from
the hot, early days of the universe, which have left an imprint on
the distribution of galaxies today -- a 500-million-light-year
"bump" -- as a cosmic yardstick for measuring the universe as it
grew.

Each of these methods has its own strengths and weaknesses, and
experts agree that it will be necessary to marry the results from
many methods to zoom in on the properties of dark energy. They also
agree that the best place to do that is in space.

The Big Bake-Off

Last year a committee from the National Academy of Sciences
recommended that a dark energy observatory be the next mission in an
astrophysics program called Beyond Einstein.

There are now three competitors angling for the job: Dr.
Perlmutter's SNAP, for Supernova Acceleration Probe; Adept, or
Advanced Dark Energy Telescope, led by Charles Bennett of Johns
Hopkins; and Destiny, for Dark Energy Space Telescope, led by Tod
Lauer of the National Optical Astronomy Observatory in Tucson.

Also in the works, just to add spice, is a European mission known as
Euclid, which could fly in 2017, if it is approved by the European
Space Agency. NASA and the Department of Energy, working together,
expect to make a final selection for the dark energy mission --
known colloquially as J-dem for Joint Dark Energy Mission -- next
spring and launch it in the middle of the next decade.

That sounds like progress, but some astronomers, including the
former members of the academy committee itself, have complained that
$600 million is less than half of the $1.2 billion to $1.5 billion
the academy committee estimated was necessary to do the job. In a
recent letter to Michael Salamon, NASA scientist in charge of the
project, 11 of the committee members, including both of its
chairmen, urged NASA to raise the cost cap on the mission, writing,
"Cutting the budget in half would probably make the attainment of
these goals impossible."

NASA's $600 million does not include the cost of launching the
satellite, so the discrepancy is not as big as it looks. But in
Baltimore, Jon Morse, director of astrophysics at NASA headquarters,
warned that if the astronomers wanted to spend a billion dollars,
some other astronomy mission would have to come off the table.

NASA has to live within its means, Dr. Morse said in an interview.

"Otherwise," he said, "Beyond Einstein becomes beyond reality."

A Hole in the Future?

Whatever proposal is eventually selected, the dark energy satellite
will return a tidal wave of data about the universe and its weird
denizens, both visible and invisible. This data is likely to
transform astronomy in unpredictable ways, but there is no guarantee
that it will nail the mystery of dark energy.

Both alternatives to the constant -- some weird energy field in
space, or a modification to Einstein's theory of gravity -- could
vary wildly over the course of history. But Paul Steinhardt, a
theorist from Princeton University, argued that they would tend to
mimic the cosmological constant so closely that the different models
cannot be distinguished within the projected error limits, of a few
percent.. He called this blur of ignorance "the J-dem hole." The
specter of the J-dem hole dominated a panel discussion later in the
week devoted to the question, "How well do we have to do?"

The answer, said Dr. Krauss of Case Western, was "better than you
will be able to do."

The only real job, he said, is to distinguish dark energy from the
cosmological constant. "If we don't answer that question, we won't
have learned a thing," Dr. Krauss said.

He compared the present situation with the development of quantum
mechanics, the paradoxical sounding rules that govern inside the
atom, which overturned science in the 1920s.

That revolution, he pointed out, stemmed from theorists' inability
to explain the so-called black body radiation emitted from a hot
glowing object. The solution did not come from more and more precise
measurements of the black body spectrum, but rather from the heads
of people like Niels Bohr and Werner Heisenberg, who envisioned new
ways that atoms could work and weird new laws of nature.

"We really need new theory, and we have none," Dr. Krauss said.

In the meantime, astronomers could get lucky. Despite Dr.
Steinhardt's analysis, measurements of dark energy's strength could
converge on a value not quite the same as Einstein's constant. Or it
could turn out that it has changed over cosmic time and is not
constant. Einstein and Dr. Witten would be off the hook.

Michael Turner, a University of Chicago cosmologist who coined the
term "dark energy," said you could measure the health of a field by
the big questions it takes on, and addressing Dr. Morse of NASA, who
was moderating the discussion, as well as his colleagues, he said,
"You have a job, to go knock on everyone's door and say this is the
opportunity of a lifetime."

Dr. Krauss said, "It would be crazy to talk ourselves out of this."

He added: "You have to do what you can. You would be crazy not to
look."