The puzzle of dark energy

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 institutes 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 wont 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 Einsteins 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

Einsteins constant is the most economical explanation for dark
energy, Dr. Witten said. The others, involving new force fields or
tinkering with Einsteins 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 1060. No fundamental principles can explain why Einsteins
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 10500 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 natures 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.

Id be happy if that happened, he said. Our reward would be to goback 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 wasnt 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 worlds 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. Perlmutters results, along with all the others that were
presented over the next four days, were consistent with Einsteins
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 Einsteins
constant. Adam Riess of the Johns Hopkins space telescope institute, a
key member of Dr. Schmidts team, said, The biggest thing we could
learn is by ruling that out.

He added, We have a suspect, but were 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. Perlmutters
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.

NASAs $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 Einsteins 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 dont answer that question, we wont
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. Steinhardts
analysis, measurements of dark energys strength could converge on a
value not quite the same as Einsteins 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 everyones 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.

date: Tue, 3 Jun 2008 02:42:09 -0700 (PDT) author: Lance