9 Billion-Year-Old 'Dark Energy' Reported
By DENNIS OVERBYE

A strange thing happened to the universe five billion years ago. As if
God had turned on an antigravity machine, the expansion of the cosmos
speeded up, and galaxies began moving away from one another at an ever
faster pace.

Now a group of astronomers using the Hubble Space Telescope have
discovered that billions of years before this mysterious antigravity
overcame cosmic gravity and sent the galaxies scooting apart like muscle
cars departing a tollbooth, it was already present in space, affecting
the evolution of the cosmos.
"We see it doing its thing, starting to fight against ordinary gravity,"
Adam Riess of the Space Telescope Science Institute said about the
antigravity force, known as dark energy. He is the leader of a team of
"dark energy prospectors," as he calls them, who peered back nine
billion years with the Hubble and were able to discern the nascent
effects of antigravity. The group reported their observations at a news
conference yesterday and in a paper to be published in The Astrophysical
Journal.

The results, Dr. Riess and others said, provide clues and place new
limits on the nature of dark energy, a mystery that has thrown physics
and cosmology into turmoil over the last decade.
"It gives us the ability to look at changes in dark energy," he said in
an interview. "Previously, we knew nothing about that. That's really
exciting."
The data suggest that, in fact, dark energy has changed little, if at
all, over the course of cosmic history. Though hardly conclusive, that
finding lends more support to what has become the conventional theory,
that the source of cosmic antigravity is the cosmological constant, a
sort of fudge factor that Einstein inserted into his cosmological
equations in 1917 to represent a cosmic repulsion embedded in space.

Although Einstein later abandoned the cosmological constant, calling it
a blunder, it would not go away. It is the one theorized form of dark
energy that does not change with time.
Sean Carroll, a cosmologist at the California Institute of Technology
who was not on the team, said: "Had they found the evolution was not
constant, that would have been an incredibly earthshaking discovery.
They looked where no one had been able to look before."

The paper by Dr. Riess and his colleagues represents a sort of progress
report from the dark side, where astrophysicists have found themselves
more and more as they try to understand what is happening to the
universe.
This encounter with the invisible began eight years ago, when two
competing teams of astronomers were using exploding stars known as Type
1a supernovas as cosmic distance markers to determine the fate of the
universe.

Ever since the Big Bang 14 billion years ago, the galaxies and the rest
of the universe have been flying apart like a handful of pebbles tossed
in the air. Astronomers reasoned that gravity would be slowing the
expansion, and the teams were trying to find out by how much and, thus,
determine whether all would collapse one day into a "big crunch" or
expand forever.
Instead, to their surprise, the two teams, one led by Saul Perlmutter of
the University of California, Berkeley, and the other by Brian Schmidt
of the Mount Stromlo and Siding Spring Observatories in Australia, found
that the universe was speeding up instead of slowing down.

But the ground-based telescopes that the two teams used could track
supernovas to distances of just seven billion light-years, corresponding
to half the age of the universe, and the effect could have been mimicked
by dust or a slight change in the nature of the supernova explosions.
Since then, Dr. Riess, who was a member of Dr. Schmidt's team, and his
colleagues have used the Hubble telescope to prospect for supernovas and
dark energy farther out in space or back in time.

The new results are based on observations of 23 supernovas that are more
than eight billion years in the past, before dark energy came to
dominate the cosmos. The spectra of those distant supernovas, Dr. Riess
reported, appear to be identical to those closer and more recent
examples. By combining the supernova results with data from other
experiments like the NASA Wilkinson Microwave Anisotropy Probe, Dr.
Riess and his colleagues could begin to address the evolution of dark
energy.

"That's one of the $64,000 questions," he said. "Is dark energy
changing?"
So far, he said, the results are consistent with the cosmological
constant, but other answers are also possible. The possibility that it
is the cosmological constant is a mixed blessing. Physicists concede
that they do not understand it.

Dr. Carroll of Caltech said, "Dark energy makes us nervous."
Einstein invented his constant to explain why the universe does not
collapse. After he abandoned it, the theory was resuscitated by quantum
mechanics, which showed that empty space should be bubbling with
staggering amounts of repulsive energy. The possibility that it really
exists in the tiny amounts measured by the astronomers has flummoxed
physicists and string theorists.

Because it is a property of empty space, the overall force of Einstein's
constant grows in proportion as the universe expands, until it
overwhelms everything. Other theories of dark energy like strange force
fields called quintessence or modifications to Einstein's theory of
gravity can change in more complicated ways, rising, falling or
reversing effects.

Astronomers characterize the versions of dark energy by their so-called
equation of state, the ratio of pressure to density, denoted by the
letter w. For the cosmological constant, w is minus one.
Dr. Riess and his group used their data to make the first crude
measurement of this quantity as it stood nine billion years ago. The
answer, he said, was minus one ? the magic number ? plus or minus
about 50 percent. By comparison for more recent times, with many more
supernovas observable and thus more data, the value is minus one with an
uncertainty of about 10 percent.

"If at one point in history it's not minus one," Dr. Riess said, "then
we have killed the very best explanation."
Getting to the precision needed to kill or confirm Einstein's constant,
however, will be very difficult, he conceded. One of the biggest sources
of uncertainty is the fact that the Type 1a explosions are not
completely uniform, introducing scatter into the observations.

The Hubble is the sole telescope that can pursue supernova explosions
deeply enough to chart the early days of dark energy. The recent
announcement that the National Aeronautics and Space Administration will
send astronauts to maintain and refurbish the Hubble once again,
enabling it to keep performing well into the next decade, is a lift for
Dr. Riess's project.

A new camera could extend observations to 11 billion or 12 billion years
back.