By X-Raying Galaxies, Researchers Offer New Evidence of Rapidly
May 19, 2004
By DENNIS OVERBYE
Observations of giant clouds of galaxies far out in space
and time have revealed new evidence that some mysterious
force began to push the cosmos apart six billion years ago,
astronomers said yesterday.
The results constitute striking confirmation of one of the
weirdest discoveries of modern science: that the expansion
of the universe seems to be accelerating, the galaxies
flying apart faster and faster with time, under the
influence of some antigravitational force. The work,
astronomers said, opens up a powerful new way of
investigating the nature of this "dark energy" and its
effect on the destiny of the cosmos.
The astronomers used an orbiting X-ray satellite called
Chandra to observe hot gases in the distant galactic
clusters. By analyzing the X-rays emitted by those gases,
they could calculate the distance from Earth and the speed
of each of the clusters and thus trace the history of the
expansion of the universe over the last 10 billion years,
"The universe is accelerating," said Dr. Steve Allen of
Cambridge University in England, leader of the
international team that did the work. "We have found strong
new evidence for dark energy."
They announced their results at a news conference at NASA
headquarters in Washington. A paper describing the work has
been submitted to the journal Monthly Notices of the Royal
Other astronomers hailed the X-ray cluster method as a
potential complement to other ways of investigating dark
energy but said they would withhold judgment about this
particular calculation until they could study the details.
Most of the previous studies, including those that led to
the discovery of dark energy, used exploding stars known as
Type 1a supernovas as cosmic distance markers.
Dr. Adam Riess of the Space Telescope Science Institute in
Baltimore, an original discoverer of dark energy, hailed
the work as another sign of the new age of "precision
Dr. Riess said in an e-mail message: "Cosmologists are all
from Missouri, the Show-Me State. It appears that X-ray
clusters have been added as a new tool in our surveyor's
tool kit. All tests point to a strange form of gravity we
call dark energy. Some love it, some hate it; it appears we
have to deal with it."
Dr. Martin Rees, a cosmologist at Cambridge who was not
part of the team, called the results "neat work and a
promising method," which, he noted, involved "very
straightforward assumptions and simple physics."
Another cosmologist who was not part of the team, Dr.
Michael Turner of the University of Chicago, said: "We can
now be quite confident that the expansion of the universe
is speeding up. It's not a fluke, it's not going away."
Dark energy has confounded experts and everybody else since
two competing groups of astronomers discovered six years
ago that the expansion of the universe was not slowing down
due to cosmic gravity, as had been presumed, but was
At the news conference, Dr. Andrew Fabian of Cambridge, a
team member, compared the phenomenon to tossing an apple in
the air and watching it go up faster and faster rather than
falling back down. "It requires new physics beyond everyday
experience, even the experience of an astronomer," Dr.
In recent years theorists have filled the journals with
ever more fanciful explanations of what might be causing
One possibility, first suggested and then rejected by
Einstein, is that space itself has a repulsive force. But
according to modern particle physics theory, this
cosmological constant, as this force is known, should be
about 1060 times bigger than what astronomers have
measured, causing theorists seek other explanations. Among
them have been extra unseen dimensions to space,
interactions with other, parallel universes and
as-yet-undiscovered particles or forces.
Or perhaps, some theorists say, Einstein's theory of
gravity, general relativity, which has been the backbone of
cosmology for nearly a century, needs modification.
Astronomers hope that some answers will come if they can
find out whether the density of dark energy - estimated to
make up 75 percent of the universe - is changing with time.
If dark energy were constant, it would mean that Einstein's
cosmological constant is in effect, and that most of the
galaxies would move away too fast to be seen a mere 100
billion years from now.
If dark energy is increasing, it could mean the universe
could end in a "big rip," in which even atoms would be torn
apart. On the other hand, the dark energy could decrease
and even turn into an attractive force, drawing the
universe to an end in a "big crunch."
The new results are consistent with Einstein's cosmological
constant but also allow for the possibility that the dark
energy could be changing, echoing recent results from the
"The nice thing is that this is a completely independent
method based on very simple physics," Dr. Allen said. "It's
the physics of hot gas and the physics of gravity."
Clusters of galaxies are the largest objects in the
universe, containing thousands of galaxies and trillions of
stars. But in a big cluster, the stars themselves are
greatly outweighed by intergalactic gas, which has been
condensed and heated to 100 million degrees or so by the
cluster's immense gravity.
The X-rays that are spit out by this gas can be seen far
across the universe. From their brightness astronomers can
gauge the amount of gas in the cluster, and from the
temperature of the gas, they can estimate the total mass in
the galaxy cluster. Most of that mass is mysterious dark
matter, which has been detected only by its gravitational
effects on the luminous parts of the universe.
The astronomers made what they said was the simple
assumption that clusters were a fair sample of the universe
as a whole and that the cosmic ratio of dark matter to
ordinary matter applied in each individual cluster. That
allowed them to calculate distances to 26 clusters, from 1
billion to 10 billion light-years away, and thus measure
how fast the universe was expanding when the light left
those far-away galaxy clouds, confirming the cosmic
"It's nice our results agree with previous experiments,"
Dr. Allen said. "It lets you feel rather more secure that
everything is as it should be in those experiments."