May 19, 2004
By X-Raying Galaxies, Researchers Offer
New Evidence of Rapidly Expanding Universe
By DENNIS OVERBYE
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, they said.
"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 Astronomical
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 cosmology.''
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
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 speeding up.
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. Fabian said.
In recent years theorists have filled the
journals with ever more fanciful explanations of what might be causing this
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
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
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 supernova
"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 acceleration.
"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."
CNN.com - Astronomers: 'Dark' galaxy discovered - Feb 24,
By Robert Roy Britt, SPACE.com
(SPACE.com) -- Astronomers have discovered an invisible galaxy that could be the
first of many that will help unravel one of the universe's greatest mysteries.
The object appears to be made mostly of "dark matter," material of an unknown
nature that can't be seen.
Theorists have long said most of the universe is made of dark matter. Its
presence is required to explain the extra gravitational force that is observed
to hold regular galaxies together and that also binds large clusters of
Theorists also believe knots of dark matter were integral to the formation of
the first stars and galaxies. In the early universe, dark matter condensed like
water droplets on a spider web, the thinking goes. Regular matter -- mostly
hydrogen gas -- was gravitationally attracted to a dark matter knot, and when
the density became great enough, a star would form, marking the birth of a
The theory suggests that pockets of pure dark matter ought to remain sprinkled
across the cosmos. In 2001, a team led by Neil Trentham of the University of
Cambridge predicted the presence of entire dark galaxies.
The newfound dark galaxy was detected with radio telescopes. Similar objects
could be very common or very rare, said Robert Minchin of Cardiff University in
"If they are the missing dark matter halos predicted by galaxy formation
simulations but not found in optical surveys, then there could be more dark
galaxies than ordinary ones," Minchin told SPACE.com.
In a cluster of galaxies known as Virgo, some 50 million light-years away,
Minchin and colleagues looked for radio-wavelength radiation coming from
hydrogen gas. They found a well of it that contains a hundred million times the
mass of the Sun. It is now named VIRGOHI21.
The well of material rotates too quickly to be explained by the observed amount
of gas. Something else must serve as gravitational glue.
"From the speed it is spinning, we realized that VIRGOHI21 was a thousand times
more massive than could be accounted for by the observed hydrogen atoms alone,"
Minchin said. "If it were an ordinary galaxy, then it should be quite bright and
would be visible with a good amateur telescope."
The ratio of dark matter to regular matter is at least 500-to-1, which is higher
than I would expect in an ordinary galaxy," Minchin said. "However, it is very
hard to know what to expect with such a unique object -- it may be that high
ratios like this are necessary to keep the gas from collapsing to form stars."
Long road to discovery
Other potential dark galaxies have been found previously, but closer
observations revealed stars in the mix. Intense visible-light observations
reveal no stars in VIRGOHI21.
The invisible galaxy is thought to lack stars because its density is not high
enough to trigger star birth, the astronomers said.
The discovery was made in 2000 with the University of Manchester's Lovell
Telescope, and the astronomers have worked since then to verify the work. It was
"The universe has all sorts of secrets still to reveal to us, but this shows
that we are beginning to understand how to look at it in the right way," said
astronomer Jon Davies of Cardiff University in the UK. It's a really exciting
discovery." Additional radio observations were made with the Arecibo Observatory
in Puerto Rico. Follow-up optical work was done with the Isaac Newton Telescope
in La Palma. Astronomers from the UK, France, Italy and Australia contributed to
the research. The project is now searching for other possible dark galaxies.
Dark matter makes up about 23 percent of the universe's mass-energy budget.
Normal matter, the stuff of stars, planets and people, contributes just 4
percent. The rest of the universe is driven by an even more mysterious thing
called dark energy.
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Week of Feb. 26, 2005; Vol. 167, No. 9
Ghostly Galaxy: Massive, dark cloud intrigues scientists
It looks like an empty patch of space, but astronomers say it holds a galaxy
that contains no stars. If Robert Minchin of Cardiff
University in Wales and his colleagues are right, they have found the first
member of a population of galaxies that theorists have proposed but observers
had never seen.
In 2000, Minchin's team noticed two apparently isolated hydrogen clouds in a
radio telescope survey of the Virgo Cluster of galaxies. Follow-up observations
with visible-light telescopes showed that one of these clouds was associated
with a faintly glowing galaxy. However, long exposures taken with the 2.5-meter,
visible-light Isaac Newton Telescope in the Canary Islands offered up a
surprise: The second cloud had no partner glowing galaxy.
"It's a very intriguing object," comments galaxy researcher Richard S. Ellis of
the California Institute of Technology in Pasadena. "It's puzzling how this ball
of hydrogen hasn't got any stars in it."
It could be that several smaller knots of gas fall along the same telescopic
line of sight and are masquerading as a single, much bigger cloud, Ellis
cautions. Gravitational tugs-of-war between galaxies frequently pull small
clouds of hydrogen out of galaxies. But Minchin's team says that, in the case of
VIRGOHI21, as their object is called, there are no suitable galaxies nearby to
have donated the gas.
Assuming that the hydrogen is contained in one big cloud, its motion suggests
that it's a small part of a massive object weighing as much as a galaxy of 100
billion suns. And yet this object remains invisible.
"Seeing a dark galaxy -- a galaxy without any stars -- is like seeing a city
without any people," says Minchin. "We want to know why nobody lives there."
Ordinary galaxies seem to be made of about 10 percent ordinary matter -- the
kind that forms stars that shine -- and 90 percent dark matter, an invisible
substance whose nature still eludes astronomers. In an upcoming Astrophysical
Journal Letters, Minchin's team reports that VIRGOHI21 has an ordinary-to-dark
matter ratio of about 1 to 1,000.
Computer simulations of galaxy formation suggest that there should be many more
small galaxies in the universe than observations indicate. Some theorists have
suggested the missing galaxies elude observation because they're rife with dark
matter yet all but devoid of ordinary star-forming matter.
VIRGOHI21 just might be one of these elusive bastions of dark matter, says
Gregory Bothun of the University of Oregon in Eugene.
If there are any ordinary-matter stars in VIRGOHI21, they're few and extremely
faint. Minchin's group has requested use of the Hubble Space Telescope, which
may be just powerful enough to detect individual stars in the object. The
researchers plan to get higher-resolution radio observations with the Very Large
Array radio telescopes near Socorro, N.M., to settle the question of whether the
object is a single entity.
If you have a comment on this article that you would like considered for
publication in Science News, send it to firstname.lastname@example.org. Please include
your name and location.
Minchin, R., et al. In press. A dark hydrogen cloud in the Virgo cluster.
Astrophysical Journal Letters. Preprint available at
Davies, J., R. Minchin, et al. 2004. A multibeam HI survey of the Virgo
cluster—two isolate d HI clouds? Monthly Notices of the Royal Astronomical
Society 349(April):922-932. Abstract available at
O'Neil, K., G. Bothun, et al. 2004. A new HI catalog of Low Surface Brightness
galaxies out to z=0.1: Tripling the number of massive LSB galaxies known.
Astronomy & Astrophysics 428(December):823-835. Abstract available at
http://dx.doi.org/10.1051/0004-6361:20047091 . Preprint available at
Additional information about the discovery of a "dark" galaxy can be found at
University of Oregon
Eugene, OR 97402
Richard S. Ellis
California Institute of Technology
Pasadena, CA 91125
School of Physics and Astronomy
Cardiff CF24 3YB
From Science News, Vol. 167, No. 9, Feb. 26, 2005, p. 131.
Copyright (c) 2005 Science Service. All rights reserved.