May 19, 2004 By X-Raying Galaxies, Researchers Offer New Evidence of Rapidly Expanding Universe By DENNIS OVERBYE   bservations 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 Society. 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 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 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 behavior. 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 supernova surveys. "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, 2005
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 galaxies.

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 galaxy.

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 the UK.

"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 announced today.

"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.


Links referenced within this article

SPACE.com
http://space.com/


Find this article at:
http://www.cnn.com/2005/TECH/space/02/24/dark.galaxy

Science News Online

Week of Feb. 26, 2005; Vol. 167, No. 9
Ghostly Galaxy: Massive, dark cloud intrigues scientists

David Shiga

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 editors@sciencenews.org. Please include your name and location.


References:

Minchin, R., et al. In press. A dark hydrogen cloud in the Virgo cluster. Astrophysical Journal Letters. Preprint available at http://arxiv.org/abs/astro-ph/0502312 .

Further Readings:

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 http://dx.doi.org/10.1111/j.1365-2966.2004.07568.x .

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 http://arxiv.org/abs/astro-ph/0406505 .

Additional information about the discovery of a "dark" galaxy can be found at http://www.astro.cardiff.ac.uk/groups/galaxies/press.html .

Sources:

Gregory Bothun
Physics Department
University of Oregon
Eugene, OR 97402

Richard S. Ellis
105-24 Astronomy
California Institute of Technology
Pasadena, CA 91125

Robert Minchin
School of Physics and Astronomy
Cardiff University
Cardiff CF24 3YB
United Kingdom


http://www.sciencenews.org/articles/20050226/fob1.asp

From Science News, Vol. 167, No. 9, Feb. 26, 2005, p. 131.

Copyright (c) 2005 Science Service. All rights reserved.