Imagine the Universe!

Chandra Opens New Line Of Investigation On Dark Energy

26 May 2004

Astronomers have detected the mysterious dark energy by applying a powerful, new method that uses images of galaxy clusters made by NASA's Chandra X-ray Observatory. The results trace the transition of the expansion of the universe from a decelerating phase to an accelerating phase that occured several billion years ago.

"Dark energy is perhaps the biggest mystery in physics," said study leader Steve Allen of the Institute of Astronomy (IoA) University of Cambridge, England. "As such, it is extremely important to make an independent test of its existence and properties," he said.

NASA Chandra X-ray Observatory/Optical Composite of Abell 2029 This optical (blue) and NASA's Chandra X-ray (red-orange) composite image shows Abell 2029, a cluster of galaxies. A large elliptical galaxy is visible in the center of the image, surrounded by smaller galaxies. The red diffuse emission shows hot intergalactic gas, heated to about 100 million degrees by the enormous gravity in the cluster, and visible only in X-rays.

(Optical: NOAO/Kitt Peak/J.Uson, D.Dale; X-ray: NASA/CXC/IoA/S.Allen et al.)

Allen and his colleagues used Chandra to study 26 clusters of galaxies which lie between one and eight billion light years away from Earth. These data span the time when the universe initially slowed from its original expansion, but then expanded rapidly because of the repulsive effect of dark energy.

"We're directly seeing that the expansion of the universe is accelerating by measuring the distances to these galaxy clusters," said IoA scientist and study co-author Andy Fabian. "The new Chandra results suggest the dark energy density does not change quickly with time and may even be constant, consistent with the "cosmological constant" concept first introduced by Albert Einstein," he said. If so, the Universe is expected to continue expanding forever, so that in billions of years only a tiny fraction of the known galaxies will be visible.

If the dark energy is constant, more dramatic fates for the universe would be avoided. These include the "Big Rip," where dark energy increases until galaxies, stars, planets and, finally, even atoms are torn apart, and the "Big Crunch," where the universe eventually collapses on itself.

Chandra's probe of dark energy uses X-ray observations to detect and study the hot gas in galaxy clusters. From these data, the ratio of the mass of the hot gas to the mass of the dark matter in a cluster can be determined. The observed values of the gas fraction depend on the assumed distance to the cluster. This distance in turn depends on the curvature of space and the amount of dark energy in the universe.

Since galaxy clusters are so large, they are thought to represent a fair sample of the matter content of the universe. If so, the relative amounts of hot gas and dark matter should be the same for every cluster. Using this assumption, Allen and colleagues adjusted the distance scale to determine which one fit the data best. They derived distances that show the expansion of the universe was first decelerating, but began to accelerate about six billion years ago.

animation of the effects of
dark energy on the expansion of the universe
This animation (1.5 MB) shows the expansion history of the Universe. The Big Bang is immediately followed by rapid expansion of the Universe. This expansion then slows down because of the gravitational attraction of the matter in the Universe. The expansion speeds up again because of the effect of dark energy.

(Credit: NASA/STScI/G. Bacon)
(Description)

Chandra's observations agree with observations of distant supernovae, including those from NASA's Hubble Space Telescope (HST), which first showed dark energy's effect on the acceleration of the universe. Chandra's results are completely independent of the supernova technique. This independent verification helps to dispel any remaining doubts that the supernova technique might be flawed.

"Our Chandra method has nothing to do with other techniques, so we're definitely not comparing notes, so to speak," said Robert Schmidt of the University of Potsdam, Germany, another co-author of the study. Better limits on the amount of dark energy, and how it varies with time, are obtained by combining the X-ray results with data from NASA's Wilkinson Microwave Anisotropy Probe (WMAP). WMAP used observations of the cosmic microwave background radiation to discover evidence for dark energy in the very early universe. Using the combined data, Allen and his colleagues found dark energy makes up about 75% of the universe, dark matter about 21%, and ordinary matter about 4%.

Energy Distribution of the Universe
The new Chandra observations include estimates of the total energy content of the Universe. As shown in this illustration, dark energy is estimated to contribute about 75% of the energy in the Universe, dark matter about 21% and normal matter about 4%. Only the normal matter can be directly detected with telescopes, and about 85% of this is hot, intergalactic gas, as detected in Chandra observations of galaxy clusters.

(Credit: NASA/CXC/M.Weiss)

Allen and his colleagues stress that the uncertainties in the measurements are such that the data are consistent with dark energy having a constant value. The present Chandra data do, however, allow for the possibility that the dark energy density is increasing with time. More detailed studies with Chandra, HST, WMAP and future X- ray missions like Constellation-X, should provide much more precise constraints on dark energy.

"Until we better understand cosmic acceleration and the nature of the dark energy, we cannot hope to understand the destiny of the universe," said Michael Turner, assistant director for mathematical and physics sciences, National Science Foundation, Arlington, Va.

The research team also included Harald Ebeling of the University of Hawaii and the late Leon van Speybroeck of the Harvard-Smithsonian Center for Astrophysics. These results appear in an upcoming issue of the Monthly Notices of the Royal Astronomy Society.

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