Cosmic Push: X-ray study confirms universe’s dark side

Culling clues from X rays emitted by distant clusters of galaxies, astronomers report new evidence that some mysterious force overcame gravity’s tug about 6 billion years ago and ever since has been pushing galaxies apart at an accelerating rate. The results add to previous evidence for one of the strangest properties of the cosmos: Cosmic expansion is speeding up, rather than slowing down, in response to the mutual gravity of all material in the universe.

DARK FUTURE. If the density of dark energy is constant, cosmic expansion will accelerate forever but galaxies will remain intact (red curve). If the density increases, galaxies will be torn apart in the Big Rip (blue curve). If the density decreases, the universe may recollapse in a Big Crunch (yellow curve). Inset: X-ray image of hot gas bathing a galaxy cluster. Allen, et al., CXC, NASA; (Inset) M. Weiss, CXC, NASA

This bizarre state of affairs came to light in 1998, when scientists found that some distant exploded stars are dimmer, and therefore farther way, than expected. That was an indication that some cosmic push, referred to as dark energy, has been revving up the expansion of the universe (SN: 5/22/04, p. 330: Dark Doings).

The new X-ray study of galactic clusters provides an independent method of detecting that cosmic push, say Andy Fabian and Steve Allen of the University of Cambridge in England and their colleagues. At a press briefing last week in Washington, D.C., they announced their findings, which are based on observations by NASA’s orbiting Chandra X-ray Observatory. The researchers also describe their work in an upcoming Monthly Notices of the Royal Astronomical Society.

“This is the first time that clusters have been [successfully] used to measure . . . dark energy,” comments X-ray astronomer J. Patrick Henry of the University of Hawaii in Honolulu.

X rays emanate from hot gas that bathes each galaxy cluster. To look for signs of dark energy, the Cambridge astronomers and their colleagues examined the brightness and energy spectra of X-ray emissions from 26 clusters to calculate the exact distances to the clusters, known to lie between 1 billion and 8 billion light-years from Earth.

The team traced a history of cosmic expansion by combining the new distance measurements with other astronomers’ observations of how fast the clusters are receding from Earth.

The results support the standard view that gravity put the brake on expansion of the universe in the first several billion years of cosmic history. They’re also in agreement with recent supernova studies showing that some 6 billion years ago, an unknown entity stepped on the cosmic accelerator. Furthermore, both the supernova and the new galaxy-cluster studies suggest that the density of dark energy doesn’t vary over time.

“It is always encouraging when independent experiments yield similar results,” says Louis-Gregory Strolger of the Space Telescope Science Institute in Baltimore, an astronomer who participated in the recent supernova studies.

If the density of dark energy is indeed constant, the universe will continue to expand faster and faster. Eventually, every galaxy will become so distant that it will be out of sight of all the others.

Allen notes that the findings don’t entirely rule out a dark-energy density that increases with time. In that case, cosmic expansion would ultimately speed up so rapidly that every galaxy and atom in the universe would be torn asunder in a Big Rip.

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