Glowing testimony. XMM-Newton observations of a cluster of galaxies (center) at billions of light-years distance.

A Shadow Over Dark Energy

The mysterious dark energy, believed to accelerate the expansion of the universe, may not exist, according to an international team of astronomers; instead, the universe may be mainly composed of ordinary matter. Recent x-ray observations carried out by the European XMM-Newton Observatory are in conflict with the increasingly popular model of a universe dominated by dark energy, the team says. But other astronomers aren't swayed.

Several studies this year have convinced many astronomers that the ordinary matter we all know--the kind that creates gravity--makes up only a few percent of the universe, while almost three-quarters consists of a strange dark energy that pushes galaxies ever farther apart (ScienceNOW, 18 December).

To check those figures, the researchers took a close look at massive clusters of galaxies. With so little matter around to pull them together, they reasoned, such clusters would have stopped growing quite early in the universe's 14-billion-year history, and new ones would have stopped forming. In a high-matter-density universe, however, massive clusters would continue to form and grow over time. So by counting galaxy clusters at various epochs (corresponding with various distances, because distant objects are seen as they were long ago) and calculating how massive they are, astronomers can tell where ordinary matter really ranks in the cosmic recipe. "Galaxy clusters are very important cosmological tools," says Neta Bahcall of Princeton University, "provided you know their masses."

But estimating cluster masses turns out to be hard. X-ray satellites observe the glow of the hot gas inside clusters, but the relation between the observed x-ray luminosity of galaxy clusters and their derived total mass may have been different in the past than it is now. Recently, a team led by David Lumb of the European Space Research and Technology Centre (ESTEC) in Noordwijk, the Netherlands, used XMM-Newton to quantify this evolutionary effect. Using these data, Sebastien Vauclair and Alain Blanchard of the Observatory Midi-Pyrénées in France and their colleagues carried out a new analysis of galaxy cluster number counts and conclude that they are consistent with a high-density universe. That means there's little room for dark energy.

Bahcall lauds the new detailed observations of distant galaxy clusters, but, she says, "it's not straightforward to draw conclusions. It's always difficult to convert x-ray luminosity into mass." What is needed, says Bahcall, is a complete sample of distant clusters for which masses are known exactly, for instance through gravitational-lens observations. Hans Böhringer of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, agrees that it's too early to draw conclusions. "Better work published soon will show that there is not yet a major conflict" between the XMM data and the concordance model, he says.

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