Clocking galaxy clusters to gauge dark energy

The universe's oldest light and its largest objects could provide a new way to study dark energy, the mysterious entity believed to be pushing the universe apart at an ever-faster rate.

The discovery of the universe's accelerating expansion earned three physicists a Nobel prize last year, but no one knows its source. "What's causing the acceleration?" asks Rachel Bean of Cornell University in Ithaca, New York. "We really don't have a clue."

The leading explanation holds that there is a set amount of repulsive energy, represented by the cosmological constant, in every spot in space ? as the cosmos expands, more space is created, leading to ever-faster expansion. But other models suggest that dark energy may vary or that gravity itself behaves strangely over vast distances. Measuring dark energy's strength over time could provide some answers.

To do so, researchers need to determine the expansion rate of the universe at different cosmic eras by looking further into space. That's because light travels at a finite speed, so looking further away means looking back further in time. So far, astronomers have used techniques such as measuring the distances from Earth to supernovae.

Galaxy clusters

Now Nick Hand at the University of California, Berkeley, and colleagues are the first to use the motion of galaxy clusters to gauge cosmic expansion. Because gravity pulls objects together at known rates and dark energy pushes them apart, studying whether two neighbouring clusters fall together as expected sheds light on dark energy's strength.

First the researchers overlaid a map of 7500 galaxy clusters spotted by the Sloan Digital Sky Survey on top of a map of the universe's oldest light, called the cosmic microwave background radiation, made with the Atacama Cosmology Telescope (ACT) in Chile.

When photons of this ancient light, which permeates all of space, pass through a galaxy cluster, they get squashed or stretched depending on whether the cluster is moving towards or away from Earth, in the same way an ambulance siren sounds higher in pitch as it approaches and then lowers as it zooms past.

Next the team measured the difference in how squashed or stretched the light was in pairs of clusters, allowing them to gauge the clusters' relative motions along the line of sight to Earth.

Dark energy changes

There is not yet enough data to determine if dark energy changes in time, but the measurement shows the technique could test this in the future, says team member Arthur Kosowsky of the University of Pittsburgh in Pennsylvania. "This is the first step on a road to using velocities as a way to measure the development of structure in the universe," he says.

Team member David Spergel at Princeton University agrees. "We can't say anything profound about dark energy yet. But the fact that we now have this new tool means we're in a better position to do something interesting," he says.

Fortunately more information will be available soon. The ACT is getting a new detector in May, and the Sloan survey continues to spot more clusters. "In a year or two we'll be able to redo this analysis with much higher sensitivity," Spergel says. "That's all we need to do now: wait for the data to get better."

Journal reference: arxiv.org/abs/1203.4219

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