AFTER billions of years of runaway expansion, is the universe starting to slow down? A new analysis of nearby supernovae suggests space might not be expanding as quickly as it once was, a tantalising hint that the source of dark energy may be more exotic than we thought.
For more than a decade, astrophysicists have grappled with evidence of a baffling force that seems to be pushing the universe apart at an ever-increasing rate. Exactly what constitutes the dark energy responsible for this cosmic speed-up is unknown, says Michael Turner at the University of Chicago. “The simplest question we can ask is ‘does the dark energy change with time?'”
So far, the evidence has suggested that dark energy is constant, though its effect on the universe has become stronger as the universe has expanded and the gravitational force between objects weakens with distance.
Now an analysis of supernovae suggests dark energy may actually be on the wane. In a paper on the physics preprint website, a team led by Arman Shafieloo at the University of Oxford examined a newly released catalogue of supernova explosions, including a number of relatively recent blasts nearby (www.arxiv.org/abs/0903.5141). They found that the new data made the best fit with a universe in which dark energy is losing strength. “It seems acceleration is slowing down,” says Shafieloo.
The first evidence of dark energy emerged in 1998, when two teams of astronomers spotted distant supernova explosions that appeared dimmer than expected, and so further away. The find suggested the exploding stars were receding from Earth faster than anticipated, and therefore so was the rest of the universe. “Dark energy” was invoked to explain the apparent anomaly. Since then more supernovae have been catalogued to help build up a picture of how the universe has expanded over time…
… a potential bias could have been introduced thanks to dimmer objects being easier to see if they are nearby. It is possible that the Harvard team happened to catalogue a disproportionate number of nearby supernovae that were faint or obscured by dust. Astronomers must correct for the dimming effect of dust and other subtleties in order to estimate a supernova’s true peak brightness. But the team may have overcompensated in this correction, producing a catalogue of nearby supernovae that are slightly too bright for their distance. That would create the illusion that the universe’s acceleration has been slowing.