Published online 28 October 2009 | Nature | doi:10.1038/news.2009.1043

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Observations suggest the early Universe is ripe for exploration.

The GRB 090423 γ-ray burst happened just 630 million year after the Big Bang. NASA/Swift/Stefan Immler

Two teams have spied a huge blast from the far reaches of our early Universe. Such γ-ray bursts occur when certain massive stars violently explode. The latest burst happened a mere 630 million years after the Big Bang (that's 13.1 billion years ago) and is the youngest such blast to have been spotted — the previous record-beater happened 825 million years after the Big Bang.

NASA's space-based Swift telescope spotted the burst, named GRB 090423, on 23 April this year. "Gamma-ray bursts are rather rare," says Nial Tanvir of the University of Leicester, UK, who led one of the efforts to characterize the burst. The Swift telescope spots around 100 a year. "It's hard work to pin them down," he says.

The telescope automatically sent the news back to Earth. "The spacecraft sends us a text," says Tanvir. Once they receive the call from Swift, astronomers must quickly decide whether to follow up the burst with observations from ground-based telescopes.

“We're now starting to approach the time when we think the very first galaxies turned on.” Nial Tanvir

University of Leicester

Tanvir's team used the United Kingdom Infrared Telescope (UKIRT) and the Gemini North 8-metre telescope, both on Hawaii, to track the burst from about 20 minutes after it was first seen. Strong winds, which can damage the telescope if it is in use, made it too dangerous to use UKIRT for long. "The weather was pretty bad that night," says Tanvir. They then waited until it was night in Chile, where the same team remotely used the Very Large Telescope in the Atacama desert to follow the burst's afterglow.

Meanwhile, a team led by Ruben Salvaterra at the National Institute for Astrophysics in Merate, Italy used the Telescopio Nazionale Galileo on La Palma in the Canary Islands to watch the burst.

Burst of excitement

Both teams measured the spectrum of the light being received from the γ-ray burst, and both saw the same thing: light below a certain wavelength was missing. "We saw the light was visible only down to one micrometre; below that there was no light," says Salvaterra. This cut-off is caused by absorption of the light by hydrogen along the line of sight between the object and Earth, suggesting light from the burst had travelled a long way.

The teams were able to use that observation to calculate the light's 'redshift' — a measure of how far the light has travelled. Light stretches out over the time it takes to travel to Earth because the Universe is expanding. The stretching makes the light appear in the redder end of the electromagnetic spectrum — the bigger the redshift, the farther away the object is.

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Light from GRB 090423 had a redshift of 8.2. A redshift of around 8 suggests that the light came from the Universe when it was nine times smaller than it is today. Before this detection, the farthest γ-ray burst seen was at a redshift of 6.7. The two latest studies are published in Nature1,2.

Not only is the burst a record-breaker, but the work also shows that astronomers can effectively probe the early Universe from the ground, says Salvaterra. "We knew by our models that these kinds of objects should exist," he says. "Being one of the people that actually detected it is quite amazing."

Others in the community are impressed. "These are spectacular discoveries, and open up unprecedented new windows on the early Universe," says Edison Liang, an astrophysicist at Rice University in Houston, Texas.

"We're now starting to approach the time when we think the very first galaxies turned on," Tanvir adds.