NASA/ESA/STScI
Most of the history of the universe is a history of light – of stars blinking on and shining throughout the cosmos. Now, we have used that light, which pervades all of observable space and builds up over time, to create a timeline of stars and galaxies starting billions of years ago.
The light of all the stars and galaxies is called the extragalactic background light. Among its properties is its ability to interfere with the high-energy gamma rays propagating through space. Some of the photons from the gamma rays smash into the photons from the background light and do not make it to our telescopes.
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So, by comparing the properties of the gamma ray sources with the number of high-energy gamma rays they actually manage to send to us, it’s possible to measure how dense the background light is, and thus the rate of formation of the stars that create it.
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An international team of researchers using the Fermi Gamma-ray Space Telescope has done just this. To get a comprehensive picture, the team looked at 740 cosmic objects that emit high-energy gamma rays. The range of distances from Earth to these objects means that they have reconstructed the history of star formation over 90 per cent of cosmic time.
They found that star formation peaked 10 billion years ago, less than 4 billion years after the big bang. “We kind of missed the party and it’s been declining ever since,” says team member Kári Helgason at the University of Iceland.
This is consistent with results from other surveys which measure the light coming from individual galaxies rather than all of them at once. “A galaxy survey always misses some faint galaxies,” says Helgason. “There was the possibility that we could have discovered a new mystery of missing light, but as it turned out we confirmed the picture.”
The next step is to observe more sources of high-energy gamma rays at higher distances, says team member Abhishek Desai at Clemson University in South Carolina. These observations hinted that there might be fewer galaxies in the early universe than some models suggest, but we cannot be sure until we have higher-quality data on extremely distant gamma ray bursts.
Journal reference: Science, DOI: 10.1126/science.aat8123
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