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Fermi measures light content of universe

By Mo-Yu Zhou | November 15, 2012

Even after stars cease to shine, their light continues to make its way throughout the universe. Astronomers have been able to make the most accurate measurement of extragalactic background light (EBL), the aggregate starlight in the cosmos, to date. They used data from NASA’s Fermi Gamma-ray Space Telescope, whose primary mission was to measure the EBL of our universe.

The research was led by Marco Ajello, a postdoctoral researcher Kavli Institute for Particle Astrophysics and Cosmology at Stanford University in California and the Space Sciences Laboratory at the University of California at Berkeley, and was recently published in Science Express.

Since its launch in 2008, Fermi’s Large Area Telescope (LAT) has observed and collected data from the sky for every three hours. It uses gamma rays, the most energetic form of light, to create the most detailed map of the universe ever made. Gamma rays can be used to explore the fossil radiation field of optical and ultraviolet light from stars that no longer shine. In other words, it detects the light that continues to travel through the universe. Thus, the EBL serves as a cosmic ‘fog’ of sorts to these gamma rays.

The ‘fog’ works as follows: Every once in a while, starlight collides with a gamma ray, causing it to transform into a pair of two particles, an electron and a positron, which is the antimatter counterpart of the electron. When this happens, the gamma ray disappears. Just as fog dims the light from a lighthouse, this cosmic ‘fog’ dampens the gamma ray signal.

To study the EBL, Ajello and his team examined gamma rays from 150 blazars, which are galaxies that are powered by black holes. Supermassive black holes can cause some matter that enters to accelerate outward in jets pointed in different directions at the speed of light. Some of these jets may appear to be especially bright since they happen to be aimed in the direction of Earth. When this happens, the galaxy is classified as a blazar.

The blazars examined in this study were detected at energies more than a billion times the energy of visible light, or over 3 billion electron volts (GeV). Fermi has detected over a thousand blazars since its launch, but it is rare to find blazars at such high energies. Thus, the researchers needed to use as much as four years’ worth of data from Fermi.

On their way to Earth – a journey of billions of light-years – the gamma rays produced from blazar jets pass through a cosmic ‘fog’ of visible and ultraviolet light from stars that exist, or once existed, in the history of the universe.

Scientists know how many gamma rays ought to be emitted given different energies from studying nearby blazars. Due to absorption by the cosmic ‘fog,’ more distant blazars show fewer gamma rays, especially at energies higher than 25 GeV.

In fact, most of the higher-energy gamma rays are missing from the farthest blazars.

The researchers determined the average reduction in gamma-rays across three distance ranges between 9.6 billion years ago and today. In so doing, they could measure the thickness of the cosmic ‘fog.’

In the cosmos, the average density of stars is about 1.4 stars per 100 billion cubic light-years, indicating an average distance of 4,150 light-years between stars.

These results make the launch of NASA’s James Webb Space Telescope even more exciting. While Fermi’s results hint at what the first stars were like, Webb will be powerful enough to detect them, telling us even more about the earliest phases of star formation in the universe.

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