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Scientists Count How Many Photons The Universe Has Produced Since The Big Bang

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An artist's sketch of the Fermi Gamma-Ray Space Telescope in orbit outside of Earth. Scientists used data from the Fermi Gamma-Ray Space Telescope to indirectly measure photons that have been produced in the observable universe since its formation.   ( NASA )

Scientists from the Clemson University in South Carolina have successfully measured all the photons that have been produced throughout the history of the observable universe.

It was no easy task. Astronomers believe that the universe was formed about 13.7 billion years ago and since then, many stars were born and died.

However, Marco Ajello and his team devised a method using data from NASA's Fermi Gamma-Ray Space Telescope, a spacecraft launched in 2008 that studies gamma rays — the most energetic form of radiation. They used a decade worth of data from the space telescope.

Their findings were published in the journal Science.

All The Stars In The Universe

The Fermi Gamma-Ray Telescope, which celebrated its 10th year in space in June, provided data on the interaction of gamma rays with the extragalactic background light or EBL, known as the "cosmic fog." The researchers said that the EBL is a good area to explore for this experiment because it records both stellar activity and the evolution of the galaxy. Other scientists also use EBL to understand how the universe evolved.

However, measuring EBL has always been a challenge because it is so much dimmer than the Milky Way. The brighter light in the foreground drowned out light emitted from far away galaxies, making the task almost impossible.

The solution devised by the team is to indirectly measure stellar activity by looking at blazars, a distant galaxy with a supermassive black hole at its center.

"By using blazars at different distances from us, we measured the total starlight at different time periods," said Vaidehi Paliya, a postdoctoral fellow and a co-author of the study.

Total Photons Produced Since The Big Bang

The team arrived at the conclusion that there have been a total of 4×10^84 photons produced in the history of the observable universe. That is 4,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,
000,000,000,000,000,000,000,000,000,000 photons.

The researchers explained that the approach has allowed them to peek into the first 1 billion years of the universe — a period that has never been studied by current satellites. They hope that one day, the method they created can also be used to probe into the early years of the universe and perhaps all the way to the Big Bang.

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