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Counting all the photons in the universe (spoiler alert: it’s a big number)

3 Dec 2018, 00:34 UTC
Counting all the photons in the universe (spoiler alert: it’s a big number)
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The Hubble Ultra Deep Field shows some 10,000 galaxies, some of them dating back to less than a billion years after the Big Bang. New research using the Fermi Gamma-ray Space Telescope allowed astronomers to estimate the total amount of starlight generated across the history of the observable universe. Image: NASA, ESA, and S. Beckwith (STScI) and the HUDF Team
Astronomers studying the effects of gamma ray jets emitted by 739 super-massive black hole-powered blazars shining across the depths of space and time have calculated the total amount of starlight generated over 90 percent of the history of the known universe.
Fans of “The Hitchhiker’s Guide to the Galaxy” will no doubt be disappointed the answer is not 42. It’s 4 X 1084. That’s 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 emitted by a trillion trillion stars over most of the lifetime of the cosmos.
Despite the stupendously large number, the light reaching Earth at any given moment, not counting contributions from the Sun and the Milky Way, is remarkably dim – equivalent to a 60-watt light bulb viewed from a distance of about 4 kilometres (2.5 miles).
“From data collected by the Fermi telescope, we were able to measure the entire amount of starlight ...

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