Fermi telescope: scientists measured the light produced in the observable Universe
We know how much light has been emitted by stars in the observable Universe: an extremely large number, and long to write, that is 4 followed by 84 zeros.
The estimate – made by an international group of researchers – was obtained by analysing data collected in over 9 years by the Fermi Large Area Telescope: NASA’s gamma-ray space telescope. Italy made a primary contribution to the mission through the Italian Space Agency (ASI), the National Institute for Nuclear Physics (INFN), and the National Institute for Astrophysics (INAF).
"Thanks to data collected by the Fermi telescope, we were able to measure the entire amount of starlight ever emitted”, said Marco Ajello, from Clemson University and lead author of the study. “Stars create most of the light we see and synthesize most of the heavy elements of the Universe, such as silicon and iron. Understanding the Universe in which we live depends largely on our understanding of how stars evolved”.
One of the main goals of the Fermi mission, which this year has celebrated its tenth anniversary, was precisely to calculate the amount of light emitted by all galaxies over the history of the Universe, known as extragalactic background light (EBL).
This background light – a type of cosmic “fog” composed of ultraviolet, visible and infrared light – makes gamma radiation opaque, that is, it attenuates it, thus leaving an observable imprint.
“In general, we use gamma radiation detected by our space telescopes to study celestial objects. In this case, however, we quantify the absence of gamma radiation to measure the light that pervades the Universe”, commented Patrizia Caraveo, responsible for INAF for the exploitation of the Fermi LAT data. “It is fascinating to see what can be derived from the evidence of an absence”.
In particular, the researchers measured this attenuation using 739 active galaxies and one gamma-ray burst detected by the Fermi Large Area Telescope. This allowed them to reconstruct the evolution of the EBL, determining the star formation history of the Universe for over 90% of its evolution.
“Gamma ray photons travelling through a fog of starlight produced by the EBL have a large probability of being absorbed”, added Ajello. “By measuring how many photons have been absorbed, we were able to measure how thick the fog was and also measure, as a function of time, how much light there was in the entire range of wavelengths”.
The result is an independent confirmation of previous estimates from other missions, such as Hubble. This is an important signal that we are on the right track.