All the electromagnetic energy ever in the universe is carried by particles called photons, and they are all collectively called the Extra-galactic Background Light (EBL). These include energy in the visible, radio, ultraviolet, etc. wavelengths. They are mostly produced by stars and galaxies.

Knowing their distribution throughout the observable universe would give us important clues about how mass is distributed and how galaxies have formed and evolved. One other energy distribution being studied is the cosmic microwave background radiation born out of the Big Bang, but it is more useful for determining the shape of the early universe rather than the more recent, which EBL represents well.

Since Earth is inside a bright Solar System and a bright galaxy, it is extremely difficult to distinguish and detect the faint EBL. Current instruments have failed in doing so in a reliable way, both from ground and space. Furthermore, it is impossible to send an instrument so far way as to separately detect the EBL.

Astronomers from the University of California, Riverside, have come up with an ingenious workaround. Their work was published in The Astrophysical Journal on May 24.

Novel approach

Instead of directly measuring the EBL, they have measured how much light from other sources is blocked by the EBL. Specifically, they measure how effective a curtain EBL is to light of specific wavelengths coming from gamma-radiation-spewing supermassive black holes called blazars.

They have found that the EBL is fainter than expected. “What we also found from our work is that most of the EBL we see today is coming from galaxies we have already detected,” Alberto Dominguez, a postdoctoral researcher in the Department of Physics and Astronomy at the university, and lead author of the paper, told this Correspondent via email.

This renders other potential sources of EBL less important.

For their analysis, Dominguez and other co-authors relied on an event from particle physics. There is an effect called pair-production where two photons that satisfy certain energy conditions interact, producing an electron and a positron.

This interaction happens between gamma-ray and EBL photons, producing an aberration in the gamma-ray flux coming from blazars. The strength and location of these aberrations are then measured to receive clues about the presence and sources of EBL.

Since scientists are currently grappling with different ways to find out how much gamma-rays blazars actually emit, Dominguez’s analysis is not final. At the moment, he uses “data taken from X-ray satellites and the Fermi Space Telescope,” which detect photons of a lower energy not distorted by the EBL.

The astronomers chose blazars because they consistently emit radiation at or over 30 giga electron-volts (GeV), which the EBL effectively blocks.

Further fine-tuning the nature of this interaction, as well as going farther than the 5 billion light-years he already has, will be the next stages of Dominguez's work.

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