Scientists Measure the Total Brightness of the Universe: 4 x 10⁸⁴ Photons

Our local star is very bright by human standards—just looking at it for a few seconds can wreck your vision. That’s just an infinitesimal fraction of the total brightness of the universe. How bright is it? Well, we finally have an answer to that. Using new measurement techniques, researchers have counted almost all the photons ever emitted in the universe. The final tally is 4 x 10⁸⁴, or a four followed by 84 zeros. In short: it’s a lot of light.

Measuring the brightness of the universe isn’t as simple as pointing a telescope upward and counting photons. The light from local sources will interfere with any attempt to recording the so-called Extragalactic Background Light. If you filtered out all the light from Earth, the sun, and other stars in the Milky Way, the sky would only be as bright as a 60W light bulb as soon from 2.5 miles away.

The team led by Clemson University astrophysicist Marco Ajello instead turned to a phenomenon called blazars. These objects are a specific class of quasars, active galactic nuclei with black holes spewing out streams of particles. The only difference between a regular quasar and a blazar is that the latter is pointed at Earth. It turns out, that gives us a window into light emissions in far away corners of the universe.

By staring into these giant galactic particle accelerators, the team managed to glean data about the local environment around the stream of energy. Researchers examined nine years of data from the Fermi Space Telescope, which records gamma-ray emissions from distant sources like blazars. The signal from blazars drops off as it plows through light on its way to Earth. Therefore, we can measure the brightness along the signal’s path just by knowing where the blazar is and its apparent intensity at Earth.

According to the study, a collection of 739 blazars scanned by Fermi gives us a picture of virtually all the photons in the universe. The team managed to tabulate light for 90 percent of the universe’s history. The data doesn’t include light that hit solid particles of dust or gas, which would be re-emitted as heat in infrared. That’s roughly half of the energy in background light, but the team compensated for that in the model.

Knowing the brightness of the universe is more than an interesting factoid. This data could help scientists understand star formation and evolution, as well as how the universe came to be in its current state. Additional research may even be able to fill in the oldest 10 percent of the universe’s light map.

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