Our local star is very bright by human standards – just looking at it for a few seconds can destroy 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 for that. With new methods of measurement, the researchers have counted almost every photon ever emitted in the universe. The final count is 4 x 10⁸⁴ or 4, followed by 84 zeros. In short, it's a lot of light.
Measuring the brightness of the universe is not as easy as pointing a telescope up and counting photons. The light from local sources interferes with any attempt to record the so-called extragalactic background light. If you filter out all the light of the earth, the sun and other stars in the Milky Way, the sky is only as bright as a 60W bulb at a distance of 2.5 miles.
The team around the astrophysicist Marco Ajello of Clemson University turned instead to a phenomenon called Blazars. These objects are a particular class of quasars, active galactic cores with black holes that spew out particle streams. The only difference between a regular quasar and a blazar is that it is focused on the earth. It turns out that we have a window into the light emissions in far corners of the universe.
Staring at these huge galactic particle accelerators, the team managed to gather data about the local environment around the energy stream. The researchers spent nine years studying Fermi Space Telescope data, which detects gamma-ray emissions from distant sources such as blazars. The signal from blazars falls off as it plows through light on its way to Earth. Therefore, we can measure the brightness along the signal path by knowing only where the blazar is and its apparent intensity on Earth.
According to the study, a collection of 739 blazars scanned by Fermi delivers virtually all the photons in the universe. The team managed to shed light on 90 percent of the history of the universe. The data does not include light that strikes solid particles of dust or gas that are released in the infrared as heat. That's about half the energy in the backlight, but the team compensated for that in the model.
Knowing the brightness of the universe is more than an interesting factoid. These data could help scientists understand star formation and evolution, and how the universe is in its present state. Additional research could even fill in the oldest 10 percent of the universe's light map.
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