We finally know how bright the universe is. Astrophysicists measured all of the lights.

Pedro_P

A black hole shoots gamma rays out into the universe.
ESO/M. Kornmesser
▼ The Earth glows faintly with the bustle of humanity. From far away you can’t pick out individual homes, or even cities, but by tracking the collective photons that our spotlights and streetlights throw out over time, you might be able to get a rough sense of the rise of technological civilization—and you might notice if all of the lights started going out.
The same story applies to the universe at large.
Stars are the ultimate light bulbs, and while some of their rays dead-end into dust-particles, others get away intact. Space has a reputation for being cold and dark, but out in the comparatively empty void between galaxies, these escaped particles of light collectively produce a diffuse glimmer everywhere. This glow tells you what’s out there without the hassle of counting all the stars and galaxies one by one. Borrowing tools from particle physics, an international team of astrophysicists has carried out the most accurate and sweeping measurement yet of this light, the collective shining of all the universe’s stars.
Their results, published Thursday  in Science, tell an epic tale covering most of the universe’s history, and even poke at the veil of the cosmos’s first billion years—an epoch invisible to traditional astronomers. “I would have never believed that a measurement of this kind would be possible,” says Marco Ajello, an astrophysicist at Clemson University and the team’s leader.
To find the Extragalactic Background Light, as those in the know call it, you can’t just point your telescope at a patch of black sky and count photons: you’ll have no way of telling local sunbeams from truly external rays of light. Rather, the team took advantage of hundreds of cosmic accidents.
Huge black holes lie in the center of most galaxies, and some of the most monstrous let unimaginably violent jets of gamma rays rip into space—an area smaller than our solar system slinging out as much energy as our whole galaxy. When these jets happen to be pointed straight at Earth, astronomers call them blazars. They are some of nature’s most powerful particle accelerators, and NASA’s Fermi  Space Telescopeis one of humanity’s best gamma-ray detectors.
“Thanks to Fermi and our work, we can combine two different fields, high energy physics and classical astronomy,” says Alberto Domínguez, an astronomer at the Computense University of Madrid in Spain and coauthor. (▪ ▪ ▪)

► Please, read the full note/article here: Source