Since the first stars first started flickering about 100 million years after the Big Bang our universe has produced roughly one trillion trillion stars, each pumping starlight out into the cosmos. That’s a mind-boggling amount of energy, but for scientists at the Fermi Large Area Telescope Collaboration it presented a challenge. Hannah Devlin at The Guardian reports that the astronomers and astrophysicists took on the monumental task of calculating how much starlight has been emitted since the universe began 13.7 billion years ago.
So, how much starlight is there? According to the paper in the journal Science, 4×10^84 photons worth of starlight have been produced in our universe, or 4,000,000,000,000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000,000,000,000,000,000,000,000,000,000 photons.
To get to that stupendously ginormous number, the team analyzed a decades worth of data from the Fermi Gamma-ray Space Telescope, a NASA project that collects data on star formation. The team looked specifically at data from the extragalactic background light (EBL) a cosmic fog permeating the universe where 90 percent of the ultraviolet, infrared and visible radiation emitted from stars ends up. The team examined 739 blazars, a type of galaxy with a supermassive black hole in its center that shoots out streams of gamma-ray photos directly toward Earth at nearly the speed of light. The objects are so bright, even extremely distant blazars can be seen from Earth. These photons from the shiny galaxies collide with the EBL, which absorbs some of the photons, leaving an imprint the researchers can study.
Looking at blazars ranging in age from 2 million to 11.6 billion years old allowed the researchers to use the sensitive instruments on the Fermi telescope to analyze their light, measuring how much radiation it lost as it moved through the EBL. This let them create an accurate measure of the density or thickness of the EBL over time, essentially creating a history of starlight in the universe since, in deep space, distance and time are almost the same thing.
“By using blazars at different distances from us, we measured the total starlight at different time periods,” co-author Vaidehi Paliya of Clemson University says in a press release. “We measured the total starlight of each epoch – one billion years ago, two billion years ago, six billion years ago, etc. – all the way back to when stars were first formed. This allowed us to reconstruct the EBL and determine the star-formation history of the universe in a more effective manner than had been achieved before.”
Researchers have tried to measure the EBL in the past, but were unable to get past the localized dust and starlight close to Earth, making it almost impossible to collect good data on the EBL. The Fermi telescope, however, finally allowed the team to minimize that interference by looking at gamma rays. The data they collected is in line with previous estimates for the density of the EBL.
The study shows that the peak of star formation in the universe took place about 11 billion years ago. Over time, it has slowed drastically, but stars are still forming, with about seven new stars lighting up in the Milky Way every year alone.
The study was not just an exercise in smashing the zero key either. Ryan F. Mandelbaum at Gizmodo reports that the measurement gives scientist an upper limit to the number of galaxies that were floating around 12 billion years ago during the Epoch of Reionization, the period when dark matter, hydrogen and helium first coalesced into stars and ordinary matter. It’s also possible that the EBL measurement could help develop new ways to look for unknown particle types.
Clemson astrophysicist and lead author Marco Ajello says in the release that the study is also good step toward understanding the universe’s earliest days.
“The first billion years of our universe’s history are a very interesting epoch that has not yet been probed by current satellites,” he says. “Our measurement allows us to peek inside it. Perhaps one day we will find a way to look all the way back to the big bang. This is our ultimate goal.”