This is How Many Photons Are in the Entire Observable Universe

Researchers from Clemson University used almost a decade of gamma ray data to map the entire observable universe for the very first time.

Nov 29 2018, 7:28pm

We now have a head count of every single photon that’s ever existed in the observable universe, according to new research from Clemson University published in Science today.

The net total? 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. That’s four sextillion photons.

In counting photons, the researchers hoped to learn more about how and when stars formed in the earliest days of the universe. According to the study, there are “systematic uncertainties” in converting observable light into the mass of stars in galaxies. To get a handle on this, the researchers looked to blazars—galaxies with powerful black holes at their centres which blast high-energy gamma ray radiation towards Earth. The way this energy propagates through space depends on the total amount of light emitted by all galaxies, according to the study.

“It’s like following the rainbow and discovering a bucket of gold,” Vaidehi Paliya, one of the study’s authors, told Motherboard in an email. In this case, blazars are the rainbow and all of the light in the observable universe is the pot of gold.

According to the study, star formation peaked about three billion years ago. This finding is similar to previous estimates, but provides valuable confirmation.

The data analysis took place over ten months, but the data itself had been accumulating for almost a decade. The researchers used gamma ray data from the Fermi telescope, developed by General Dynamics and launched in 2008. The telescope continuously measures gamma rays from about 350 miles above Earth’s surface, enabling it to chart the entire sky every three hours.

Paliya told Motherboard that for him, it was fascinating to explore the dynamics of photons emitted within the first billion years after the Big Bang, when many galaxies were zealously creating new stars. However, Paliya said there’s still much more to be discovered about that era in our universe’s history.

“[The first billion years after the Big Bang] is an epoch we can’t really probe yet (we need the James Webb Space Telescope for that),” Paliya said, referring to the Hubble Space Telescope successor scheduled to launch in 2021. “So our measurement set the benchmark for the science of future space missions like [the James Webb Space Telescope].”