This is the extrapolated amount of light that has ever been emitted in the universe.

As you know, every star you’ve ever seen at night is just like the sun — a very huge broiling furnace of fusing hydrogen atoms to produce electromagnetic energy thousands of lightyears away. Understanding the nature of stars, that is, their formation, luminosity, and collapse, is really crucial. But what about all that light they produced, how do we know that amount of luminosity? A team of scientists from Clemson University did just that, and what they discovered is really mind-blowing.


Related media: Stars: Crash Course Astronomy #26


Twinkle Twinkle Little Stars

The first stars ever formed were roughly a few hundred million years old. The universe is approximately 13.8 billion years old today, and stellar population is now well over the trillion-trillions. Scientists know that stars formation didn’t just trickle randomly through time; instead, the rate of stellar formation has shifted in our universe’s history. The team of scientists sought to precisely measure all the light in the universe to figure out more about when and how those shifts happened.

For this study, the scientists used the Fermi Space Telescope of the National Aeronautics and Space Administration (NASA), instead of observing visible light just by gazing up upon the sky. (Good luck on that anyway). Fermi’s design is to measure gamma rays — the highest form of light on the electromagnetic (EM) spectrum — invisible to the human eye.

That’s just nature. The most prolific source of gamma rays are so ridiculously powerful; these are powered by the supermassive black holes at the centers of galaxies. As these cosmic beasts go about their gravitational thug-of-war game, all that matter pulled over gets heated up and thus produces light which then emitted. Sometimes, not always, these black holes end up blasting a ridiculously bright jet stream of gamma radiation out of each end, this is only detected when that beam of light is aimed at us. This is what scientists term as a blazar.


Oh! I Know How Bright You Are

As you read this article, right now, there are hundreds of black holes emitting gamma rays directly towards Earth, every second. Yikes! NASA’s Fermi telescope does an impressive job of detecting all of them — only if its direction is towards Earth. The telescope keeps track of possible interactions between gamma rays, such as weaker glows known as extragalactic background light (EBL). This is a sort of ancient cosmic fog that formed from the first starlight in the early universe and has since absorbed photons (particles of light) from newer stars and all other sources of light throughout the course of the universe.

Even when these sources of light run out of fuel, their photons keep on traveling through the void of space like a ghost ship with a long lost crew. Knowing when and where these photons formed could provide clues about the history of stellar formation in the universe. But here’s the catch: when gamma rays interacts with the EBL fog, their high-energy photons shatter into pairs of electrons and positrons — which dims the gamma ray so slightly that the Fermi telescope could then measure. These measurements are what scientists used to extrapolate the amount of light ever produced in any particular period in time.

“By using blazars at different distances from us, we measured the total starlight at different time periods,” Vaidehi Paliya, a post doctoral researcher at Clemson University’s physics and astronomy department, and the study’s co-author, said in a statement. “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.”


How Bright Is The Entire Universe?

Image: NASA / Hubble Space Telescope / ESA

(Cue the drumroll!)

The number of photons emitted in the history of the universe is 4×1084. That’s 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 — a 4 with 84 zeroes (you didn’t read it all, did you?). That’s too ridiculous for a number to even comprehend. In comparison, the Sun emits roughly 3×1052 photons per year, whereas the average 60-watt light bulb emits roughly the range of 2×1020 photons every second. Mind-blown?

You’re now wondering, with all that light, why is the night still dark for all purpose? Good thought! As a matter of fact, that thought once gave rise to a quandary that puzzled some great thinkers in the 19th century. This is the idea known as Olbers’ Paradox: it states that in an infinite universe with infinite stars, every modicum of empty space should be filled by a star out there somewhere — yet the night sky isn’t blazing with light, and ever since we’ve figured out why. 

The first obvious thought: The universe isn’t infinite, and neither is the number of stars. The second: The universe is so old, and vast expanding at an unprecedented rate that the visible light from many stars cannot reach us, or hasn’t yet. Maybe they never will. As galaxies drift apart from each other faster and further, a term known as cosmic inflation, their lights are redshifted on the end of the EM spectrum, and eventually into infrared wavelengths, which is totally invisible to the human eye.


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Written by: Nana Kwadwo, Tue, Jun 04, 2019.

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