Science & Technology

Scientists Measured All the Light Ever Produced in the Universe

Almost every point of light you see in the night sky is a star — a huge, broiling sun smashing atoms together to produce electromagnetic energy many light-years away. If we're going to understand more about what makes those stars form, shine, and die, we need to start by measuring them. Recently, scientists from Clemson University made one particularly awe-inspiring measurement: they figured out how much starlight has ever been produced in the universe. And it's a very, very big number.

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When You Wish Upon a Blazar

Scientists believe the first stars formed when our universe was only a few hundred million years old. Today, the universe is about 13.7 billion years old, and that handful of stars now numbers in the trillion-trillion. But scientists know that those stars didn't just form in a steady trickle through time. Instead, the rate of star formation has shifted in our universe's history. By precisely measuring the light in the universe, these researchers aimed to figure out more about when and how those shifts happened.

To do that, they used NASA's Fermi space telescope. Rather than observing visible light like your garden-variety backyard telescope, Fermi is designed to measure gamma rays, which are invisible to the human eye. But they pack a punch: Gamma rays are the most energetic kind of light on the electromagnetic spectrum.

It's only natural, then, that the most prolific sources of gamma rays are also incredibly powerful: the supermassive black holes in the centers of galaxies. As these spinning cosmic kitchen drains pull matter into their gravitational pull, that matter heats up and emits light. Sometimes, those black holes end up shooting incredibly bright jets of gamma radiation out of each end, and if a jet is pointed in our direction, it's what scientists call a blazar.

There are hundreds of these galactic death beams shooting gamma rays directly at Earth as we speak — and the Fermi telescope is ready to catch 'em all. Specifically, the telescope keeps watch for interactions that gamma rays make with another, much weaker glow known as extragalactic background light (EBL). This is an ancient cosmic fog that formed from the first starlight in the universe and has since absorbed photons, or particles of light, here and there from new stars and other light sources throughout the universe's history. Even after those light sources have been snuffed out, their photons keep traveling through the void like a ghost ship with a long-dead crew. Learning where and when these photons formed gives us priceless information about the history of star formation in the universe.

When gamma rays meet the fog of the EBL, their high-energy photons shatter into pairs of electrons and positrons, dimming the gamma-ray jet slightly in a telltale shift that Fermi can measure. Those measurements let scientists track changes in the fog's composition, which helps them estimate the amount of light produced in a particular period of the universe's history.

Four Octagintillion Bottles of Beer on the Wall

The scientists performed this analysis on all 739 blazars across the sky. "By using blazars at different distances from us, we measured the total starlight at different time periods," said Vaidehi Paliya, a co-author on the study and a postdoc in Clemson University's physics and astronomy department, 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."

So what did they find? The number of photons emitted in the history of the universe is 4x1084. 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.

As this is an inconceivably large number, here are some other numbers for comparison. Our sun emits about 3x1052 photons per year, and a 60-watt light bulb emits something in the range of 2x1020 photons every second.

With all that light, doesn't it seem strange that the night sky is so dark? In fact, that hits on a conundrum that once puzzled a great many scientists in the 19th century. The idea is known as Olbers' Paradox, and it says 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!

Since then, we've figured out why. One is obvious: The universe isn't infinite, and neither is the number of stars. The second reason is that our universe is so old and vast and expanding at such a rate that the visible light from many stars hasn't had a chance to reach us yet. Some of it never will: As the furthest galaxies race away from us faster and faster, their light shifts to the red end of the spectrum and eventually into infrared wavelengths, which we can't see with the naked eye. Photons, photons everywhere, and only some to see.

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Go back before the first starlight in Simon Singh's "Big Bang: The Origin of the Universe." We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Ashley Hamer December 28, 2018

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