Extragalactic background light: Scientists measure the universe's first starlight

The universe was just a kid at 4 billion years old. Thursday, scientists said that they have a measurement for all of the light that was around at that time that's still traveling to us.

It's called the extragalactic background light. This includes light from stars that existed when the universe was even younger than 4 billion years old. Researchers report in the journal Science that this can help with understanding how stars formed and how galaxies evolved.

"I think it's amazing to be able to probe our universe when it was so young, when the very first stars formed," said Marco Ajello, researcher at Stanford University and study co-author.

Researchers write in the study that there have been several attempts in the past to detect this phenomenon, but none were successful.

The finding is important for estimating the number of smaller, fainter galaxies that current telescopes cannot detect, said Claude-Andre Faucher-Giguere, researcher in the Department of Astronomy at the University of California, Berkeley, who was not involved in the study.

Here's why: Every telescope has limitations, especially its size. So astronomers can use them to detect the big, luminous galaxies, but there are more galaxies that the tools will miss.

"Studying the extragalactic background light allows us to overcome this limitation, because the background light is the sum of the light produced by all galaxies, including the ones that are too faint to be detected individually by traditional methods," he said.

How they did it:

To study this, scientists focused their efforts at high-energy gamma rays using NASA's Fermi Gamma-ray Space Telescope. Specifically, they looked at "blazars," which are galactic nuclei that spew jets associated with supermassive black holes.

When this extragalactic background light absorbs gamma rays, the process produces electron-positron pairs. A positron is an anti-matter particle.

This is the inverse reaction from what's described in Dan Brown's novel "Angels and Demons," explains Faucher-Giguere. In that book, the villains' bomb would harness the extraordinary energy from matter and anti-matter annihilating each other.

Based on how many gamma rays are expected to be present, compared to how many were observed, scientists calculated the number of gamma rays that appeared to be absorbed by the starlight from the early universe.

In that sense, these gamma-ray sources are like "lighthouses" and the starlight is like the fog, Ajello said. Scientists know that starlight is absorbing the gamma rays when the "lighthouse" is dimmer.

Scientists can therefore add up the light from the galaxies they can detect, and compare that to the extragalactic background light. This subtraction is a clue to how many galaxies we haven't yet directly detected with our telescopes, and how luminous they are, Faucher-Giguere said.

According to this study, the galaxies observed directly via telescope accounts for most of the extragalactic background light measured. That means there cannot be much more light coming from fainter galaxies, Faucher-Giguere explains. This also puts limits on how many black holes and massive stars were in the early universe.

"This is a new and unique constraint that all future models of galaxy and black hole evolution will have to satisfy," Faucher-Giguere said.

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