How Astronomers Discovered the Universe's Hidden Light

Galaxies in every corner of the universe have been sending out photons, or light particles, since nearly the beginning of time. Astronomers are now beginning to read this extragalactic background light

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Why is the night sky dark? After all, if the universe is filled with billions of galaxies, every one of them swirling with billions of stars that have been emitting photons of light for billions of years, why would the universe not be awash with light? German astronomer Wilhelm Olbers pondered that question in the 1820s, and the riddle became known as Olbers's paradox. By then, astronomers and philosophers had wondered for centuries why the sky was dark and what the darkness implied about the nature of the universe. It turns out that these scholars were on to something truly profound.

More light is out there than we can easily see. Even from deep space, far away from the lights of Earth and the stars of the Milky Way, the sky of intergalactic space is absolutely black. It glows with what is called the extragalactic background light (EBL). The EBL consists of all the photons of light radiated by all the stars and galaxies that have ever existed, at all wavelengths from the ultraviolet through the far infrared, during all of cosmic history to the present. The EBL from distant galaxies is faint because extragalactic space is vast compared with the number of galaxies that glow (or have ever done so). Because the universe is expanding, the photons emitted by galaxies over the history of the cosmos have spread throughout the cavernous volume of space and become dilute. And because of the expansion, light from distant galaxies undergoes a “redshift”—wavelength increases, pushing the light toward the red side of the electromagnetic spectrum, outside the visible realm.

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