Missing gamma rays were abducted by light from the first stars

Diagram of the process by which high-energy gamma rays were annihilated by ultraviolet light from the earliest stars.

NASA/GSFC

The first stars in the Universe were evidently cosmic hooligans, knocking electrons off atoms, making much of the hydrogen gas in interstellar space ionized. This reionization happened between the formation of the first atoms (about 380,000 years after the Big Bang) and some point roughly a billion years later. However, observations of this era are challenging, so the precise conditions of reionization are not well understood.

Now a study using the Fermi Gamma-ray Space Telescope has used the light emanating from supermassive black holes known as blazars to measure the diffuse light produced by reionization. When high energy gamma rays interacted with the ultraviolet photons produced by early stars, they were converted to particle/antiparticle pairs, and this creates a dropoff at a specific point in the blazar spectrum. This absorption was evident in a sample of 150 blazars, and the data can help constrain models of the very first stars in the Universe.

The first stable atoms formed about 380,000 years after the Big Bang, in an event called recombination. This primarily means that electrons joined with protons to form hydrogen. Subsequently, many of those atoms were broken apart again in an event cosmologists refer to as reionization.

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