American Scientist

To the Editors:

I am puzzled by the note “Furthest and Oldest Galaxy” on page 378 of the November–December issue (Briefings). It had been my understanding that the early universe was too hot to sustain un-ionized hydrogen for the first half-billion years or so, and was therefore a necessarily opaque plasma of protons and electrons. I further thought that as the universe expanded, it cooled, so that protons captured electrons and space became transparent. So, I thought that the microwave background is a relic of that time, appropriately red-shifted. Now, it seems, the radiation from protostars ionizes hydrogen atoms, which permits transparency. What is (was?) going on?

Timothy Weakley
Dundee, UK

The Editors respond:

Weirdly, both parts are true. In the very high density of the early universe, ionized matter and radiation were tightly coupled until the first atoms formed, creating a neutral medium that allowed radiation to pass. From then on the universe was mostly transparent, but it was dark, because there were no sources of light. When the first stars kicked in, they generated light and re-ionized the universe. At this point the cosmic density was so low that matter-radiation coupling was no longer an important factor. Instead, the ionization of hydrogen eliminated a major radiation absorption line.

We did err in implying that the dark ages were dark due to the opacity of hydrogen. They were dark primarily because of the lack of starlight—but we are correct that re-ionization wiped out the radiation-absorbing qualities of large clouds of neutral hydrogen.