The early Universe was so hot and energetic that ordinary matter was fully ionized for a long time; that is no neutral atoms existed, only positively charged nuclei and negatively charged electrons. As the Universe expanded, however, it cooled. When the temperature of the Universe fell below a certain threshold, protons and electrons could, at last, combine together to form neutral atoms, mostly hydrogen and helium. This event occurred about 380,000 years after the initial Big Bang, and the Universe then stayed neutral until much later. At 380,000 years, the Universe also suddenly became transparent. Earlier, the free electrons scattered light, and the Universe was opaque in the same way a cumulus cloud is. Once the electrons entered neutral atoms, the scattering stopped, the Universe became transparent, and light from this early moment could stream freely towards us. This is the moment we “see” when our radio telescopes, like those used by the Simons Observatory, observe the CMB. 380,000 years compared to the 13.8 billion year age of the Universe is about the same fraction as one day is of the lifetime of a 70-year-old person.
Then, for another 500 million years, the Universe stayed neutral and dark. These “dark ages” ended when the first stars exploded as the first galaxies were being assembled. The energy released once again ionized the ordinary matter in the Universe, breaking atoms up into nuclei and electrons. Scientists call this important moment in the history of our Universe “the epoch of reionization.” with free electrons again available, some of the CMB light was again scattered, leaving an imprint on the CMB we can study. Understanding of reionization is till far from complete. When exactly did it happen? How long did the process take? The Simons Observatory will help answer these questions, and thus help complete the picture of how the very first stars and galaxies formed in our Universe.