Briefly explain why radiation was trapped for 300,000 years during the era of nuclei, and why the cosmic background radiation broke free at the end of this era

Question

What will be an ideal response?

Answer 1

During the era of nuclei, the universe consisted of a hot plasma of hydrogen nuclei, helium nuclei, and free electrons. Throughout this era, photons bounced rapidly from one electron to the next, just as they do deep inside the Sun today, never managing to travel far between collisions. If a nucleus managed to capture an electron to form a neutral atom, one of the photons quickly ionized it. This era came to an end when the expanding universe had cooled down to about 3,000 K. At this temperature, the hydrogen and helium nuclei were able to capture electrons and form stable, neutral atoms for the first time. With electrons bound into atoms, the photons began to stream freely across the universe.