Answer to Question 1
Originally, the Universe was so hot that the gas was completely ionized, and electrons were not attached to nuclei. Photons could not travel far without being deflected by an electron.
At an age of around 400,000 years, free electrons were spread so far apart that photons could travel for thousands of parsecs before being deflected. At this point, the Universe began to become transparent to photons.
At about this time, the temperature of the gas dropped to the point where electrons could join with protons to form neutral hydrogen in a process called recombination. As the free electrons were converted, the gas became close to transparent and the photons could pass through undeflected.
These photons were emitted from a gas of 3000K and retained the blackbody temperature of the gas. This radiation, redshifted to an apparent temperature of 2.7K, is the cosmic microwave background radiation we observe today.
Answer to Question 2
Energy flowing toward the Sun's surface backs up like water behind a dam, and the gas begins to churn in convection currents. Hot blobs of gas rise, and cool blobs sink. In this region, known as the convective zone, the energy is carried outward not as photons but as circulating gas. Rising hot gas carries energy outward, but sinking cool gas is a necessary part of the cycle.
Heat flows upward as rising currents of hot gas and downward as sinking currents of cool gas. The rising currents heat the solar surface in small regions seen from Earth as granules.
Era of Recombination
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