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Foothill College *

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Astronomy

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Oct 30, 2023

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20

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Lecture # 5 First Atoms and Dark Ages
Formation of First Atoms • Big Bang nucleosynthesis ended at 10 mins after the big bang with 75% of the matter ending up in hydrogen and 25% in Helium. • At this point the temperature of the Universe was low enough to forbid formation of elements heavier than berylium and lithium but still high enough to forbid electrons joining the nuclei to form atoms. They would be ejected as soon as they joined the protons. • This continued until the Universe became cold enough so that the photons no longer had the energy to kick out the electrons. This happened when the temperature of the Universe reached 3740 degrees Kelvin at 240,000 years after the Big Bang
The Origin of Helium • Immediately after the Big Bang, only protons and electrons existed. • Shortly after the BB, temperature and density was high enough for deuterium (heavy Hydrogen) to form by fusion. • After 100 seconds or so, temperature cooled enough so that deuterium could fuse into helium nuclei. • The temperature continued to cool, and fusion stopped after a few minutes. • Big Bang theory predicts that around 24% of the matter in the early universe was helium, which matches what we see.
“To make an apple pie from scratch, you should first invent the Universe.” Carl Sagan
Radiation and First Atoms • The Universe at this time was dominated by electrons and radiation (photons) with the photons continuously scattered by the electrons. • The rate with which photons scatter competes with the rate of the expansion of the Universe and rapidly decreases as the Universe expands. • This continues until the time between subsequent scatter of photons (by electrons) exceeds the age of the Universe at that epoch. At this point photons cease to interact with electrons and therefore, decouple. This epoch is called decoupling .
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