The Story of Astrophysics in Five Revolutions showed up in my kindle library, probably because another family member checked it out from the local library. I read it in a handful of days because it was relatively easy to read (no equations), and mostly covered stuff I already knew, from Newton to Einstein to Hubble to the Big Bang Theory.
Towards the end of the book, as she discusses the Big Bang theory, I started getting into stuff I either didn't know or didn't remember, such as the origins of Inflation, and the confirmation that the Big Bang Theory is correct mostly because of the proportion of the light elements that are present in the universe.
Helium, deuterium, and lithium nuclei are formed. The cosmic quantities of these elements are set, along with that of hydrogen, whose density has decreased as a result of the formation of helium. If we were to calculate how many of these elements were produced by nucleosynthesis, we would find astonishing results. The mass ratio between hydrogen and helium that we still observe in the Universe today is exactly that established in those first 3 minutes. For every three hydrogen atoms, there is one helium atom. This is one of the most surprising confirmations of the Big Bang theory. Without primordial nucleosynthesis, the amount of helium we observe in the Universe today could not be explained. (kindle loc 1218)
Cosmology is great fun, because it's astonishing how much we've been able to discover about the universe from mostly being Earthbound, and the astonishing idea that so much of what we know about the expanding universe will become unknowable in the far future, once the last external galaxy has been expanded out of our light cone. We truly live in interesting times!
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