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The first compound/molecule formed was helium hydride. Why was helium hydride ion HeH$^+$, not hydrogen H$_2$, the first molecule formed in the early universe? Helium hydride started to form at redshifts of $z \sim 2000$, after helium had recombined, but all the hydrogen was still ionised. $${\rm He} + {\rm H}^{+} \rightarrow {\rm He H^+}$$ This is about 260,...

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There is a simple geometric interpretation of the quantity that is called the age of the universe. Spacetime has a large-scale shape. It looks something like this: Later times are at the top, earlier times at the bottom. You shouldn't take this too literally because it isn't a proper embedding (I flipped the sign of the metric to make it Euclidean, and ...

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No. The age of the universe does not depend on any referential system. In order to measure time, you need some physical quantity that's changing to measure time against. In the case of cosmology, it's the time perceived by a typical observer based on the expansion parameter $a$ --see below. In a manner of speaking, it's the time that follows the galaxies in ...

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Anti-protons have a rest-mass energy of 938 MeV. They are not created in reactions at temperatures of just 1 MeV and neither are protons or neutrons. The protons were created much earlier (at around $10^{-6}$s and $k_BT\sim 1$ GeV, along with anti-protons), but most of them annihilated with anti-protons as the universe cooled, leaving the protons that remain ...

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