In looking at the answers to this question regarding light from distant galaxies ever being visible to us:

Expansion of the Universe, will light from some galaxies never reach us?

I came across a few concepts that were quite surprising to me. In particular:


Movement faster than the speed of light
The big bang was not an explosion outwards from a single point.


Granted I am just a rank beginner and self-studier, yet I did study a QM course from Oxford, have read several sets of notes on SR, and readily went through the first hundred pages of "Student Friendly QFT."

Yet I have never encountered these notions.

My question is where does one acquire this type of information. Not necessarily the technicalities (of, e.g., GR); but just a correct awareness.

The bottleneck in Solar fusion is getting two hydrogen nuclei, i.e. two protons, to fuse together.

Protons collide all the time in the Sun's core, but there is no bound state of two protons because there aren't any neutrons to hold them together. Protons can only fuse if one of them undergoes beta plus decay to become a neutron at the moment of the collision. The neutron and the remaining proton fuse to form a deuterium nucleus, and this can react with another proton to form $^{3}\text{He}$. The beta plus decay is mediated by the weak force so it's  relatively slow process anyway, and the probability of the beta plus decay happening at just the right time is extremely low, which is why proton fusion is relatively slow in the Sun. It takes gazillions of proton-proton collisions to form a single deuterium nucleus.

Nuclear fusion weapons bombs fuse fast because they use a mixture of deuterium and tritium. They don't attempt to fuse $^{1}\text{H}$ so they don't have the bottleneck that the Sun has to deal with.
