Jim @sullybiker@sully.site

Today in 1969, 55 years ago: in London, the British band Pink Floyd releases the album Music from the film "More".

#OnThisDay

When a proton and neutron crash into each other, they can stick together and make a deuterium nucleus. After another proton and neutron hit this, you get helium. That's how helium was made in the early Universe. But there was a bottleneck which limited this process!

About 15 seconds after the Big Bang, it cooled down enough for antimatter to go away. Then all we had was protons, neutrons, electrons and a shitload of light.

But neutrons are unstable - they decay into protons - so it was a race against time to make deuterium by smacking protons and neutrons together. And it was still way too hot. All that energetic light destroyed the deuterium nuclei as fast as they got made!

Only later, 3 minutes and 45 seconds after the Big Bang, did it cool down to the point where deuterium nuclei become stable. Alas, by then most of the neutrons had decayed. There were about 7 protons for every neutron.

At this point, most of the remaining neutrons quickly combined with protons and made deuterium and then helium. But a lot of protons were left without dance partners! So the universe was roughly 3/4 hydrogen and 1/4 helium.

By then, it had cooled down too much for heavier elements to be formed. This only happened much later, in stars. And the universe is still roughly 3/4 hydrogen and 1/4 helium... with a tiny bit of other stuff.

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