Hello, SFIA audio listeners. In this month's Nebula exclusive, big alien theory,

we're asked at the reason alien civilizations might be rare is because most aliens are huge.

To hear it and every episode early and ad-free, plus hours of bonus content, check out

go.nebola.tv slash Isaac Arthur and use my code, Isaac Arthur.

Many civilizations aspire to be eternal to outlive their rivers and mountains, and even the stars themselves,

but even they are thinking much too short term.

As we get ready to head in the new year and the future ahead of us, it doesn't hurt

to reflect on what's come before, and not much comes before the beginning of time.

It's strange to think, but back when the universe was hotter and denser, not long after

the Big Bang, more fusion took place in the first 10 minutes than in the next 10 billion years.

All of our hydrogen and lithium and most of our helium dates from that period.

Indeed, it'll probably take the next 10 trillion years to surpass the amount of fusion that

occurred in that window of time that's shorter than this episode.

Quite a lot of fusion occurred after that too, as the universe cooled down to something

more like normal stellar fusion rates and eventually down to the temperature as we can see

at the surface of stars rather than their ultra-hot cores.

About the time that happened, things finally thinned and cooled enough to let atoms

with electrons form, and we stopped seeing every single photon created About the time that happened, things finally thinned and cooled enough to let atoms with

electrons form, and we stopped seeing every single photon created to get rapidly reabsorbed

by its neighbors. Indeed nowadays, most photons will never get absorbed, and will meander

off to the edge of eternity, so there will never will be a true dark age in this universe,

even when the last star burns out.

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