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 add 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 ten minutes than the next ten billion years.

All of our hydrogen and lithium, and most of our helium, dates from that period. Indeed, it will probably take the next

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 temperatures 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 stop 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. It is that dark age we are going to discuss today, the one that comes after the stars go away,

and it is thousands of times further ahead in our future than the Big Bang is in our past,

but each epoch of our universe is longer than the last, with that brief ten-minute one where

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