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.

In the ancient past, before the first stars formed, all was darkness, and the universe was a very

different place, but there may have been strange dark stars in that primordial era at the beginning

of time.

They say 13.8 billion years ago there was a giant explosion and the universe came into

existence, and sometime later stars and galaxies formed.

It wasn't truly an explosion, even if the early universe of that era was more terrifyingly

energetic than ground zero of an atomic bomb blast, and indeed it was

a while after the Big Bang before things spread out and cooled down enough to even resemble

the surface of a star, let alone a place where stars and planets existed.

That early universe before the stars wasn't dark back then, the first few seconds it was too hot for nuclear

fusion to even occur, that process that requires such insanely high temperatures and densities

that we are just barely able to achieve them in a lab, and whose upper ranges are at the

course of supernovae.

It took a minute just to spread and cool enough to get into that range.

Almost all the matter of modern times comes from this period where those early protons

banged into each other, when it was just cool enough to stick together and form neutrons

and deuterium, which could fuse to form helium and a tiny bit of lithium.

Had we spread even a little slower, there might have been no hydrogen left to form stars.

...