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

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

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

This episode is sponsored by Auduble.

Carl Sagan famously said the universe and cosmos is all that is or ever was or ever will be,

but what if it's just the tip of the iceberg?

When we look at the night sky, we can see several thousand stars.

As we reached the modern era, we learned there were millions of times more stars in our galaxy,

itself one of trillions we can see.

Our universe was not a few thousand stars, but billions and billions of them, but this is

probably not the sum of the universe, which expands outward

in all directions equally until we see a wall of ancient light, the cosmic microwave

background radiation, left over when the universe was rather young and hot, and red shifted

from its journey through nearly 14 billion years.

This we call the observable universe, and we suspect it goes beyond that, probably far beyond

that and possibly even infinitely so.

And yet that's just this universe in this iteration and timeline.

We don't know if anything preceded this universe, or if our Big Bang was merely a relatively

local affair in some other, older, larger universe, or if every possible timeline of our

universe has happened, or both.

We don't know if other places with other rules might exist, extra dimensions or different

...