> Who's to say that other "loud" aliens haven't already expanded and begun spacefaring, but simply are not expanding out to where we are? By definition if they

> Who's to say that other "loud" aliens haven't already expanded and begun spacefaring, but simply are not expanding out to where we are? By definition if they're loud and we can't see them, they're not within the volume of our visible universe. > I might be missing something here, but why would alien (or any) civilizations continually expand at a constant rate? This assumes they keep multiplying and colonizing outward. There is no real upper limit - except, eventually, the accelerating expansion rate of the universe that keeps them from reaching further. Because the colonized volume is so large, we don't really assume these colonized volumes form a coordinated empire. It would be more like a loose tangle of city states spread across a vast landscape. They might share common heritage, technology and some amount of culture, but be otherwise independent.