Hart–Tipler, Kipping, and the Rare-Intelligence Resolution to the Fermi Paradox: A Cosmological Perspective on Intergalactic Probes

To the point

David Kipping’s three-parameter model shows that if intelligent life can spontaneously arise at rate λ, spread at rate u, and start at time t, then self-replicating probes could colonize galaxies well before now, implying we should have observed them unless such civilizations are extraordinarily rare or blocked by a strong filter.

Why extraterrestrial civilizations may be rare in the Universe
universemagazine.com

Why extraterrestrial civilizations may be rare in the Universe

David Kipping, a researcher at Columbia University, recently published a study in which he revisited one of the older solutions to the Fermi paradox.