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Ambitious civilizations that expand for resources at an intergalactic scalecould be observable from a cosmological distance, but how likely is one to bevisible to us? The question comes down to estimating the appearance rate ofsuch things in the cosmos --- a radically uncertain quantity. Despite thisprior uncertainty, anthropic considerations give rise to Bayesian updates, andthus predictions. The Self-Sampling Assumption (SSA), a school of anthropicprobability, has previously been used for this purpose. Here, we derivepredictions from the alternative school, the Self-Indication Assumption (SIA),and point out its features. SIA favors a higher appearance rate ofexpansionistic life, but our existence at the present cosmic time means thatsuch life cannot be too common (else our galaxy would long ago have beenoverrun). This combination squeezes our vast prior uncertainty into a feworders of magnitude. Details of the background cosmology fall out, and we areleft with some stark conclusions. E.g. if the limits to technology allow acivilization to expand at speed $v$, the probability of at least one expandingcosmological civilization being visible on our past light cone is$1-\frac{v^3}{c^3}$. We also show how the SIA estimate can be updated from theresults of a hypothetical full-sky survey that detects "$n$" expandingcivilizations (for $n \geq 0$), and calculate the implied final extent of lifein the universe.

On the Likelihood of Observing Extragalactic Civilizations: Predictions from the Self-Indication Assumption