On the Likelihood of Observing Extragalactic Civilizations: Predictions
  from the Self-Indication Assumption
  Ambitious civilizations that expand for resources at an intergalactic scale
could be observable from a cosmological distance, but how likely is one to be
visible to us? The question comes down to estimating the appearance rate of
such things in the cosmos --- a radically uncertain quantity. Despite this
prior uncertainty, anthropic considerations give rise to Bayesian updates, and
thus predictions. The Self-Sampling Assumption (SSA), a school of anthropic
probability, has previously been used for this purpose. Here, we derive
predictions from the alternative school, the Self-Indication Assumption (SIA),
and point out its features. SIA favors a higher appearance rate of
expansionistic life, but our existence at the present cosmic time means that
such life cannot be too common (else our galaxy would long ago have been
overrun). This combination squeezes our vast prior uncertainty into a few
orders of magnitude. Details of the background cosmology fall out, and we are
left with some stark conclusions. E.g. if the limits to technology allow a
civilization to expand at speed $v$, the probability of at least one expanding
cosmological 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 the
results of a hypothetical full-sky survey that detects "$n$" expanding
civilizations (for $n \geq 0$), and calculate the implied final extent of life
in the universe.
