Nick Bostrom: How big is the cosmic endowment?

Superintelligence, pp. 122-3. 2014. Consider a technologically mature civilization capable of building sophisticated von Neumann probes of the kind discussed in the text. If these can travel at 50% of the speed of light, they can reach some stars before the cosmic expansion puts further acquisitions forever out of reach. At 99% of c, they could reach some stars. These travel speeds are energetically attainable using a small fraction of the resources available in the solar system.