Also, his assumption that life could spread at nearly the speed of light may be overly optimistic. If, by contrast, one assumes that light can spread at only 1/

Also, his assumption that life could spread at nearly the speed of light may be overly optimistic. If, by contrast, one assumes that light can spread at only 1/10th the speed of light, and that the earliest star-faring life could have arisen was a billion years ago (due to as the time needed for a second-generation yellow star to brighten enough to keep an earth-like planet warm after its conversion to an oxygen/nitrogen atmosphere), then one only needs to explain why star-faring life has not yet arisen in relatively nearby galaxies, rather than why it has not yet arisen anywhere in the visible universe. This is a much less daunting hurdle.