Well, it's not really about locales that might support intelligent life, but
this new article may be interesting related reading for anyone interested in this thread. (Caveat: I haven't read it and probably don't have time to at the moment.)
Fun to read. Select paragraphs:
Estimates for the number of visible galaxy-spanning civilizations and the cosmological expansion of life
S. Jay Olson
Department of Physics, Boise State University, Boise
P4:
"A basic input of an aggressive expansion scenario is the appearance rate of expanders per unit coordinate volume, per unit cosmic time, f(t). We will consider three such models for the time-dependence of f(t), leaving the overall proportionality constant as a parameter to be fixed by assumptions on the relative time of arrival of humanity in the next section. The baseline “non-catastrophic” model will set the appearance rate at time t to be proportional to the number of earthlike planets formed between 4.5 Gyr − 6 Gyr prior to t. This means that we assume it takes at least 4.5 Gyr for maximally advanced life to appear on a newly formed earthlike planet, and that the window for life to evolve is no more than 6 Gyr."
P5:
"In addition to the baseline model, we also introduce two models that include galaxywide extinction events, with a rate that changes as a function of cosmic time. These models assume that a life-harboring planet will be subject to a high-energy event, nearby gamma ray bursts (GRB’s) being the prototype example [17], that destroys planetary ozone layers, causing a mass extinction event which sets back the evolution of life by some amount of time. We will assume that such events are severe if they occur in the final stages of evolution towards intelligence, so that we reduce the pool of potential planets to those which have not seen such an extinction event in the last .2 Gyr."
P10:
"It is interesting to visualize just how rarely aggressively expanding civilizations arise, according to this analysis. A typical value for the appearance rate parameter α in a GRB-tracking scenario is of order 10−3 appearances per Gly3 per Gyr.
In other words, it would take a sphere of radius ≈ 5 Gly to produce a single aggressive expander in a billion years. This is a volume encompassing many thousands of superclusters and perhaps a hundred million large galaxies. Similar numbers are implied by [10], in their calculation of the number of galaxies that could have reached and colonized the Milky Way in the last few Gyr. Any sufficient great filter [20] must be very great indeed."
Bold added by me.
Thx!
TomB
