I agree with most of the points you made in response to Mitchell_Porter, but there's a plausible answer to this:
But why haven't they already taken all of the matter and energy we're made of? Why is the cosmos in a pristine, isotropic, homogeneous state?
Because there are tremendous costs to coming all the way here to take our sun's energy. Any inconvenience we find in the years necessary to travel between the stars is worse for a species achieving superhuman intelligence because it feels longer to them.
I remember a writer on edge.org -- I will find it when I get a chance -- who proposed that a species that reaches a singularity will find a higher return to living in a virtual reality on their home world, where they don't have such communication problems. In that case, they would Dyson-sphere at most their own star, and therefore not interfere with the cosmos's isotropy or homogeny.
This was one such idea that I had. I don't get the feeling that a superior intelligence is going to be so petty as to just waltz around the universe sucking up resources. That smacks, to me, of Hollywood horror stories. And, considering how poorly we understand Game Theory in relation to non-human species... It might be that they have a much more cooperative solution to the universe's problems.
After all, any post-Singularity Society should realize that the ultimate goal of their intelligence is to merge with the rest of the universe. Why would that mean th...
An uplifting message as we enter the new year, quoted from Edge.org:
A few thoughts: when considering the heavy skepticism that the singularity hypothesis receives, it is important to remember that there is a much weaker hypothesis, highlighted here by Tegmark, that still has extremely counter-intuitive implications about our place in spacetime; one might call it the bottleneck hypothesis - the hypothesis that 21st century humanity occupies a pivotal place in the evolution of the universe, simply because we may well be a part of the small space/time window during which it is decided whether earth-originating life will colonize the universe or not.
The bottleneck hypothesis is weaker than the singularity hypothesis - we can be at the bottleneck even if smarter-than-human AI is impossible or extremely impractical, but if smarter-than-human AI is possible and reasonably practical, then we are surely at the bottleneck of the universe. The bottleneck hypothesis is based upon less controversial science than the singularity hypothesis, and is robust to different assumptions about what is feasible in an engineering sense (AI/no AI, ems/no ems, nuclear rockets/generation ships/cryonics advances, etc) so might be accepted by a larger number of people.
Related is Hanson's "Dream Time" idea.