Carrigan argues that we should broaden SETI's scope to include the archeological remnants of Kardashev III civilizations, namely those civilizations who successfully tapped into their Galaxy's entire energy output. At first blush, one would assume that a K3 galaxy would be immediately obvious, with every one if its stars enclosed in a Dysonian structure of some sort. But Carrigan makes the case that this might not be the case:
…what would happen for a civilization on its way to becoming a type III civilization, a type II.5 civilization so to say? If it was busily turning stars into Dyson spheres the civilization could create a “Fermi bubble” or void in the visible light from a patch of the galaxy with a corresponding upturn in the emission of infrared light. This bubble would grow following the lines of a suggestion attributed to Fermi… that patient space travelers moving at 1/1000 to 1/100 of the speed of light could span a galaxy in one to ten million years. Here “Fermi bubble” is used rather than “Fermi void”, in part because the latter is also a term in solid state physics and also because such a region would only be a visible light void, not a matter void.As Gilster notes, this is long-term thinking in the richest sense; a patient, long-lived civilization could envelop a galaxy on a time-scale comparable to or shorter than the rotation period of the galaxy (considerably >~250 million years).
Civs who are busy turning stars into Dyson spheres should leave vast Fermi 'bubbles' whose infrared signature would flag their existence. But as Carrigon notes, detection might still elude us.
For example, we see M51, the Whirlpool galaxy, face-on at a distance of 30 million light years. We can say with some confidence that we see no unexplained voids larger than about five percent of M51's area, but any void features below this level would be hard to identify because of spiral galaxy structure. Elliptical galaxies might be better places to look for Fermi bubbles, because they display little structure, and potential voids should be far more pronounced.
And then there's the difficulty in separating artificial structure from natural phenomena where the tendency is to defer to the latter.
I come back around to the premise behind interstellar archaeology, that unlike conventional SETI it does not require a civilization to have any intention of contacting us. There are numerous ways to proceed, involving the kind of Dyson sphere search Carrigan has himself conducted within our own galaxy, or looking at planetary atmospheres in hopes of finding not only biosignatures but the markers of an advanced industrial or post-industrial culture. As we continue the SETI hunt, keeping in mind how planetary change or deliberate decisions to expand into the galaxy could leave visible traces allows us to hunt for things advanced intelligence might do.More.
How many civilizations in our galaxy, for example, have already faced the end of their main sequence star’s lifetime? If the number is high, it may be that we can find evidence of their response in the form of planetary or stellar engineering, making stars of this description interesting targets for future searches. In any case, our model of SETI is changing as not only our technologies but our assumptions become more sophisticated, leaving us to ponder a universe in which the need for expansion or simple survival may have left its own detectable history.