When I was under the velvet black skies of western Texas a few months ago I had a magnificent view of the star-studded bulge of our galaxy, in the direction of the summer constellation Sagittarius.
How many advanced civilizations might be in this hub of the Milky Way? I pondered. After all, this is the direction where the mysterious "WOW" radio signal that was detected three decades ago came from.
The problem is that we are embedded in a thick forest of stars, and identifying the location of an extraterrestrial civilization -- one that's attempting to contact us -- is the proverbial needle-in-haystack search as the SETI scientists always say.
Therefore, it would make sense to go looking at a neighboring "forest," or rather nearby galaxy, for evidence of extraterrestrial intelligence.
The image above from NASA's Wide-field Infrared Survey Explorer is an example. The red object at upper left is an aging star shrouded in dust. Nevertheless, in a survey of 250,000 infrared sky sources cataloged in the 1970s, 17 "quasi-plausible" Dyson sphere signatures came up, according to Richard Carrigan of Fermilab.
It’s imaginable that a super-civilization would begin a wave of colonization that spread out to neighboring solar type stars from its home base. Each offshoot would "astro-form" the colonized planetary system by constructing a Dyson sphere around the host star.
Carrigan envisions seeing "Dyson bubbles" in nearby galaxies. These would be clusters of Dyson spheres that enclosed a grouping of stars colonized by a Type II Kardashev civilization. The logic is that after you've built a backyard fence you can start to conceptualize building the Great Wall of China and still hope to gain perspective on the process, Carrigan writes.
These would be detected as anomalous dark voids in a galaxy's disk. When these voids were observed in infrared light they would glow brightly with the heat radiation from the surfaces of Dyson spheres. This would show that they are not that simply voids where solar-type stars are conspicuously missing.
NASA's Hubble Space Telescope is conducting a multi-year survey across a swath of the neighboring Andromeda galaxy (M31). The images are filled with so many resolved stars that they resemble at grains of sand on a beach. This could make an excellent citizen science project, to scour the Andromeda fields for anomalous-looking regions.
Gigantic elliptical galaxies (shown above), which are completely devoid of light-blocking dust, would look very odd indeed if dark voids were detected. However the nearest ellipticals are 60 million light-years away and so would require a space telescope much larger that Hubble to yield enough resolution.
An apparent lack of any evidence for large-scale artifacts in galaxies as old as ours would begin to set an upper limit on just how technologically advanced alien civilizations can evolve to become.
Kardeschev hypothesized about Type III civilizations that would harness the energy of an entire galaxy. The observational evidence of astro-engineering a complete galaxy is lacking, and so it's fair to say that Type III civilizations either don't exist at all, or at least not yet.
The universe has had 12 billion years to evolve Type II or Type III civilizations. If there's not obvious archeological evidence, then maybe intelligent beings don't evolve all that far beyond mega-engineering to the scale of a single planetary system.
Maybe extraterrestrials simply don't have the motivation, know-how, or the budget.
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