Suppose there were amazingly advanced alien civilizations out there -- aliens capable of harnessing the energy of entire galaxies. (These are the Kardashev Type III civilizations). Surely such a civilization would easy to spot?
I've noticed a surprisingly large number of serious astronomy papers on this subject recently. As you might imagine, nobody has seen any evidence for such a civilization -- if they had seen something, you probably would have heard about it by now. But here's a fun new idea from Beatriz Villarroel and collaborators at Uppsala University: searching for disappearing stars. These scientists compared two surveys of sky -- one from the US Naval Observatory, based on observations of the sky between 1950 and 1999, and the more recent Sloan Digital Sky Survey. They examined 10 million objects, and found exactly one object in the first survey that seems to have disappeared in the later survey.
Does this mean that an advanced civilization has caused a star to vanish?
Probably not, and the authors of the paper don't make such a claim. Maybe the star was simply much brighter in the past and then dimmed rapidly. On the other hand, it's a rather intriguing observation....
You might expect that the USNO survey might have missed objects picked up by the Sloan; but this says that something spotted previously hasn't been seen again. Where'd it go? Has it been enclosed in a Dyson Sphere since the last time it was spotted? Or perhaps it was the exhaust of a starship that is now coasting?