Dyson spheres still a thing

Oh, we have scientists still wondering about what super high energy use civilisations might be using, and it might be...wait for it...a Dyson Sphere-y thing around a black hole.  Let Science magazine explain:

But astronomer Tiger Hsiao of National Tsing Hua University says we might be looking for the wrong thing. In a new study, he and colleagues set out to calculate whether it would also be possible to use a Dyson sphere around a black hole. They analyzed black holes of three different sizes: those five, 20, and 4 million times the mass of our Sun. These, respectively, reflect the lower and upper limits of black holes known to have formed from the collapse of massive stars—and the even more enormous mass of Sagittarius A*, the supermassive massive black hole thought to lurk at the center of the Milky Way.

Black holes are typically thought of as consumers rather than producers of energy. Yet their huge gravitational fields can generate power through several theoretical processes. These include the radiation emitted from the accumulation of gas around the hole, the spinning “accretion” disk of matter slowly falling toward the event horizon, the relativistic jets of matter and energy that shoot out along the hole’s axis of rotation, and Hawking radiation—a theoretical way that black holes can lose mass, releasing energy in the process.

From their calculations, Hsiao and colleagues concluded that the accretion disk, surrounding gas, and jets of black holes can all serve as viable energy sources. In fact, the energy from the accretion disk alone of a stellar black hole of 20 solar masses could provide the same amount of power as Dyson spheres around 100,000 stars, the team will report next month in the Monthly Notices of the Royal Astronomical Society. Were a supermassive black hole harnessed, the energy it could provide might be 1 million times larger still.

If such technology is at work, there may be a way to spot it. According to the researchers, the waste heat signal from a so-called “hot” Dyson sphere—one somehow capable of surviving temperatures in excess of 3000 kelvin, above the melting point of known metals—around a stellar mass black hole in the Milky Way would be detectible at ultraviolet wavelengths. Such signals might be found in the data from various telescopes, including NASA’s Hubble Space Telescope and Galaxy Evolution Explorer, Hsiao says.

Meanwhile, a “solid” Dyson sphere—operating below 3000 kelvin—could be picked up in the infrared by, for example, the Sloan Digital Sky Survey or the Wide-field Infrared Survey Explorer. The latter is no stranger to looking for the infrared signals of traditional, star-based Dyson spheres. But, like all other such searches, it has yet to find anything conclusive.

Opatrný says using the radiation from accretion disks would be particularly clever, because the disks convert energy more efficiently than the thermonuclear reaction in conventional stars. Aliens concerned with the sustainability of their power supply, he suggests, might be better off encapsulating small stars that burn their fuel slowly. However, he continued, “The fast-living civilizations feeding on black hole accretion disks would be easier to spot from the huge amount of waste heat they produce.”

It is kind of hard imagining what this type of civilisation would even look like, though.  Here's the amusing last paragraph:

As for what the aliens might use this energy for, Opatrný has some thoughts. “Mining cryptocurrency, playing computer games, or just feeding the ever-growing bureaucracy?” he jokingly muses.