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author: niplav, created: 2024-11-15, modified: 2025-07-08, language: english, status: in progress, importance: 6, confidence: unlikely

One might be able to solve two problems far-future civilizations will face (most baryonic matter is intergalactic hot plasma; intergalactic high-c probes are damaged by dust) with one idea—use black holes as shielding.

Black Hole Mop

Baryons Are Scattered

Baryons make up only 4.5% of the current massenergy of the universe. >80% of that baryonic massenergy is intergalactic dust or plasma—mostly hydrogen and helium with temperatures from $10^5$ to $10^7 K$, and if compressed to water density would have temperatures from $10^{31}$ to $10^{34}K$ (Sandberg 2023, p. 746).

We or our descendants might care a lot about this massenergy, since if we can't use dark matter or dark energy, it would represent most of the resources available in the universe.

So it would be cool if we had a method of collecting this intergalacic plasma & dust, and ideally one that fulfills these properties:

1. Cools down the collected plasma and uses the heat for useful work.
2. Leaves it in a state that admits energy-generation via nuclear fusion.
3. Is fast enough that most of the useful energy is extracted before protons start decaying.

Relativistic Probes Are Damaged By Dust

For intergalactic colonization, an advanced civilization would want to move through intergalactic space at very high speed—the closer to $c$, the better. But at such velocities, collisions with dust and small particles create explosions releasing as much energy as a hand grenade.

Synthesis

For a black hole civilization, One option to fulfill some of the criteria for collecting the IGM and shielding a probe would be to create intergalactic "ships" that are composed of a coordinating, baryonic center and a "shell" of black holes orbiting that center, moving at high speed through the intergalactic medium, in a setup similar to a Dyson swarm. Alternatively, the setup could be two or more black holes orbiting each other, with the baryonic center stationary at a Lagrange point where it is shielded by one or more of the black holes in the direction of travel.

This, but with black holes instead of mirrors. Sandberg 2023, p. 460

The black holes would "mop up" the intergalactic medium as the craft would move through it, increasing in mass¹. The heat of the intergalactic medium would be converted to the mass of the black hole, as black holes don't have any temperature and are excellent heat sinks; instead they emit Hawking radiation in the black body spectrum.

One option is to use such black hole mops in intergalactic colonization, where the black holes act as shielding for the center from collisions with intergalactic gas. I think this would slow this construction down, but usually not change its structure, though I haven't thought about it very much.

How does this fare on our list of criteria?

1. The heat of the plasma & dust is converted perfectly into mass, but not used for anything productive.
2. The plasma is unavailable for fusion, and the form in which it later is emitted again Hawking radiation is less useful, as it happens over extremely long timespans.
3. I haven't yet calculated how much of the reachable universe could be plausibly covered this way.

Black hole mops would have some other disadvantages:

1. They require advanced control over black holes, setting them up in stable complicated orbit around a center.
2. They probably require large numbers of small black holes, which increase in mass during the journey. 1. This is a downside because small black holes are likely much more valuable, since they only come about after long evaporations of large black holes, or have to be constructed synthetically through extremely precise focusing of light. 2. Potentially one could create stable orbits of three or more large black holes "covering" the construction in the direction of travel, similar to multiple star systems.
3. The black holes can probably not cover the central craft completely at all times, so some intergalactic medium would get lost and/or damage the central craft.
4. If the black holes are orbiting the central craft, it must have a very high mass.

Despite having mostly listed downsides of black hole mops, I think they're an attractive option to do further reseach on.

I don't know whether such constructions are possible or desirable, but I would like to hear some feedback by someone more knowledgeable about physics.