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Astronomers Find Star-Less Galaxy
Wohngebaeudeversicherung writes "Astronomers have discovered a galaxy about 50 million lightyears away from earth that appears to be composed entirly of dark matter. This galaxy, dubbed VIRGOHI21 is rotating like a real galaxy, at speeds only explainable through massive amounts of matter, thought no single visible star could be detected."
...and it doesn't require exotic quarks, leptons, or baryons to work.
Okay, that's an enormous (and highly unlikely) exaggeration, but I *have* thought of an interesting possibility. A Dyson Sphere surrounding several stars (or in a Type 3 civilization, an entire galaxy) would block visible light - the problem is it would glow in the infrared, so it wouldn't really be dark. Black holes are dark, but they tend to fling stuff around, and matter sucked into them gives off bursts of energy before they disappear.
The solution: a dark bubble. At the center of our galaxy there is a supermassive black hole, which is (according to some estimates) roughly three million solar masses. A civilization putting a bubble around it would have 1 (earth) gravity a little beyond the orbit of Pluto, perhaps 40-45 A.U. or so. The problem is that you still would need to stick some stars around it to supply energy, and a Klemperer rosette would be pretty noticeable.
Well, light falling onto a blackhole blue shifts, increasing its energy. Increase the bubble enough (remember, we're talking a civilization that can harness the energy of a galaxy), and the mass of the bubble itself starts to warp space around it. There comes a point where the size of the bubble and the mass that makes it up can be just under the Schwarzschild limit - a bit more massive and it would be a black hole - even without a central singularity. For humans, we'd want a bubble that has a surface gravity equal to earth's, and a blue-shifted energy equal to the average output from our sun.
As a back-of-the envelope calculation, using v^2=2*g*R, where v is the escape velocity, g is the gravitational attraction at the earth's surface, and R is the radius from the center of mass, and setting v=c (the speed of light) for the maximum size, you get a bubble with a diameter just a bit under a light-year across (354 light days, if I figured correctly). The surface area would be about 3 square light-years, 2.6 x 10^26 square kilometers, or 5.2 x 10^17 times the surface area of the earth. The mass would be equivalent to 1.5 trillion suns - roughly twice the mass of our galaxy. Assuming you use buckytubes as the material of choice, you'd have a shell 7000 kilometers thick of solid buckminsterfullerene.
Of course, this is the absolute maximum size and mass just before it becomes a black hole, so the actual construct would be a bit smaller and less massive, balancing surface gravity and blue-shifted energy hitting the surface. You'd also want to carve out mountain ranges and oceans for a bit of variety - a galactic Kansas would be kind of boring. For safety reasons, you would have to stick these bubbles in the empty space between galaxies, or just use all of the mass in one large galaxy (you'd have to be careful, though, to keep relativistic rocks from flying at the completed project). You'd have a sky that would look kind of like a slow-moving aurora, perhaps -- infrared would be shifted into visible light, visible stars would have their peak shifted to ultraviolet -- especially since the gravitational warping would slow down time considerably compared to the rest of the galaxy.
To detect them, you'd have to aim telescopes at the "empty" parts of the sky and see if there was any gravitational lensing. If something was there that was far too massive to be a neutron star but didn't have the characteristics of a supermassive black hole, that could be a sign of it. The largest ones would have the gravitational mass of a large galaxy, so if a supercluster appears to be missing a galaxy's worth of stars that stellar motions demand, it might not be exotic matter but instead bubbles of normal matter from some vast engineering project.
Of course, it might be too early in the evolution of the universe for a type 3 civilization to appear, or you might not be able to make a buckytube bubble big enough that would also support its own weight, so exotic forms of matter might still be necessary. One thing's for certain, though - a bubble like this would make Ringworld look as spacious as a phone booth.
It was easy to disprove the existence of aether with the Michelson-Morley experiment. Had that experiment not been possible it would have been very premature to jump to the conclusion that there is no aether. When it comes to dark matter, there is no easy experiment to disprove its existence and so it would be very rash to conclude that our understanding of gravity, which has worked extremely well for us for hundreds of years, is wrong.
...or they could be a Type III civilization, capturing the energy output of that entire galaxy with one giant device.
"...always new atoms but always doing the same dance, remembering what the dance was yesterday." -Richard Feynman