Could Dyson Spheres Be the Universe’s Missing Mass?

The search for “missing matter” in the universe has failed so many times that some exotic suggestions are being taken more seriously than ever. As Sherlock Holmes famously said, “When you have eliminated the impossible, whatever remains, however improbable, must be the truth.” In this case, many improbable ideas are being tested to see if they are impossible. One idea that has attracted enough attention that IFLScience was asked to discuss it is Dyson Spheres. There are good reasons to conclude that these hypothetical spheres are not the matter you’re looking for, but also to investigate how we know that.

What is a Dyson sphere?

Only a small portion of the sun’s energy falls on its planets, the rest escapes into space. In 1937, science fiction writer Olaf Stapledon wrote a book, The star makerwhich explored ideas about the search for energy by much more advanced civilizations. The book inspired physicist Freeman Dyson to propose that such civilizations could build gigantic thin surfaces in space to capture more of their stars’ energy, eventually partially or completely circling the star.

Dyson noted that such structures would block the star’s visible light for observers elsewhere, but would emit it in the infrared. Consequently, he argued, one way to find advanced alien civilizations might be to look for spectra dominated by the infrared.

The idea captured the imagination of many, and gained traction when the mystery of KIC 8462852 (also known as Boyajian’s Star) came to light in 2015. KIC 8462852 undergoes significant dips in brightness at irregular intervals, far too large to be the result of planets blocking its light. There was so much speculation that the observed behavior could be caused by a partially constructed Dyson Sphere that another nickname, the “Alien Megastructure Star,” became common.

What is the missing mass?

There are actually two kinds of mass that our surveys of the local universe have failed to find. The most familiar of these is dark matter, the mass needed to explain the motions of galaxies under the laws of gravity. The other kind of missing mass is more regular material, probably composed mainly of hydrogen and helium, as opposed to dark matter, which is most likely to be exotic particles.

When astronomers talk about “missing mass,” they mean the second kind. We know that this category consists of ordinary elements because evidence from shortly after the birth of the universe allows us to calculate how much ordinary matter there should be in the universe today. When we look around us, we see only about two-thirds of that amount.

There is much less mass missing in this category than dark matter, but still a lot. Explanations include vast filaments of gas stretching between galaxies

Could Dyson spheres explain both types of missing mass?

Unfortunately, almost certainly not.

Once people realized how cool Dyson Spheres would be, and had fun with the possible science fiction ideas of life in something so mind-bogglingly large, physicists started thinking about the practicalities. And it turns out that full-blown Dyson Spheres just don’t make sense.

The material for a Dyson Sphere would have to come from somewhere. It is highly unlikely that even the most advanced civilization would be able to scoop matter from their star and convert it into something solid. Even if they could, they probably wouldn’t be dependent on stellar energy. Therefore, the material for the Sphere would have to be made from planets, moons, and asteroids.

Some galaxies have more orbital mass than ours, others probably less. But there is no reason to think we are unusually light in that regard.

That means there wouldn’t be that much mass in the sphere itself, even if you used every bit of solid material in the planetary system. If the question was meant to mean “Could the material in Dyson Spheres be so massive that it makes up a large fraction of the missing matter?” then you’d have to explain where that matter came from in the first place. Combing the space between the stars and finding renegade planets or other sources of material so that they can be used as substrates for solar panels is probably not practical.

Another way to interpret the question is, “Could it be that there are billions of stars surrounded by Dyson spheres that catch all their light, so we can’t see them, making the Milky Way much more densely packed with stars than we think?” That’s generally what people mean.

The popular, but almost certainly incorrect, view of the Dyson sphere is one that steadily builds until the star is surrounded by a complete sphere.

The popular, but almost certainly incorrect, view of the Dyson sphere is one that steadily builds until the star is surrounded by a complete sphere.

However, given the amount of solid material in the solar system, any completely enclosing sphere would have to be very thin. So thin, in fact, that it would be gravitationally unstable. The only way to avoid disaster would be to expend enormous amounts of energy, making the whole idea a net loss.

If Dyson Spheres exist, they are very incomplete, either thin “Dyson Rings” or networks of patches that capture a few percent or less of the star’s light. These are sometimes called Dyson Swarms.

If a star were orbiting it in a Dyson Swarm, we would see it, obscured by the occasional blip as the fragment comes between us and it – the hypothetical situation that made KIC 8462852 famous. Dozens of stars have been identified where this could happen, though other explanations are more likely.

In such a case, the star would not have disappeared for an extended period. Consequently, our estimates of the number of stars in the galaxy would not be far, if at all, off. A small underestimation would account for only a small fraction of the missing matter.

Even if a complete Dyson Sphere were built, a key feature of the concept is that it would radiate in the infrared. Dyson wanted us to look for that kind of infrared signal. The JWST and our few other infrared telescopes can’t see everywhere, so they may have missed a few such radiators. But if they were common enough to solve the mystery of the missing mass, we should have seen them by now.

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