The search for the “missing matter” in the universe has been so often unsuccessful that some exotic suggestions are being taken more seriously than they might have been before. As Sherlock Holmes 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 of them has attracted enough attention that IFLScience was invited to discuss it: Dyson spheres. There are good reasons to conclude that these hypothetical spheres are not the matter you are looking for, but also to explore how we know that.
First of all, what is a Dyson Sphere?
Only a tiny fraction of the Sun’s energy falls on its planets, the rest escaping into space. In 1937, science fiction writer Olaf Stapledon wrote a book, The Star Makerwhich explored the ideas of much more advanced civilizations seeking energy. This book inspired physicist Freeman Dyson to propose that these civilizations could build giant thin surfaces in space to capture more energy from their stars, possibly partially or entirely encircling the star.
Dyson noted that such structures would block visible light from the star to observers elsewhere, but would radiate in the infrared. Therefore, he argued, one way to find advanced alien civilizations might be to look for spectra dominated by infrared.
The idea captured the imagination of many people and gained renewed popularity when the mystery of KIC 8462852 (also known as Boyajian’s Star) emerged in 2015. KIC 8462852 experiences significant dips in brightness at irregular intervals, far too significant 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 types of mass that our studies of the local universe have failed to find. The most famous of these is dark matter, the mass needed to explain the motions of galaxies under the laws of gravity. The other type of missing mass is more ordinary matter, probably composed mainly of hydrogen and helium, as opposed to dark matter, which is most likely made up of exotic particles.
When astronomers talk about “missing mass,” they’re referring to the second category. We know that this category is made up of ordinary stuff because evidence collected shortly after the universe was born allows us to calculate how much ordinary matter there should be in the universe today. When we look around us, we can only see about two-thirds of that amount.
There is much less mass missing in this category than dark matter, but it is still very important. Explanations include huge filaments of gas extending between galaxies
Could Dyson spheres then explain either type of missing mass?
Unfortunately, this is almost certainly not the case.
Once people realized how cool Dyson spheres would be and toyed with the potential science fiction ideas of living inside something so incredibly large, physicists considered the practicalities. And it turns out that full 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 harvest material from its star and turn it into something solid. If it could, it probably wouldn’t rely on stellar energy anyway. Therefore, the material for the sphere would have to be planets, moons, and asteroids.
Some star systems have more orbital mass than we do, others probably less. But there is no reason to think that we are abnormally light in this area.
This means that there wouldn’t be that much mass in the sphere itself, even if we used every bit of solid matter in the planetary system. If the question were to mean “Could the matter in the Dyson spheres be so huge that it makes up a large portion of the missing matter?” then we would first have to explain where that matter came from. It’s unlikely that we could scour the space between stars and find wandering planets or other sources of matter to turn into solar panel backing.
The other way to interpret the question is, “Could there be billions of stars surrounded by Dyson spheres that capture all their light so that we can’t see them, making the galaxy much more densely populated with stars than we think?” This is usually what people mean.
The popular, but almost certainly incorrect, view of the Dyson sphere is that it gradually builds up until the star is surrounded by a complete sphere.
However, given the amount of solid matter in the solar system, any sphere that completely surrounded it 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 use enormous amounts of energy, which would make the idea a net loss.
If Dyson spheres exist, they are very incomplete: they are either thin “Dyson rings” or networks of spots collecting a few percent or less of the star’s light. They are sometimes called Dyson swarms.
If a star were orbiting a Dyson swarm, we would see it, dimmed by an occasional tiny dot as the part between us and it shines – the hypothetical situation that made KIC 8462852 famous. Dozens of stars have been identified in which this phenomenon could occur, although other explanations are more likely.
In such a case, the star would not disappear for a long time. Therefore, our estimates of the number of stars in the galaxy would not be very wrong, if at all. A small underestimate could be responsible for only a tiny proportion of the missing matter.
Even if a complete Dyson sphere were built, a key feature of the design is that it would emit infrared radiation. Dyson wanted us to be on the lookout for this type of infrared signal. The JWST and our few other infrared telescopes can’t look everywhere, so they may have missed some of these radiators. However, if these were common enough to solve the mystery of the missing mass, we would have seen them by now.