Thursday, July 18, 2024

Could Dyson Spheres Be The Universe’s Missing Mass?

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

First, What’s A Dyson Sphere?

Only a tiny fraction of the Sun’s energy falls on its planets, with the rest escaping into space. In 1937, science fiction writer Olaf Stapledon wrote a book, The Star Maker, that explored ideas of vastly more advanced civilizations’ quest for energy. The book inspired the physicist Freeman Dyson to propose that such civilizations might build giant thin surfaces in space to capture more of their stars’ energy, eventually partially or entirely encircling the star. 

Dyson noted that such structures would block the visible light from the star to observers elsewhere, but would radiate in infrared. Consequently, he argued, a way to find advanced extraterrestrial civilizations might be to look for infrared-dominated spectra.

The idea captured a lot of people’s imaginations and achieved a surge in popularity when the mystery of KIC 8462852 (also known as Boyajian’s star) emerged in 2015. KIC 8462852 undergoes significant dips in brightness on irregular intervals, far too large to be the result of planets blocking its light. There was so much speculation that the observed behavior might 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 sorts of mass our surveys of the local universe have failed to find. The more famous of these is dark matter, the mass needed to explain the movements of galaxies under the laws of gravity. The other sort of missing mass is more regular material, probably mostly composed of hydrogen and helium, unlike dark matter, which is most likely to be exotic particles

When astronomers talk about “missing mass”, they mean the second sort. We know that this category is made of regular 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 can only see about two-thirds of that amount.

There is a lot less mass missing in this category than dark matter, but still an awful lot of it. Among the explanations are enormous filaments of gas stretching between galaxies

So Could Dyson Spheres Account For Either Sort Of Missing Mass?

Sadly, almost certainly not.

Once people got over how cool Dyson Spheres would be, and having fun with the potential science fiction ideas of living on the inside of something so mind-blowingly huge, physicists contemplated the practicalities. And it turns out that complete Dyson Spheres just don’t make sense.

The material for a Dyson Sphere would need to come from somewhere. It’s very unlikely that even the most advanced civilization would be able to scoop matter from their star and turn it into something solid. If they could, they probably wouldn’t be relying on stellar energy anyway. Therefore, the material of the Sphere would need to be made of planets, moons, and asteroids.

Some star systems have more mass in orbit than ours, others probably less. But there’s no reason to think we’re unusually light in that department.

That means that there wouldn’t be all that much mass in the sphere itself, even if you used every scrap of solid material in the planetary system. If the question was intended to mean “Could the material in Dyson Spheres be so enormous it accounts for a large portion of the missing matter?” then you’d have to explain where that matter came from in the first place. Scouring the space between the stars and finding rogue planets or other sources of material so they could be turned into backing for solar panels is unlikely to be practical.

The other way to interpret the question is: “Could there be billions of stars surrounded by Dyson Spheres that catch all their light so we can’t see them, thus making the galaxy much more densely packed with stars than we think?” That’s generally what people mean.

The popular, but almost certainly incorrect, vision of the Dyson Sphere, is one that gets steadily built up until the star is surrounded by a complete sphere.

However, given the amount of solid material in the Solar System, any completely encircling 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 use vast amounts of energy, making the whole idea a net loss.

If Dyson Spheres exist at all, they’re very incomplete, either thin “Dyson Rings”, or networks of patches collecting a few percent or less of the star’s light. These are sometimes referred to as Dyson Swarms

Were a star orbited by a Dyson Swarm, we would see it, dimmed by the occasional blip as the portion got between us and it – the hypothetical situation that made KIC 8462852 famous. Dozens of stars have been identified where this could be happening, although other explanations are more likely.

In a case like this, the star would not go missing for any extended period. Consequently, our estimates of the number of stars in the galaxy would not be wrong by much, if at all. Any small undercount could only be responsible for a tiny proportion of the missing matter.

Even if a complete Dyson Sphere was built, an essential feature of the concept is that it would radiate in the infrared. Dyson wanted us to be on the lookout for that sort of infrared signal. The JWST and our few other infrared telescopes cannot be looking everywhere so they may have missed a few such radiators. However, if these were common enough to solve the mystery of the missing mass, we should have seen them by now.

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