[LINK] "Are we surrounded by Dyson spheres?"

[rfmcdonald]

io9 shared the welcome news that there are no supercivilizations using Dyson spheres--briefly, either a solid shell or a dense halo of artificial constructs orbiting a star, capturing all of the energy it produced--in our galactic neighbourhood. Physicist Dave Goldberg has three reasons.

1) We could still see them, just not with our regular eyes

Dyson introduced his Spheres not so much as a "how to become a supercivilization" instruction manual as a "how to find a supercivilization" manual. Everything that takes in energy ultimately re-radiates it. This is true, on average, of the earth, for instance, and if it weren't we'd heat up at an alarming rate. Likewise, you absorb light and take in fuel and as a result, you heat up and glow in the dark, though not in the wavelength range that our eyes are sensitive to. You glow in the infrared rather than the visible. This is how night-vision goggles work.

[. . .]

2) Gravity leaves a trace.

One of the reasons that I like this question is that it goes far beyond the specifics of Dyson Spheres, and gets at something much more general. Most cosmologists think that Dark Matter is some sort of particle — just one we haven't yet discovered. But think about it, couldn't there be some giant spheres: maybe Dyson Spheres, maybe ordinary black holes, flying around our Galaxy? If they're dark, we'd never see them.

These objects have a name: Massive Compact Halo Objects (or, their juvenile acronym MACHOs). Fortunately, we don't need to detect MACHOs from their light alone. In 1936, Einstein predicted an effect of general relativity known as "microlensing." Microlensing uses the gravitational focusing power of gravity to temporarily brighten a star when it passes behind a massive object — any massive object. This would include anything: ordinary stars, black holes, brown dwarves, and even Dyson Spheres. Einstein wasn't terribly optimistic about actually detecting this since only about 1 in a million stars gets microlensed. Fortunately, modern observatories are able to observe millions of stars simultaneously, and we get to observe lots and lots of microlensing events.

[. . .]

3) The Big Bang Limits our Chemistry Set

The problem with Dyson Spheres (and any other MACHOs) is that they're ultimately made from boring old atoms, and it turns out, there's just not enough atoms out there. In many ways, physics in the early universe was way simpler than it is now. With a few relatively simple assumptions, we can make a lot of predictions. This may perhaps be a question for another day, but we're able to get extraordinarily accurate measurements of the baryon (aka ordinary matter) to Dark Matter ratio from the Cosmic Microwave Background, and completely independently get the same ratio by measuring the amount of various light elements in the universe today.

For instance, suppose we assume that there's about 5 times as much Dark Matter as baryonic matter (the number that we get from the Cosmic Microwave Background estimates). From that simple input, we can predict all sorts of things, including the fraction of atoms that are in the form of helium, the fraction that are in the form of deuterium (a sort of super hydrogen with a neutron as well as a proton), and so on, for the other light elements. These numbers are dead-on with the ratios that we actually observe. Increase the fraction of "ordinary matter" (or equivalently decrease the amount of dark matter) by even a few percent and a bunch of observations suddenly become totally inconsistent. What's more, these estimates of the baryons are pretty much in line with the numbers that we get by just counting up the mass in gas and stars in galaxies.