A team searched 129 galaxies for the heat that could be produced by a civilization using a large share of the energy from its stars.
The idea relies on a simple physical consequence. Technology that captures enormous amounts of energy must ultimately release some of it as heat. This heat could appear strongly in the infrared, a form of light invisible to our eyes. The researchers therefore looked for an anomalous thermal signature on the scale of entire galaxies.
Example of a Dyson structure around a star.
Wikimedia image
To understand what they are looking for, we can suppose that advanced civilizations might build installations surrounding their star in order to collect its radiation. These are known as Dyson swarms. They do not necessarily consist of solid spheres. Billions of collectors could, in theory, intercept a portion of starlight and re-emit some of the absorbed energy as heat.
The problem comes from the galaxies themselves. Young stars, clouds of heated dust, or an active black hole can also produce large amounts of infrared radiation. A simple detection is therefore not enough. The team simultaneously modeled the light from stars, gas, and dust, then added the expected signature of Dyson swarms.
This method was applied to 129 nearby galaxies with highly varied profiles. The researchers also performed 1,419 tests by artificially adding a technological signal to the data. They then checked whether their method could recover it. In quiet galaxies, it detected installations intercepting about 4 to 5% of their star's light.
The measurements become particularly strong for quiet galaxies without a dominant active nucleus. For these galaxies, the median limit corresponds to less than 0.3% of the starlight re-emitted by hot swarms.
The authors identify the outer regions of quiet galaxies in particular as interesting targets. These areas contain less hot dust and are less affected by the activity of a central black hole.
Future infrared observatories could further reduce the ambiguities. The study specifically mentions observations capable of better distinguishing natural dust from possible artificial heat. Targeted images with the James Webb Space Telescope could then examine the most unusual candidates.