Cosmic Giants: How Alien AI May Be Building Megastructures Around Dark Matter Black Holes
For generations, the profound question, "Are we alone?", has driven human curiosity, evolving from ancient philosophical debates to the sophisticated scientific investigation known today as astrobiology. While the initial search for extraterrestrial life focused on finding simple biological systems or listening for intelligent radio signals (SETI), modern research acknowledges a critical possibility: intelligent life has a nonzero chance of eventually being succeeded by, or even dominated by, advanced Artificial Intelligence (AI). This recognition refocuses the cosmic hunt onto the Search for Extraterrestrial Artificial Intelligence (SET-AI). Since a primary characteristic of AI is its capability for extensive calculation and storing data, advanced AI civilizations are expected to have an enormous, persistent demand for energy. This leads researchers to hypothesize that the most promising way to find ET-AI is not by listening for faint messages, but by detecting the monumental power plants they would need to build.
Measuring the Might of Cosmic Civilizations
To systematically categorize civilizations based on their energy use, scientists employ the Kardashev scale. This scale, proposed by Nikolai Kardashev in 1964, defines three main levels:
Type I Civilization: Consumes all the energy available on its home planet.
Type II Civilization: Utilizes all the energy generated by its host star, often achieved through massive artificial constructions known as Dyson spheres.
Type III Civilization: Harnesses the entire energy output of its host galaxy.
In this framework, advanced civilizations are placed on the same footing as those dominated by AI. To quantify the actual physical scale of these advancements, a new metric has been introduced: the Space Exploration Distance (SED) ($\Delta_{sed}$). The SED measures how far a civilization explores space to collect the massive energy needed for its operations.
Humanity, for instance, currently sits far below Type I, with a civilization parameter (K) estimated at around 0.7. Our current SED is modest, tied to stable structures like space telescopes placed at the Sun–Earth Lagrangian 2 point ($\sim$0.01 au, where au stands for Astronomical Unit, the distance from the Earth to the Sun). In contrast, a Type I civilization's structures would reach astronomical unit scales ($\sim$1–2 au). A Type II civilization, which uses all the energy of its star, would have an SED extending to the gravitational boundaries of its solar system, such as the Oort cloud (around $10^5$ au). Finally, a Type III civilization would explore space on the scale of the entire Milky Way disk (approximately $4 \times 10^9$ au).
Researchers suggest that ET-AI will likely be situated between Type II and Type III. If they explore space significantly beyond their own star system, they must seek energy from sources beyond their local star. The relationship between the power consumed ($P$) and the SED is tightly linked, meaning greater power demands require a larger exploration distance.
Why Black Holes Are Better Than Stars
For civilizations operating far from their home stars, other energy sources must be harnessed. Recent studies indicate that black holes (BHs) can be more effective and promising energy sources than main-sequence stars. The primary energy from a BH comes from its accretion disk, where matter is heated by friction due to the immense gravity before falling in.
The most intriguing candidates for energy harvesting are Primordial Black Holes (PBHs). PBHs are a potential candidate for Dark Matter (DM), the mysterious substance making up most of the matter in the universe. If PBHs constitute even a fraction of DM ($f_{PBH}$), they would be abundant and thus represent the most reliable and copious energy source available in the cosmos.
A solar-mass PBH, even with a conservative assumption about its accretion efficiency ($\eta_{disk} \sim 10^{-4}$), can generate power ranging from 0.032 to 320 times the luminosity of the Sun ($L_e \sim 3.8 \times 10^{26}$ W). Furthermore, a major architectural advantage is that Dyson sphere–like structures can be built closer to a PBH than to a star because the PBH’s accretion disk is much smaller.
The AI Blueprint: Harvesting Low-Temperature Heat
An AI-based society, being fundamentally computational, needs low temperatures for high performance. To achieve this, an ET-AI civilization would build its Dyson sphere–like structures at a specific distance from the PBH or star to capture energy at the desired low temperature.
Since these megastructures act as blackbodies, they must radiate away waste heat, primarily in the infrared spectrum. The required distance scales dramatically depending on the target temperature.
If the civilization seeks temperatures suitable for liquid water or classical computing ($\sim$300 K), the structure would be $\sim$1.7 au from a star, or $\sim$3.0 au from a solar-mass PBH.
For advanced, low-temperature computation, such as 30 K, the structure must be enormous and far away: $\sim$170 au from a star, or approximately 300 au from a solar-mass PBH.
The feasibility of using PBHs to achieve high Kardashev levels depends critically on the fraction of PBHs ($f_{PBH}$) present in the Dark Matter halo, rather than the mass of the individual PBH. The more PBHs there are, the greater the energy that can be harvested to meet the demands of advanced civilization.
The Observational Fingerprint: Submillimeter Excess
The strategy for finding ET-AI hinges on detecting the waste heat radiated by these distant, low-temperature Dyson spheres. While searching for Dyson spheres around stars is challenging due to the potential confusion with natural infrared sources like circumstellar dust, searching around PBHs offers a potentially cleaner signal.
If an advanced civilization operates its Dyson sphere at an extremely low temperature, such as 10 K, the waste heat will be radiated as blackbody radiation peaking at a wavelength of approximately 0.3 millimeters (mm). This is known as the submillimeter excess, emitted from an extended structure, potentially reaching $\sim$2700 au around a solar-mass PBH.
This submillimeter signal is precisely what the Atacama Large Millimeter/submillimeter Array (ALMA), particularly its Band 10 detector, is designed to observe. The resulting spectrum would be a multiblackbody spectrum with two distinct peaks: high-frequency X-ray and UV radiation originating from the PBH accretion disk, and the low-frequency submillimeter radiation coming from the vast, low-temperature Dyson sphere megastructure.
ALMA's high angular resolution ($\sim$0.5 arcseconds) means researchers can theoretically search our own Milky Way galaxy for direct imaging of these structures up to scales of approximately 5.4 kiloparsecs (kpc). Furthermore, this search can be extended to nearby galaxies, probing up to 2 megaparsecs (Mpc) for the 0.3 mm excess. For galaxies at higher cosmological distances, direct imaging is not possible, but researchers can still look for a general sign of ET-AI by observing an excess of infrared radiation plotted against the galaxy’s UV continuum slope.
In conclusion, the proposed SET-AI strategy marks a significant new direction in the search for intelligent life. By focusing on interstellar Dyson sphere–like megastructures powered by the reliable energy source of Primordial Black Holes—which may constitute part of Dark Matter—scientists have developed a unique, observationally grounded approach. Detecting the specific submillimeter waste heat signatures offers the possibility of uncovering the existence of advanced computational civilizations far beyond our current understanding.
