Space is Colder Than We Hoped ⚡ экспресс
A search for galaxy-scale thermal emission in the mid-IR from WISE data was conducted. Based on the 2MASS Redshift Survey catalog, cross-matching with CatWISE2020 and AllWISE was performed using standard masks for AGN and starbursts. W1 and W2 bands were taken as the stellar baseline, while W3 and W4 were used to constrain blackbody radiation at temperatures of 150–600 K. Conservative (3σ) upper limits on the bolometric luminosity of technogenic heat per galaxy were obtained, with median values of (5–9)×10⁸ L_⊙. At the population level, a one-sided 95% upper limit on the fraction of galaxies with excess heat at high thresholds tends to ~1/6500. Within the AGENT formalism, for a typical luminosity similar to the Milky Way (3×10¹⁰ L_⊙), α = L_WH/L_⋆ ≤ 1.7–2.9% over the entire temperature range (e.g., α ≤ 1.8% at 300 K). At 300 K, no more than ~0.016% of nearby galaxies can contain KIII-scale systems that reprocess ≥21% of stellar energy into ~300 K radiation.
Any device, from a toaster to a smartphone, heats up when operating. This is the inevitable result of energy conversion — there's always a heat trace left, like a hot stove burner after cooking. The law of entropy applies to galaxies too: if advanced civilizations live there, reprocessing the light of entire stars, they should 'glow' with infrared heat — like a kitchen where something is stewing around the clock.
Astronomers pointed the heat-sensing WISE telescope at thousands of neighboring galaxies. Using photometry — measuring brightness in different shades of thermal radiation — they compared the actual glow with what ordinary old stars produce. Natural 'hot spots', like dust around black holes, were excluded in advance. The result: not a single galaxy showed a clear excess.
The strict conclusion: no more than 0.016% of nearby galaxies can contain civilizations that turn at least a fifth of starlight into heat. The universe seems suspiciously cool, like a kitchen after the residents have moved out, leaving us in solitude.
🎯 The temperature of 300 Kelvin they were looking for is a comfortable 27°C. Remarkably, the human body radiates about as much heat as a 100-watt light bulb. From a galactic civilization, however, they expected a glow hundreds of trillions of times more powerful.
🎬 The search for engineering structures of alien races is a popular sci-fi plot. For example, in Ivan Efremov's novel 'Andromeda Nebula', the characters find a vanished civilization by its residual radiation.