In a paper posted on arXiv on 2023-04-05, “Upper limits on transmitter rate of extragalactic civilizations placed by Breakthrough Listen observations”, the authors searched for signals from extraterrestrial civilisations of Kardashev Type II (able to harness all the energy from a single star) or Type III (able to harness the energy of one entire galaxy) present in background galaxies that appear within the field of view of stars observed by the Breakthrough Listen SETI survey. No candidate signals were found, from which the statistical conclusion is drawn that, assuming one potential civilisation per one solar mass star system and the narrowband radio frequency output of the advanced civilisation greater than 7.7\times 10^26 watts (around twice the power output of the Sun), the frequency of occurrence of such civilisations in galaxies within 969 megaparsecs (3.16 billion light years) is fewer than one per hundred trillion stars.
Here is the abstract:
The Search for Extra-Terrestrial Intelligence (SETI) has been conducted for over sixty years, yet no technosignatures have been identified. Previous studies have focused on stars in our galaxy, with few searches in the extragalactic Universe despite a larger volume being available. Civilizations capable of harvesting energy from a star or a galaxy are classified as KII or KIII on the Kardashev scale, respectively. Technosignatures from such advanced civilizations would be extremely luminous and detectable by current radio telescopes, even from distant galaxies. To explore the frontier of extragalactic SETI, we investigate the likely prevalence of extragalactic civilizations possessing a radio transmitter, known as the transmitter rate, based on observational results from the Breakthrough Listen (BL) observations. We calculated the transmitter rate by considering the background galaxies in the field of view of target stars in BL observations. We used a statistical method to derive the total mass of stars in those background galaxies from a galaxy stellar mass function. Our statistical method suggests that less than one in hundreds of trillions of extragalactic civilizations within 969 Mpc possess a radio transmitter above 7.7\times 10^26 W of power, assuming one civilization per one-solar-mass stellar system. Additionally, we cross-matched the BL survey fields with the WISE×SuperCOSMOS Photometric Redshift Catalogue and compared with the statistical method. Our result sets the strictest limits to date on the transmitter rate at such high power levels, emphasizing the high efficiency of searching for radio transmitters in galaxies and the rarity of technologically advanced civilizations in our Universe.
An earlier search for energy emitted by advanced civilisations was Ĝ (G-HAT), where the acronym stands for “Glimpsing Heat from Alien Technologies”. The link is to an overview of the project and results and publications are linked to the page “The Ĝ Search for Kardashev Civilizations”.