The first Search for Extraterrestrial Intelligence (SETI) experiment was headed by Dr. Frank Drake in 1960 at the National Radio Astronomy Observatory in Green Bank, West Virginia. Astronomers have carried out numerous surveys in the over sixty years that have elapsed, looking for technological activity (also known as technosignatures). With advanced analytics combined with data from the Robert C. Byrd Green Bank Telescope, the Parkes Murriyang Telescope, the Automated Planet Finder, and the MeerKAT Radio Telescope, Breakthrough Listen is the most ambitious SETI experiment to date.
A survey of the 100 nearest galaxies to our own, the center of our galaxy, and the entire galactic plane are all included in the program. There are also one million closest stars to Earth. Breakthrough Listen members recently published the findings of their radio technosignature search of 97 nearby galaxies' centers, which was done using the Robert C. Byrd Green Bank Telescope. Surveying trillions of stars at four frequency bands, this search was one of the biggest and most comprehensive efforts ever made to find radio evidence of extraterrestrial intelligence. Regretfully, no outstanding candidates could be identified.
Carmen Choza, a Berkeley SETI Research Center intern with Breakthrough Listen and an assistant researcher with the SETI Institute, served as the team's leader. Colleagues from the SETI Institute and Breakthrough Listen, as well as researchers from the University of Malta's Institute of Space Sciences and Astronomy, Curtin University's International Center for Radio Astronomy Research (ICRAR), and the Green Bank Observatory (GBO), joined her.
"The Breakthrough Listen Search for Intelligent Life: Technosignature Search of 97 Nearby Galaxies," a paper summarizing their research, was just released in The Astronomical Journal.
According to their analysis, Choza and her associates' experiment comprised a narrowband Doppler drift search of 97 galaxy centers at four frequencies (1.1–2.7 GHz and 4.0–11.2 GHz). Included in the 2017 Breakthrough Listen survey, which included 123 nearby galaxies representing a full sample of morphological types (i.e., spirals, ellipticals, dwarf spheroidals, and irregulars), were these galaxies. This method differs from the majority of conventional SETI surveys in that it did not concentrate on single stars or star clusters. According to Choza's email to Universe Today:
"We anticipate that life would form on planets similar to our own when looking for life elsewhere in the universe. Numerous earlier investigations have concentrated on a single star at a time, frequently stars with known planets nearby. We can search millions of stars, and possibly millions of stellar systems with planets, for the possibility of finding a signal by aiming for the stellar densities that the galaxy centers provide.
"With the catch that the signal would need to be stronger than any signal that current human technology could generate, galaxies allow us to cast an enormous net." Thus, focusing on galaxies enables us to look for civilizations that are far more technologically sophisticated than our own. If such a signal could be detected, it would be extremely encouraging even though civilizations with the ability to produce it might be extremely rare. This is because it would mean that humankind has a real chance to advance beyond its current level of technology without completely collapsing.
At the GBO in West Virginia, the 100-meter Green Bank Telescope (GBT) collected all of the data used in this experiment. The reason the team chose the GBT was because of its backend, which makes it possible to store and analyze much more SETI data than was previously feasible. Additionally, GBT observations use a "cadence" strategy, in which sample targets are observed for five minutes, followed by the observation of an offset location that is a few beamwidths away from the target. A 30-minute ABACAD cadence is produced by repeating this pattern three times with three different offset locations (each of which is observed for five minutes).
The turboSETI pipeline was then used to analyze each cadence in order to look for signals of narrowband Doppler drifting that were linearly chirped. "This search targets narrowband, drifting technosignatures; that is, signals a few Hz wide that show frequency drift, indicating that the transmitter is accelerating relative to the Earth," Choza stated. "If it drifts, it's coming from somewhere else; it could be a transmitter on a far-off planet, satellites in orbit, or a Voyager spacecraft traveling through space. In order to search a range of accelerations one might anticipate from transmitters situated on actual exoplanets, we select a drift rate of -4 Hz/s to 4 Hz/s."
Furthermore, in order to search for potential transmitters with an equivalent isotropic radiated power of 1026 W, or 10,000 zetawatts (ZW), the team placed constraints on the data. According to Choza, this power level was selected because, on the Kardashev Scale, it represents the theoretical power consumption of a civilization that is able to utilize all of the energy in its star system, or a Type II Civilization:
"By utilizing a well-characterized tool such as the Green Bank Telescope and making certain assumptions about the signals we are looking for, we can determine the lowest power that an isotropic signal—one that propagates throughout the universe in all directions—would need to send out in order for us to be able to detect it. Our search could find a hypothetical beacon transmitting at a power of about 1026 Watts, which is comparable to the sun's total power output, for the farthest galaxies in our sample. It is theoretically possible for a Kardashev Type II civilization to build a beacon large enough to facilitate interstellar communication because they are thought to be able to harness the full power resources of their host star."
Ultimately, 1,519 candidate signals that were not caused by radio frequency interference were obtained by the team. Following a thorough visual inspection, algorithmic processing, and correlation of signal characteristics with known RFI populations, they were unable to find any convincing evidence of technosignatures. But this most recent survey was innovative in a lot of ways, and it will have a big impact on future SETI research. Choza clarified that when looking for uncommon signals, it's critical to maximize the field of view and to carefully take into account foreground and background sources:
Given that we don't yet know how many or how bright extraterrestrial transmitters may exist, this survey marks a significant milestone in the accomplishment of the Breakthrough Listen mission's original search objectives. It also serves as a turning point in the development of new search techniques to enhance and re-analyze earlier searches. We impose the strongest limitations on the existence of technosignatures in neighboring galaxies to date."
This paper represents the result of a year's work and numerous authors' contributions to advancing technosignature science toward ever-deeper constraints and ever-larger numbers of star systems, as well as to improving Breakthrough Listen techniques. For me as well as other young people, the program has been a fantastic means of introducing them to science, and some of the most intriguing papers that have come out of the partnership are being led by interns, postbacs, or graduate students."
These findings may also contribute to the understanding of other Breakthrough Listen searches, such as the one we have in store for our own galactic center, a sample of almost 2,000 nearby stars, and another sample of galaxies observed with the Parkes Murriyang Telescope from the Southern Hemisphere.
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