In 1960, in preparation for the first SETI conference, Cornell astronomer Frank Drake formulated an equation to calculate the number of detectable extraterrestrial civilizations in our Milky Way. Rather than being a scientific principle, the equation was intended as a thought experiment that summarized the challenges faced by SETI researchers. This became known as the Drake Equation, which remains the basis for the Search for Extraterrestrial Intelligence (SETI) to this day. Since then, astronomers and astrophysicists have proposed many updates and revisions to the equation.
The background to this is ongoing research into the origins of life on Earth and the conditions that led to its emergence. In a recent study, astrophysicists led by Durham University have developed a new model for the emergence of life that focuses on the acceleration of the universe's expansion (also known as the Hubble constant) and the number of stars formed. Since stars are crucial to the emergence of life as we associate it, this model could be used to estimate the likelihood of intelligent life in our universe and beyond (i.e. in a multiverse scenario).
The study was led by Daniele Sorini, a postdoctoral researcher at the Institute for Computational Cosmology at Durham University, and was funded by a European Research Council (ERC) grant. She was joined by John Peacock, Professor of Cosmology at the Royal Observatory and the Institute for Astronomy at the University of Edinburgh, and Lucas Lombriser from the Département de Physique Théorique at the Université de Genève. The paper detailing their findings was recently published in the Monthly Notices of the Royal Astronomical Society.
The Drake Equation is a mathematical formula for the probability of finding life or advanced civilizations in the universe. Photo credit: University of Rochester
As previously mentioned, the Drake Equation was not intended as a tool for estimating the number of extraterrestrial intelligences (ETIs), but rather as a guide for how scientists should search for life in the universe. The formula for the equation is:
N = R* x fp x ne x fl x fi x fc x L
While N is the number of civilizations in our galaxy that we can potentially communicate with, R* is the average star formation rate in our galaxy, fp is the fraction of stars that have planets, and ne is the number of planets that can actually support life support, fl is the number of planets that will develop life, fi is the number of planets that will develop intelligent life, fc is the number of civilizations that would develop transmission technologies, and L is the amount of time available to these civilizations would stand around to transmit their signals into space.
In the same spirit, the new research does not attempt to calculate the absolute number of intelligent species in the universe. Instead, the team presents an analytical model for cosmic star formation history to measure the influence of cosmological parameters within the most widely used cosmological model. This is none other than the Lambda-Cold Dark Matter (LCDM) model, in which dark matter and dark energy (lambda) make up about 95% of the matter-energy density of the universe. The remaining 5%, the “normal” matter we see every day, is what scientists call baryonic matter (also known as “luminous matter”).
In their work, the team calculated the fraction of ordinary matter converted into stars throughout the history of the universe based on different dark energy densities. Stars are vital because they produce heavier elements through nuclear fusion, which enable planet formation, biochemistry and all life as we know it. Their model predicts that the most efficient density for star formation would be 27%, compared to the 23% that scientists have observed in our universe. In short, their results suggest that our universe is an outlier in the context of the multiverse.
Early dark energy may have caused the early seeds of galaxies (shown on the left) to give rise to many more bright galaxies (on the right) than theory predicts. Photo credit: Josh Borrow/Thesan Team
These findings could have significant implications for cosmology and the ongoing debate about whether or not our universe is “fine-tuned” for life. Like Dr. Sorini in a press release from the Royal Astronomical Society stated:
“Understanding dark energy and its effects on our universe is one of the greatest challenges in cosmology and fundamental physics. The parameters that govern our universe, including the density of dark energy, could explain our own existence. “Surprisingly, however, we found that even a significantly higher density of dark energy would still be consistent with life, suggesting that we may not live in the most likely of universes.”
The new model could also shed light on how different densities of dark energy affect the formation of the universe and the development of conditions that enable the emergence of life. The influence of dark energy drives cosmic expansion and causes the large-scale structures of the universe (galaxies and galaxy clusters) to move further and further apart. For life to evolve, matter must be able to clump together to form stars and planets and remain stable for billions of years – because evolution is a long-term process that takes billions of years.
Another insight from this research is that star formation and the evolution of the large-scale structure of the universe reach equilibrium over time. This balance determines the optimal value of dark energy density required for the emergence of life and the eventual development of intelligent life. Prof Lombriser said: “It will be exciting to use the model to explore the emergence of life in different universes and to see whether some fundamental questions we ask about our own universe need to be reinterpreted.”
The Drake equation may require additional parameters, including a lambda energy density parameter (ld) and a multiverse parameter (mv). Regardless, the search for life and the question of how it can arise remains, much like Frank Drake's equation itself!
Further reading: Royal Astronomical Society, MNRAS
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