New Study Examines Cosmic Expansion, Leading to a New Drake Equation

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 SETI researchers faced. This became known as the Drake Equation, which remains foundational to the Search for Extraterrestrial Intelligence (SETI) to this day. Since then, astronomers and astrophysicists have proposed many updates and revisions for the equation.

This is motivated by ongoing research into the origins of life on Earth and the preconditions that led to its emergence. In a recent study, astrophysicists led by Durham University produced a new model for the emergence of life that focuses on the acceleration of the Universe’s expansion (aka. the Hubble Constant) and the number of stars formed. Since stars are essential to the emergence of life as we knot it, this model could be used to estimate the probability of intelligent life in our Universe and beyond (i.e., in a multiverse scenario).

The study was led by Daniele Sorini, a postdoctoral Research Associate at Durham University’s Institute for Computational Cosmology, and was funded by a European Research Council (ERC) grant. She was joined by John Peacock, a Professor of Cosmology at the Royal Observatory and the University of Edinburgh’s Institute for Astronomy, and Lucas Lombriser, from the Département de Physique Théorique, Université de Genève. The paper that details their findings was recently published in the Monthly Notices of the Royal Astronomical Society.

As noted, the Drake Equation was not intended as a tool for estimating the number of extraterrestrial intelligences (ETIs) but as a guide for how scientists should search for life in the Universe. The formula for the equation is:

N = R^* x f_p x n_e x f_l x f_i x f_c x L

Whereas N is the number of civilizations in our galaxy that we might able to communicate with, R^* is the average rate of star formation in our galaxy, f_p is the fraction of those stars that have planets, n_e is the number of planets that can actually support life, f_l is the number of planets that will develop life, f_i is the number of planets that will develop intelligent life, fc is the number civilizations that would develop transmission technologies, and L is the length of time that these civilizations would have to transmit their signals into space.

In the same sense, 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 impact of cosmological parameters within the most widely accepted cosmological model. This is none other than the Lambda-Cold Dark Matter (LCDM) model, where Dark Matter and Dark Energy (Lambda) account for roughly 95% of the matter-energy density of the Universe. The remaining 5%, the “ordinary” matter we see every day, is what scientists refer to as baryonic matter (aka. “luminous matter”).

In their paper, the team calculated the fraction of ordinary matter that is converted into stars over the entire history of the Universe based on different Dark Energy densities. Stars are essential to life, creating heavier elements through nuclear fusion that allow for 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 23% scientists have observed in our Universe. In short, their results suggest that our Universe is an outlier in the context of the multiverse.

These findings could have significant implications for cosmology and the ongoing debate about whether or not our Universe is “fine-tuned” for life. As Dr. Sorini explained in a Royal Astronomical Society press release:

“Understanding Dark Energy and the impact on our Universe is one of the biggest challenges in cosmology and fundamental physics. The parameters that govern our Universe, including the density of dark energy, could explain our own existence. Surprisingly, though, we found that even a significantly higher dark energy density would still be compatible with life, suggesting we may not live in the most likely of Universes.”

The new model could also provide insight into how differing densities of Dark Energy affect the formation of the Universe and the development of conditions that allow life to emerge. The influence of Dark Energy drives cosmic expansion, causing the large-scale structures of the Universe (galaxies and galaxy clusters) to move farther and farther apart. For life to develop, matter must be able to clump together to form stars and planets and remain stable for billions of years – since evolution is a long-term process lasting billions of years.

Another takeaway from this research is that star formation and the evolution of the large-scale structure of the Universe achieve a balance over time. This balance determines the optimal value of Dark Energy density needed for the emergence of life and the eventual development of intelligent life. Said Prof. Lombriser: “It will be exciting to employ the model to explore the emergence of life across different universes and see whether some fundamental questions we ask ourselves about our own Universe must be reinterpreted.”

The Drake Equation may need additional parameters, including a Lambda energy density (l_d) and a multiverse (m_v) parameter. Regardless, the search for life and the question of how it can arise endure, much like Frank Drake’s equation itself!

Further Reading: Royal Astronomical Society, MNRAS