In 1961, American astrophysicist and astrobiologist Dr. Frank Drake devised an equation in which several factors are multiplied together to estimate the number of intelligent civilizations in our Milky Way Galaxy capable of making their presence known to humans. More than 60 years on, astrophysicists have produced a different model which instead focuses on the conditions created by the acceleration of the Universe’s expansion and the amount of stars formed. It is thought this expansion is being driven by dark energy that makes up more than two thirds of the Universe.
Artistic impression of a Multiverse. Image credit: Jaime Salcido / EAGLE Collaboration.
“Understanding dark energy and the impact on our Universe is one of the biggest challenges in cosmology and fundamental physics,” said Dr. Daniele Sorini, a researcher at Durham University’s Institute for Computational Cosmology.
“The parameters that govern our Universe, including the density of dark energy, could explain our own existence.”
Since stars are a precondition for the emergence of life as we know it, the team’s new model could be used to estimate the probability of generating intelligent life in our Universe, and in a multiverse scenario of hypothetical different universes.
The new research does not attempt to calculate the absolute number of observers (i.e. intelligent life) in the Universe but instead considers the relative probability of a randomly chosen observer inhabiting a universe with particular properties.
It concludes that a typical observer would expect to experience a substantially larger density of dark energy than is seen in our own Universe — suggesting the ingredients it possesses make it a rare and unusual case in the multiverse.
The approach presented in the paper involves calculating the fraction of ordinary matter converted into stars over the entire history of the Universe, for different dark energy densities.
The model predicts this fraction would be approximately 27% in a universe that is most efficient at forming stars, compared to 23% in our own Universe.
This means we don’t live in the hypothetical Universe with the highest odds of forming intelligent life forms.
Or in other words, the value of dark energy density we observe in our Universe is not the one that would maximise the chances of life, according to the model.
“Surprisingly, 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,” Dr. Sorini said.
The model could allow scientists to understand the effects of differing densities of dark energy on the formation of structures in the Universe and the conditions for life to develop in the cosmos.
Dark energy makes the Universe expand faster, balancing gravity’s pull and creating a universe where both expansion and structure formation are possible.
However, for life to develop, there would need to be regions where matter can clump together to form stars and planets, and it would need to remain stable for billions of years to allow life to evolve.
Crucially, the research suggests that the astrophysics of star formation and the evolution of the large-scale structure of the Universe combine in a subtle way to determine the optimal value of the dark energy density needed for the generation of intelligent life.
“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,” said Université de Genève’s Professor Lucas Lombriser.
The study was published in the Monthly Notices of the Royal Astronomical Society.
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Daniele Sorini et al. 2024. The impact of the cosmological constant on past and future star formation. MNRAS 535 (2): 1449-1474; doi: 10.1093/mnras/stae2236
