The radio antennas of the Very Large Array are located under the New Mexico sky. Credit: Jeff Hellerman, NRAO/AUI/NSF
In the 1960s, famed radio astronomer Frank Drake proposed his eponymous equation, which attempted to grapple with the probability of finding extraterrestrial life – at least, the kind we could identify through radio transmissions – somewhere out there in the universe. One of the key parameters of this formulation is the star formation rate. More stars equals more planets equals more possibilities for life as we know it.
Astronomers have spent decades building a precise profile of the universe’s star formation history, estimating how much available matter is converted into stars at each epoch of cosmic history. They study it, among other reasons, because the rate of star formation is sensitive to a number of cosmological and astrophysical parameters. Change anything in the universe and you end up with different galaxy assembly histories and different star formation efficiencies.
In fact, the rate of star formation in our universe peaked well over 10 billion years ago. Since then the cosmos has produced stars, but at an ever-decreasing rate. One day, about 100 trillion years from now, the last lights will go out. One of the main factors affecting this time frame is dark energy, the mysterious force that is accelerating the expansion of the universe.
Accelerated expansion is devastating for star formation. As galaxies move further apart, they have fewer opportunities to interact and accumulate new material. This prevents new gas from entering galaxies and converting into stars, and locks existing gas in hot interstellar reservoirs, where they too cannot participate in star formation.
But what if dark energy was different? Would star formation change much, if at all? And vice versa, what impact would this have on the probability of life appearing? In other words, is our value of dark energy – about 70% of the total energy budget of the universe – a typical value?
Daniele Sorini of the Institute for Computational Cosmology at the University of Durham has led a new paper that delves into this question. He said in a press release: “Understanding dark energy and its impact 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 very existence.”
But the results were not what they expected. They found that star formation was more or less similar for a wide range of dark energy values, including significantly higher values than we see in our universe.
“Surprisingly, however, we found that even a significantly higher dark energy density would still be compatible with life, suggesting that we may not live in the most probable of universes,” Sorini said.
Considering our value of dark energy, approximately only 23% of all available matter in the cosmos will eventually transform into stars over its entire lifetime. As expected, weaker forms of dark energy lead to more efficient star formation rates, but not by much: they found that the maximum possible efficiency is 27%.
They also found that values of dark energy one hundred times greater than ours would still convert 5% of the available mass into stars. This is more than enough for multiple generations of star formation, which means more than enough material to build planets and even life.
If we assume that our universe is part of a multiverse, where each individual cosmos gets its own randomly selected value for dark energy, then our particular value is highly unlikely. The vast majority of intelligent observers (assuming that any universe that forms enough stars eventually gives rise to life) would live in a universe with much higher dark energy.
This research suggests that the strength of the dark energy we observe may be somewhat unnatural, in some sense.
Professor Lucas Lombriser from the University of Geneva in Switzerland and co-author of the study explained: “It will be exciting to use the model to explore the emergence of life in different universes and see whether some fundamental questions we ask about our Universe need to be reinterpreted.” .
