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One of the greatest existential mysteries—and the most difficult to answer—is whether or not Earth is all alone in this universe, carrying a lone candle of intelligent life in the darkness. Based on what we have observed, it seems that we are unique. But there are a number of possible reasons why we haven't detected the light of an alien civilization elsewhere in the Milky Way, and a number of factors that could affect whether or not it does. A little over half a century ago, these variables were compiled into a tool known as the Drake Equation, which allowed scientists to tinker and speculate.
But one variable was missing from Drake's equation, which a group led by physicist Daniele Sorini of Durham University in Great Britain incorporated into the new calculation: the effect of dark energy on the rate of star formation in the universe.
"Understanding dark energy and its effects on our universe is one of the greatest challenges in cosmology and fundamental physics," Sorini explains. "The parameters that govern our universe, including the density of dark energy, may explain our own existence."
Dark energy is an unknown force that is accelerating the expansion of the universe. Although we do not know, with what it's made up, we can tell how much is there: about 71,4 percent of the matter and energy in the universe is dark energy.
Another 24 percent is dark matter; only the remaining 4,6 percent is ordinary baryonic matter, the stuff that makes up all the stars, planets, black holes, dust, people, and everything else we can theoretically see and touch. One of our assumptions about life is that it needs a star. This may not be the case, but the probability of life appearing on a body located far from a burning energy source is so remote that it cannot be useful in the case of the Drake equation.
So, assuming a star is necessary for life, knowing the rate of star formation in a universe like ours can tell us something about the chances of finding life in it.
Stars form from clouds of dust and gas that coalesce into dense clumps, which in turn accumulate so much mass that the density and heat in their cores trigger nuclear fusion. The outward flow of dark energy plays a role in the rate at which it can occur. It counteracts the inward pull of gravity, which would otherwise watch all the matter in the universe condense into clumps too dense to form stars.
The researchers calculated this matter conversion rate for different densities of dark energy in a model universe to determine the most efficient rate at which stars can form. And they found that the most efficient rate is when 27 percent of the matter in the universe turns into stars.
What makes it interesting is that this is not the universe we live in. Our universe has a conversion rate of 23 percent. This is not the first time we have found evidence that humanity did not originate in the most optimal conditions for life, potentially raising the possibility that intelligent life could have arisen elsewhere in the universe.
"Surprisingly," Sorini says, "we found that even a much higher density of dark energy would still be compatible with life, suggesting that we may not be living in the most likely of universes."
There are many other factors that can affect the chances of intelligent life emerging. There is only one rate of star formation. Others include the number of those stars that have planets; and the number of those planets that have conditions for life. In addition, there are variables we do not know, such as how the building blocks of life are supplied and assembled into a developing system.
But each study provides insights that will one day allow us to see a bigger picture than what we see now. This, in turn, will help us narrow down how and where to look for other civilizations that may be scattered across our galaxy.
"It will be exciting," says theoretical physicist Lucas Lombreiser of the University of Geneva in Switzerland, "to use this model to investigate the origin of life in different universes and see if we need to rethink some of the fundamental questions we ask ourselves about our own universe." The study was published in The Monthly Notices of the Royal Astronomical Society.
