One of the great questions for humanity is whether we are alone in the universe. Indeed, astrobiologists look tantalizingly close to being able to spot the signs of life on other Earths—if it exists elsewhere—using modern observatories like the James Webb Space Telescope.
Now a group of astronomers have taken this question further by asking whether life could exist in other universes. In other words, they want to know if we are alone in the multiverse. And they have developed a way to investigate this question by considering the range of conditions that might exist in other universes.
The question arises because the fundamental constants that govern the laws of physics have values that seem perfectly aligned to allow the emergence of life.
For example, carbon atoms are formed inside stars by the fusion of three helium nuclei. But the simultaneous collision of three particles seems so improbable that carbon is unlikely to form in large quantities.
Resonant frequency
In 1954, astronomer Fred Hoyle reasoned that since there is so much carbon around—life on Earth is made of the stuff—there must be a mechanism that promotes its formation in stars. It thus predicts the famous Hoyle resonance, which increases the probability of triple collisions and the abundance of carbon by several orders of magnitude. Physicists soon began to search and found this resonance almost immediately.
The curious thing about this resonance is that it is only possible because different fundamental constants take on precise values, almost as if they were fine-tuned to produce carbon.
Cosmologists have not found any reason for this fine-tuning, leading them to suggest that these constants could easily take on any other value, perhaps in other universes. But because of that, in those places, people would not have evolved to observe it. This is called the anthropic principle, the idea that humans can only evolve in a universe with fundamental constants that allow that evolution.
All of this leads to the idea of a multiverse – that other universes may exist with fundamental constants that are different from our universe.
Now McCullen Sandora of the Blue Marble Space Institute of Science in Seattle and colleagues are asking whether life could develop in any of these other universes, and if so, what conditions would support it.
In their current work, they investigate how the cores inside stars are formed and the relative proportions of the various elements that stars leave behind when they die. This mixture forms the building blocks of a new generation of planets.
Sandora and co looked at the amount of metal produced inside stars compared to other elements; in the Hoyle resonance and how much carbon it can produce in other universes; and the stability of isotopes to see how long important elements last before decaying.
In each case, the team is investigating how different these properties might be in other universes, or in other parts of our own universe, while still allowing Earth-like life to emerge.
Their answers put interesting constraints on where life might emerge. They conclude that carbon levels have to be just right—too much or too little makes life much less likely. “Our results show that carbon-rich or carbon-poor planets are uninhabitable,” say Sandora and co.
Information carriers
The amounts of other elements are said to be less important. “Life does not depend too sensitively on nitrogen abundance,” they say. “We also find suggestive but inconclusive evidence that metal-rich planets and phosphorus-poor planets are habitable.”
Sandora and co’s work is part of a larger effort to map the conditions that make Earth-like life possible. They have already looked at the types of stars that allow photosynthesis on suitably located planets, how many Earth-like planets can form, and so on.
It’s a bold project with predictions that will be difficult to test, even if the set of conditions it explores exist elsewhere in our own cosmos.
Like most thought about extraterrestrial life, it adopts a narrow definition of life built on the carbon-based life forms found on Earth.
But more general definitions of life are possible. For example, some researchers think of life in terms of the information it passes from one generation to the next through the process of evolution. In this sense, life has evolved a system for storing, processing and transmitting information.
It puts a different perspective on life. Characterizing the universes in which this kind of information transfer can take place could be a useful trick for time-strapped astrobiologists.
Ref: Multiverse Habitability Predictions: Element Abundance: arxiv.org/abs/2302.10919