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When did life first stir in the vast expanse of our universe? This tantalizing question has intrigued scientists for generations. Imagine a time, approximately 15 million years after the Big Bang, when the cosmos was bathed in radiation at room temperature. In a groundbreaking 2013 paper, Avi Loeb dubbed this era the "habitable epoch" of the early universe. Could life have emerged during this primordial period, obviating the need for a sun to keep us warm? The cosmic radiation background from our fiery beginning might have been sufficient. But did life truly begin that early?
The first 20 minutes of our hot, dense universe birthed hydrogen and helium, with a mere whisper of lithium. However, life as we know it requires water and organic compounds, which necessitated the fusion of hydrogen and helium into oxygen and carbon within the cores of the first stars. Roughly 50 million years later, the crux of life's inception was not the temperature, as it is today, but rather the synthesis of essential elements, given the meager initial supply of heavy elements.
How early did life actually begin? Most stars in the universe formed billions of years before our sun. Loeb, in collaboration with Rafael Batista and David Sloane, demonstrated that life near sun-like stars likely formed over the past few billion years. Yet, it may continue to emerge around dwarf stars like our nearest neighbor, Proxima Centauri, which could persist hundreds of times longer than our sun. Ultimately, it might be feasible for humanity to settle on a habitable planet around a dwarf star like Proxima Centauri b, where we could bask in the glow of a natural nuclear furnace for up to 10 trillion years.
Stars are merely fusion reactors, confined by gravity, offering a stability and durability surpassing our magnetically confined laboratory versions. Could water be the only liquid capable of supporting life's chemistry? A new paper by Monosphere Lingam and colleagues suggests that ammonia, methanol, and hydrogen sulfide could exist as liquids shortly after the first stars formed, with ethane and propane potentially joining the list even later. The relevance of these liquids to life remains unknown, but if we succeed in creating synthetic life, as attempted at Harvard University's Jack's Sostax Laboratory, we might uncover whether life can emerge in liquids other than water.
One method to determine when life began is to investigate whether it formed on planets orbiting the oldest stars, relics from the early universe. These stars, deficient in elements heavier than helium, are known as metal-poor stars. Often rich in carbon, these carbon-enhanced metal-poor (CEMP) stars might host planets predominantly composed of carbon, providing a fertile ground for early life. By searching for planets transiting CEMP stars and analyzing their atmospheric composition for biosignatures, we might uncover how far back in time life began.
Alternatively, we could estimate the age of interstellar technological equipment discovered near Earth or on the moon, using long-lived radioactive elements or the extent of impact scars from dust particles. Another strategy involves searching for technological signals from ancient civilizations that harnessed sufficient energy to be detected across cosmic distances. These signals might include a flash of light from a collimated light beam used to propel light sails or signs of cosmic engineering projects, such as moving stars.
Yet, the prospects for life in the distant future are grim. The accelerated expansion of the universe will likely snuff out all forms of life in 10 trillion years. Until then, we must cherish the fleeting gifts nature has bestowed upon us. Our actions today will be a source of pride for our descendants if we sustain a civilization intelligent enough to endure for trillions of years. Here's to hoping we act wisely enough to be remembered favorably in their annals of history.
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