About 15 million years after the Big Bang, the entire universe had cooled down enough that the electromagnetic radiation left over from its hot beginning was around room temperature. In a 2013 article I referred to this phase as the “habitable epoch of the early universe”. If we had lived then, we would not have needed the sun to keep us warm. this cosmic radiation background would have been sufficient.
Did life start so early? Probably not. The hot, dense conditions in the first 20 minutes after the Big Bang produced only hydrogen and helium along with a tiny trace of lithium (one in 10 billion atoms) and a negligible amount of heavier elements. But life as we know it requires water and organic compounds, the existence of which had to wait until the first stars fused hydrogen and helium into oxygen and carbon some 50 million years later. The initial bottleneck for life was not a suitable temperature, as it is today, but the production of the essential elements.
Given the limited initial supply of heavy elements, how early did life actually begin? Most of the stars in the universe formed billions of years before the sun. Based on the history of cosmic star formation, in collaboration with Rafael Batista and David Sloan, I have shown that life near sun-like stars most likely began in the last billion years in cosmic history. In the future, however, it could continue to appear on planets orbiting dwarf stars, such as our closest neighbor, Proxima Centauri, which will last hundreds of times longer than the sun. Ultimately, it would be desirable for humanity to shift to a habitable planet around a dwarf star like Proxima Centauri b, where they could keep warm for up to 10 trillion years near a natural nuclear furnace (stars are just fusion reactors) by gravity, with the advantage that they are more stable and durable than the magnetically limited versions that we make in our laboratories).
As far as we know, water is the only liquid that can aid the chemistry of life – but there is a lot we don’t know. Could alternative fluids have existed in the early universe due to cosmic ray background warming alone? In a new work with Manasvi Lingam we show that ammonia, methanol and hydrogen sulfide can be present as liquids immediately after the formation of the first stars and that ethane and propane can be liquids a little later. The relevance of these substances to life is unknown, but they can be studied experimentally. If we ever succeed in creating synthetic life, as is being attempted in Jack Szostak’s laboratory at Harvard University, we could examine whether life can arise in liquids other than water.
One way to determine how early life began in the cosmos is to examine whether it formed around the oldest stars on planets. Such stars are expected to be lacking elements heavier than helium, what astrophysicists refer to as “metals.” (In our language, for example, unlike most people, oxygen is considered a metal). Indeed, metal-poor stars have been discovered on the periphery of the Milky Way and recognized as potential members of the earliest generation of stars in the universe. These stars often have an increased amount of carbon, which makes them “carbon-reinforced metal-poor” (CEMP) stars. My former student Natalie Mashian and I suggested that planets around CEMP stars could be made mostly of carbon so that their surfaces could provide a rich foundation for early life nutrition.
We could therefore look for planets traversing or passing in front of CEMP stars and show biosignatures in their atmospheric composition. This would enable us to determine by observing the age of these stars how far life in the cosmos could have started in the past. Similarly, we could estimate the age of interstellar technological devices that we discover floating near Earth (or that may have crashed on the moon) from long-lived radioactive elements or the extent of scars from the impact of dust particles on the surface.
A complementary strategy is to look for technological signals from early distant civilizations that used enough energy to make them detectable on the vast cosmic scale. One possible signal would be a flash of light from a collimated beam of light that is generated to propel light sails. Others could be linked to cosmic engineering projects, such as moving stars. Communication signals are not expected to be recognizable throughout the universe, since the signal propagation would take billions of years in each direction and no participant would be patient enough to engage in such a slow exchange of information.
But the signatures of life won’t last forever. The prospects for life in the distant future are bleak. The dark and cold conditions resulting from the accelerated expansion of the universe by dark energy are likely to wipe out all life forms in 10 trillion years. Until then, we could appreciate the temporary gifts that nature had given us. Our actions will be a source of pride for our descendants as they sustain a civilization intelligent enough to endure trillions of years. We hope that we act wisely enough to be fondly remembered in their “great story”” Books.