Astronomers hunting for signs of alien life today look for exoplanets within the habitable zone of stars – the region around a star that is warm enough for liquid water to exist on a planet's surface. In the deep freeze of space, worlds that are too far from their host stars should be inhospitable to life as we know it.
But space wasn't always so frigid, argues Abraham Loeb of Harvard University. The very early universe was filled with superheated gas, or plasma, that gradually cooled and condensed to form stars and galaxies. We see the first light emitted by this plasma as the cosmic microwave background (CMB).
Today, the CMB's temperature is just a few degrees above absolute zero. But Loeb calculates that about 15 million years after the big bang, the radiation would have been warm enough to make the whole universe one large habitable zone. This life-friendly epoch would have lasted a few million years, enough time for microbes to emerge but not complex life, says Loeb.
A thornier issue is whether any planets could have formed so early in the universe's history, along with the complex molecules necessary for life. When the hot plasma cooled, it initially produced only hydrogen and helium atoms. Heavier stuff had to be cooked up inside the nuclear forges of the first stars then expelled when those stars exploded.
Standard cosmology says that in most parts of the universe, the amounts of heavy elements needed to make planets didn't occur until hundreds of millions of years after the big bang. But our current understanding of the early distribution of matter is incomplete, says Loeb. If some regions were much denser than average, it is possible stars and planets formed there earlier – perhaps in time for the CMB to be the right temperature. "These planets are very rare objects that are extremely unlikely, but because the universe is so large, you could still have them," he says.
Such physical conditions might have been right for life, but any microbes would also need enough time to evolve, argues Jack O'Malley-James at the University of St Andrews, UK. Our mid-sized sun is already almost 5 billion years old and is expected to live for another 5 billion. By contrast, the earliest stars would have been much more massive with shorter lifespans, maybe only a few million years or so. "These systems would have to be very calm and stable from a very early stage to give life a good chance of gaining a foothold," says O'Malley-James.
Journal Reference : http://arxiv.org/abs/1312.0613
From New Scientist
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