The Habitable Epoch of the Early Universe
In recent weeks, the world of cosmology has been buzzing with the news that gravitational waves - remnants of the Big Bang - may have been detected by the BICEP2 experiment. But did life come not long afterwards?
Abraham Loeb from Harvard University has posited in his paper The Habitable Epoch of the Early Universe that conditions were rife for life just 10 million years after the Big Bang. Life on Earth is about 3.5 billions years old, and it took about 1 billion years to first appear after the Earth was formed. So to think that life could have formed only 10 million years after the Big Bang - a blink of the eye in cosmological terms - certainly goes against conventional thinking.
To come to this conclusion, Loeb looked at the conditions needed for life to take hold. Astrobiologists talk of the Goldilocks zone - an orbit around a star where a planet is not too hot or too cold to have liquid water, just like here on Earth. On Earth, we are aided by an atmosphere that keeps temperatures mild. But there are other ways that a planet can stay warm enough for liquid water - tidal heating, for example, is thought to maintain a liquid ocean under the surface of Jupiter's moon Europa.
Loeb postulates another mechanism, one that I presume wouldn't be particularly good for your health if we could replicate it, but nonetheless would keep water in liquid form. In fact, he proposes that the whole Universe would have had these conditions. The cosmic background radiation is the afterglow of the Big Bang, fills the Universe and these days has a temperature of about 3K. But it wasn't always this cold, and moments after the Big Bang it would have had a temperature of billions of degrees (more actually). It has been cooling since then, and around 10-17 million years after the origin of the Universe, the cosmic radiation would have made the Universe nice and balmy with liquid water.
But even if the temperature was right, is 10-17 millions years long enough for rocky planets to form on which life can live? And were there enough heavy elements around to get the chemistry of life going? Loeb thinks maybe. Matter was pretty evenly spread around the Universe at this age, but some areas would have been more dense than others. Where the matter was more or less dense than the average, this is called a perturbation. Assuming these perturbations had a Gaussian distribution (the classic bell-shaped normal distribution), massive stars of Hydrogen and Helium could have formed at the very edge of the distribution - 8.5 standard deviations from the mean. This is pretty unlikely; if you've done management courses you'll know that 6 standard deviations from the mean (that is, Six Sigma) is what you are aiming for when detecting defects. If you're making a product, a Six Sigma event would happen roughly every few hundred million products. A 8.5 sigma event would occur less than once every few hundred trillion products.
What this means is that if the density of the early Universe had Gaussian perturbations (and it's not settled science that it did), it's not very likely such stars could have formed and in the process created heavy elements, but the Universe is a big place!
Imagine then that there were rocky planets formed from exploding first generation stars that contained the elements of life in this temperate Universe. Could there be life? 10 millions years is not a long time for life to form - it took a billion years on Earth, and then longer to evolve. But let's say it did. Could it still be out there? Well, the issue with having cosmic radiation as life's heat source is that it cooled down over time and when the Universe was 17 million years old, it wouldn't have been warm enough for liquid water. So the planets on which life resided needed to have their own heat source, and then perhaps life could have escaped through panspermia. Loeb recommends that astronomers look for biosignatures in really old stars to further investigate, something that is now technically possible as we discover more early generation stars.
Loeb also makes a more philosophical point. Some proponents of the anthropic principle claim that various fundamental physical constants are what they are - some say "fine tuned" - because they must be those values to bring about life. Loeb argues that anthropic arguments are weak, at least with regards the cosmological constant, which describes the density of energy in the Universe, as this habitable epoch would have existed for various values of the constant.
I was actually going to post a few other astrobiology arXiv papers, but I think this is enough for one Saturday! I'd be interested to hear what others think of this idea that life could have existed so early in the Universe's life.
References:
- Abraham Loeb (2013). The Habitable Epoch of the Early Universe arXiv arXiv: 1312.0613v2