On yesterday's www.slate.com website we have an article about the “early habitability epoch” theory of Harvard professor Abraham Loeb. We normally think of life as something that can only appear on planets revolving around stars, and only planets that have an orbit not too close and not too far from a star. Planets that have such an orbit are said to exist in the habitable zone, also called the Goldilocks zone (because planets in such a zone are neither too hot nor too cold). Loeb's theory is that a few million years after the Big Bang, there was a warm background temperature of space which may have allowed life to appear fairly soon in the early universe, only about 10 to 17 million years after the universe began. For a relatively brief slice of cosmic history (less than a thousandth of the universe's history), this warmth would have existed throughout the universe, in theory allowing life to arise on suitable planets, even planets beyond the normal habitable zone. The visual below is a highly schematic diagram illustrating the idea:
But this brief period of pleasant warmth did not exist for long. The universe rapidly cooled after the Big Bang, and there was only a relatively short period during which the background temperature would have been suitable to support life's evolution on a planet far away from a star. Loeb's scientific paper estimates that this period of warmth (which he calls an epoch of habitability) lasted only six million years. Any life that evolved (outside of the habitable zone) because of this brief period of warmth would have been killed as soon as this relatively short period ended, when the background temperature of space would have plummeted.
Six million years is, in reality, far too short a time to allow for the evolution of a visible organism. Loeb admits this. When asked when whether intelligent life could have evolved in this period, Loeb says, “No. I'm talking about very simple organisms like algae.”
However, it is most improbable that even one-celled organisms like algae or diatoms could have evolved within a six million time frame. Our planet is 4.6 billion years old, but the oldest traces of life are 3.5 billion years old. So we have about one billion years of something going on before there was even evidence of life. Life seems to require hundreds of millions of years of evolution before you get to something like algae or one-celled organisms.
This is one reason why Loeb's theory is not credible. Six million years is too short a time frame for any visible life to appear. We must also consider that the early universe was a place of high radiation, when any young planet would be bombarded by intense radiation causing the sterilization of any freshly evolved life. It is actually very unlikely that any planets would have formed by the beginning of the supposed habitability epoch Loeb imagines (between 10 and 17 million years after the Big Bang). Loeb's paper says that the possibility of such planets is “not ruled out” by the existing data, but he uses some special pleading to get to such a conclusion. We must also consider the fact that between 10 and 17 million years after the Big Bang, any newly formed small planet would be a kind of lava world still cooling from its initial formation, and would also be constantly bombarded by asteroids hanging around from the early formation of its solar system. A negative trifecta of high background radiation, the hellish inferno of a recent planet formation, and constant asteroid bombardment should have been sufficient to prevent life from appearing anywhere during the time frame imagined by Loeb.
The hellish environment of a newly formed planet
We need a proper theory for explaining how life (and the semantically rich genetic code) could have got started on Earth even given billions of years, something which is actually quite difficult to do, given reasons discussed here. In this light a new theory trying to squeeze an origin of life into a six million year time frame (less than one percent of the time it took on Earth) is rather laughable. But I understand the appeal to certain minds, the attraction of trying to minimize the difficulty of explaining the origin of life by suddenly imagining it occurring 200 times faster. It's kind of like a man saying to his skeptical girlfriend: “You don't think I'll become a millionaire in 30 years? I'll become a millionaire in 3 months!”
But far from being some great intellectual breakthrough, Loeb's theory is of little significance. One reason is that it is neither falsifiable nor verifiable. We could never possibly falsify the idea that life arose somewhere in the billions of galaxies between 10 and 17 million years after the Big Bang. We could also never verify such a theory.
For such a theory to be verified, we would need to find fossil traces that life had arisen shortly after the Big Bang. We couldn't find that in our solar system, since our solar system is less than 5 billion years old. We also could not find such fossil traces if we visited any other solar system. As there is absolutely no fossil evidence of life during the first billion years of earthly history, we can conclude that any life that appeared a few million years after the Big Bang (and disappeared a few million years later, when the background temperature became colder) would not have left behind any traces that someone could detect billions of years later. Any such life would at best be pre-cellular life such as self-reproducing molecules, something too primitive to leave behind fossil remains.
Loeb's theory doesn't explain anything, doesn't predict anything, and it also has no important implications. Since his theory imagines a relatively short “epoch of habitability” lasting only six million years near the beginning of the universe (which is merely a “one-night stand” from the standpoint of a 13-billion year cosmic history), the theory does not have any implications for the overall likelihood of life's current existence in the universe.
Loeb's theory is therefore a kind of insignificant curiosity, an idea without any deeper meaning, a sterile idea that we will never be able to confirm or disprove. Far from being the start of a new Copernican revolution, his theory seems to be a dead end.