One of the key questions for astrobiologists is where in the universe life might have taken hold. Their standard approach is to look for places that are warm enough to keep water in liquid form and so allow chemistry similar to our own.

That’s given rise to the idea of circumstellar habitable zones—regions around stars that are not too hot and not too cold but just right for liquid water. Goldilocks zones as they are sometimes called.

But in recent years, planetary geologists have pointed out that various other mechanisms might keep planets and moons warm enough for liquid water. For example, tidal heating can generate considerable heat. This is the squeezing and squashing of a body as it moves through a powerful gravitational field and the process that maintains a salty ocean beneath the ice on Jupiter’s moon Europa. Radioactive decay also generates heat and keeps the interior of our planet warm.

Now Abraham Loeb at Harvard University in Cambridge says there is another mechanism that creates a Goldilocks zone but in this case the zone is in time rather than in space. He says this mechanism would have created a Goldilocks zone that filled the entire universe for a few million years soon after the Big Bang. If he’s right, that means life could have evolved some 10 billion years before it cropped up on Earth.

The key phenomenon in Loeb’s reasoning is the cosmic microwave background radiation, the afterglow of the Big Bang which fills the universe with light.

This radiation was once blazing hot. But as the universe has expanded, the wavelength of this light has increased and become less energetic. Today, it is freezing with a temperature of around 3 Kelvin.

Loeb points out that as it cooled, at some point this radiation must have once been amenable to life. Indeed, it would have been warm enough to maintain water in liquid form on a planet, regardless of its distance from its parent star.

And Loeb has calculated exactly when. He says the cosmic microwave background radiation would have had a temperature of between 273 and 300 Kelvin (between 0 and 30 degrees C) about 15 million years after the Big Bang and this would have lasted for several million years. That would have allowed “the chemistry of life to possibly begin when the Universe was merely 15 million years old,” he says.

That’s an exciting possibility but one that comes with a number of caveats. The first is the question of whether planets could have formed at all at this stage of the universe.

The question arises because to start with, the cosmos was filled only with hydrogen and helium. The heavier elements were forged inside stars and so came later.

So an important question is whether the cycle of heavy element formation could have advanced enough in these 15 million years to allow rocky planets to have formed along with the variety of elements necessary for the chemistry of life.

Loeb is optimistic on this point. He calculates that the first stars, which would have been tens to hundreds of times more massive than the Sun, had a lifespan of about 3 million years. And although they would not have formed immediately after the Big Bang, he calculates there ought to have been time enough for heavy element formation in 15 million years.

So the conditions were certainly possible that would have “triggered the formation of rocky planets with liquid water on their surface,” he says.

A much bigger question is whether life could have evolved in these conditions and in this timescale. That’s something that Loeb is unable to help with.

Nevertheless, this a fascinating idea. If life did evolve during the Universe’s Goldilocks epoch, it would have predated life on Earth by 10 billion years.

This has implications for another debate in cosmology. Astrophysicists have long wondered why conditions in the universe seem ripe for life right now. These conditions are determined by various fundamental physical constants, the values of which physicists do not understand.

The idea that the universe is fine-tuned for life is called the anthropic principle and it is the subject of much philosophical debate among cosmologists.

That life could have emerged much earlier in the universe has a significant bearing on this. “The possibility that the chemistry of life could have started in our universe only 15 million years after the Big Bang argues against the anthropic explanation,” says Loeb.

That will keep a number of prominent cosmologists awake at night. For astrobiologists, Loeb’s ideas will raise other important questions. Foremost among them will be whether there is any mechanism that could have allowed life from this era, if it did evolve, to have survived as the universe cooled down. And if so, whether there might be evidence of it today.

On the face of it, this seems unlikely. A few million years of warmth is not much for evolution to get its teeth into. And the changes that have occurred since then to every part of the universe will have been cataclysmic.

Nevertheless, interesting food for thought.

Ref: arxiv.org/abs/1312.0613 : The Habitable Epoch of the Early Universe