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Habitability Around Ancient Stars

I see a lot to like about Abraham Loeb’s new paper “The Habitable Epoch of the Early Universe,” available as a preprint and now going through the submission process at Astrobiology. Not that it isn’t controversial, and for reasons that are patently obvious as soon as one digs into it. But the sheer chutzpah of postulating that microbial life might have started up no more than ten or fifteen million years after the Big Bang takes the breath away. This is a notion that extends life so far back that it defies our conventional models of how it formed.

Temperatures aren’t the problem, given that radiation in the early universe would have produced cozy conditions for a multi-million year window of time as what is now called the cosmic microwave background (CMB) continued to cool. The problem is that we have to get from hydrogen and the helium created by fusion in the Big Bang furnace to heavy elements that are usually explained by large stars seeding the cosmos with supernovae explosions. So we need to figure out whether or not such stars could have formed at this remarkably early epoch.


Image: An artist’s impression of how the very early universe might have looked as star formation began. Photograph: Adolf Schaller/AP.

Loeb argues that the first star-forming halos begin to collapse in this era. Certain density perturbations in this environment, rare but feasible, could have given birth to the stars we need, themselves the harbingers of the first rocky planets, which would have existed in a CMB radiation warm enough for liquid water to exist on their surface. Or as he puts it in the preprint:

Deviations from Gaussianity in the far (8.5σ) tail of the probability distribution of initial density perturbations, could have led already at these redshifts to the birth of massive stars, whose heavy elements triggered the formation of rocky planets with liquid water on their surface. And even as the CMB continued to cool, the thermal gradients needed for life would still be created through geothermal energy as well as radioactive energies from unstable elements in the primordial supernovae. A blanket of molecular hydrogen could retain that warmth.

At a redshift of z ∼ 100, which Loeb regards as ‘the earliest cosmic epoch after which life was possible in our Universe,” we are dealing with a time when the universe was far denser than it is today, perhaps a million times as much, or approximately 1 hydrogen atom per cubic centimeter. This is typical of the average density of matter in galaxies, but imagine this density extended over the entire universe in that era. I should note that I’m drawing from Loeb here but also from Edward Harrison’s Cosmology: The Science of the Universe (Cambridge, 2000). When we go this far back, to a time 10 to 17 million years after the Big Bang, the cosmic microwave background has reached a temperature of 273-373 K (0-100 degrees Celsius).

The density issue is intriguing. A certain density of matter must exist within the universe to avoid a Big Crunch, with the entire cosmos falling back in on itself because of gravity, and recent work indicates that baryonic matter combined with still mysterious dark matter can account for 30 percent of that critical density. The balance is provided by a cosmological constant, an idea introduced by Einstein and later given punch by the discovery of cosmic acceleration in 1998.

It was back in 1987 that Stephen Weinberg argued that if the cosmological constant were just one order of magnitude larger than observed, stars would never form nor, obviously, would life. Loeb looks askance at that conclusion at the end of his paper, especially given the possibility that the amplitude of the initial density perturbations might have varied in different regions of the presumed multiverse. He makes the case that life could emerge with a cosmological constant that is (1 + z)3 ~ 106 bigger than observed. “The possibility of life starting when the average matter density was a million times bigger than it is today argues against the anthropic explanation for the low value of the cosmological constant.”

So we wind up with a new kind of habitable planet, one we are now considering for the first time. Yes, we can talk about liquid water on its surface, and we can throw in geothermal activity as well and radiogenic heating, but we might not need a star to nourish it given the help of the cosmic microwave background’s heat energy. As I say, it’s a fascinating concept, and one I don’t think I’ve run into in the most speculative science fiction. That situation should change soon enough as some talented practitioners of the trade get a close look at Loeb’s conclusions.


Comments on this entry are closed.

  • M Mahin February 4, 2014, 12:36

    Loeb’s paper talks about a 2 million year time frame — way, way too short a time frame for the evolution of even one-celled life. His comments about the anthropic principle show a complete misunderstanding of it. The anthropic principle isn’t about the universe being compatible with mere microbes — it’s about the universe being compatible with intelligent observers like us (anthropic is derived from the same root word as anthropology, a root word meaning “having to do with man”).
    See my blog post for a rebuttal of his claim:

  • Anthony Mugan February 4, 2014, 13:30

    This is a fascinating concept. My initial question is over the extent to which metallicity could have increased in areas around the first supernova to a point sufficient to enable rocky planets to form at such an early point on time?

  • Ashley Baldwin February 4, 2014, 16:29

    Kepler data suggests that there is a clear link between gas giant formation and stellar metallicity as opposed to terrestrial planets ,which can and do form round low metallicity stars ,including presumably early on even in the first population II stars.

  • CharlesJQuarra February 4, 2014, 19:21

    One of the most interesting sources for heavy elements in the early universe might have been quasars. They existed for a brief period and they typically burned the mass equivalent of 10 sun-like stars per year. So there is a high change for large pockets of heavy element clouds to have existed and produced planets will all the elements required for earth-like life.

  • Rob Henry February 5, 2014, 2:50

    Ashley Baldwin makes a comment about the first population II stars, and that just increases my frustration. No one ever wants to hazard a guess over the nature of population III stars, which have never been observed, and whose theory seems to have only the most tenuous of publication histories.

  • Ashley Baldwin February 10, 2014, 17:25

    Dear Rob. Your wish is granted . Keller et al just discovered the oldest Pop 2 star yet ,and have been able to piece together some unexpected findings about the supernova and related parent Pop 3 star that helped ” seed ” it. Published in Nature but I enclose a link below to a full draft in arXiv . Very different to pre existing thinking , so a big finding. For comparison , I also enclose a second link to a more traditional (2007) view that has prevailed till now :