Monday, July 18, 2022

Early Light/Early Life



Last week NASA released the first images acquired by its new James Webb Space Telescope, including this infra-red image of the galaxy cluster SMACS 0723. The field of view is tiny, about the size of a grain of dust held at arms length, but it's packed with galaxies shining with light 4.6 billion years old, around the time the solar system was condensing out of a swirl of gas and cosmic dust. And amongst those galaxies are redder, fainter, fingernail arcs that are the light of even older star clusters and galaxies, distorted by the gravity of nearer, younger galaxies. That reddened, distorted light is around 12.8 billion years old, according to NASA: a window into the very deep past, around a billion years after the Big Bang and the universe's creation.

What was it like, then? The universe was still somewhat hotter and denser than it is now, and star formation was more intense, but there were stars and recognisable galaxies, even if they were small and irregular or simple spirals rather than elliptical giants like the Milky Way. Given what we know now about the abundance of exoplanets, some of those stars may have had planetary companions. But was there any life* on those first worlds? Was anything splashing about in some primordial ocean, under a sky crowded with stars and cauls of hot gas giving birth to stars, and pocket-sized galaxies smashing into each other?

On early Earth, the limiting factor for the kindling of life was temperature and the availability of liquid water. The earliest undisputed trace of life are fossilised microbial mats 3.5 billion years old. There are also traces of what might be stromatolites and biologically formed graphite in rocks 3.7 billion years old, and there's a claim that eyelash-sized iron-rich tubes may have been formed by microorganisms living 4.28 billion years ago. Their age and biogenic nature is still disputed, but if they really are fossil traces of life, then life on Earth began very soon after its formation by violent accretion 4.54 billion years ago: as soon as the first oceans appeared. Could life* have arisen somewhere in the universe as quickly? Here's some artless speculation.

In the early universe, the limiting factor for the first appearance of life was not temperature, but availability of water and necessary elements -- carbon, oxygen, nitrogen and so on. A brief universe-wide flare of fusion processes in the first few minutes after the Big Bang produced mostly hydrogen and helium, with a vanishingly small smattering of lithium and even tinier traces of beryllium. Everything heavier than hydrogen and helium (called metals by astronomers, so both oxygen and carbon, for instance, are metals) had to be forged by fusion in stars, so the very first stars that formed in protogalaxies a few hundred light years across were composed entirely of primordial hydrogen and helium.

Those first stars, known as Population III stars, started to form 0.2 -- 0.4 billion years after the Big Bang. None have yet been imaged (it's one of the tasks planned for the JWST), but it's likely that they were large, 30 - 300 times the size of the sun, and burned hot and briefly, lasting only a few million years. Any more than 250x the mass of the sun collapsed into black holes; the rest either burned out or blew up in supernovae that scattered newly-forged metals into space. And in addition to forging heavier elements, their fierce radiance, most of it UV light, turned the opaque soup of neutral hydrogen and helium that filled the universe into a transparent plasma by reionizing hydrogen atoms, a process that was largely over a billion years after the Big Bang.

By then, the universe had begun to be enriched by metals, too, including the stuff of life. But the composition of surviving members of the subsequent Population II generation of stars suggests that around a billion years after the Big Bang the universe was still extremely metal-poor;  even the oldest Population I stars, formed 2 - 3 billion years later, contain only a tenth of the metal content of youngers stars like our sun.

So it's likely that those dim red crescents imaged by the JWSR are ancient light from a prebiotic universe, because as far as the building blocks of life and their universal solvent, water, were concerned, that early universe was a desert. And even if a few planetary systems of early Population II stars condensed out of dust and gases excessively rich in water and the stuff of life, there'd be only a few scattered oases containing the unicellular equivalents of bacteria and archaea. It wasn't until much later (how much later is still being debated)** that cosmic metal enrichment reached levels that could support life across the universe.

And of course, there's a chance that life on Earth is the only life in the universe. That until it arose here on this little blue planet, 10 billion years after the birth of the universe, the universe contained no life at all. But given that all the galaxies in the JWST's grain-of-sand peephole are just a fraction of the two trillion or so galaxies in the universe, each with their several hundred billion stars and several thousand billion planets, how likely is it that the spark of life caught fire only once, in the billions of years following the emission of the red-shifted, gravity-lensed light of the early stars captured in that extraordinary image?

*That is, squishy carbon-based life-as-we-know-it, not life based on (say) space-time defects or dark matter, like the Xeelee and the Photino Birds in Stephen Baxter's Xeelee sequence.

**Update July 19: it turns out the JWST was able to capture the red-shifted spectra of several of those ancient proto-galaxies, which will give insights into their chemical composition and how that changes over time. It seems that even the oldest of the galaxies imaged contain oxygen and neon, but differences in relative abundance of that element between galaxies of different ages aren't yet clear.


Blogger Paul McAuley said...

Belated thanks for the links, Philip.

August 12, 2022 2:18 pm  
Blogger Philip said...

Pas de probleme! I hope you found them interesting.

August 12, 2022 7:43 pm  

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