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Starts With A Bang

What Was It Like When Life In The Universe First Became Possible?

It took more than 9 billion years for Earth to form: the only known planet housing life. But it could have happened much, much sooner.

The cosmic story that unfolded following the Big Bang is ubiquitous no matter where you are. The formation of atomic nuclei, atoms, stars, galaxies, planets, complex molecules, and eventually life is a part of the shared history of everyone and everything in the Universe. As we understand it today, life on our world began, at the latest, only a few hundred million years after Earth was formed.

That puts life as we know it already nearly 10 billion years after the Big Bang. The Universe couldn’t have formed life from the very first moments; both the conditions and the ingredients were all wrong. But that doesn’t mean it took all those billions and billions of years of cosmic evolution to make life possible. It could have begun when the Universe was just a few percent of its current age. Here’s when life might have first arisen in our Universe.

The photons, particles and antiparticles of the early Universe. It was filled with both bosons and fermions at that time, plus all the antifermions you can dream up. If there are additional, high energy particles we haven’t yet discovered, they likely existed in these early stages, too. These conditions were unsuitable for life. (BROOKHAVEN NATIONAL LABORATORY)

At the moment of the hot Big Bang, the raw ingredients for life could in no way stably exist. Particles, antiparticles, and radiation all zipped around at relativistic speeds, blasting apart any bound structures that might form by chance. As the Universe aged, though, it also expanded and cooled, reducing the kinetic energy of everything in it. Over time, antimatter annihilated away, stable atomic nuclei formed, and electrons could stably bind to them, forming the first neutral atoms in the Universe.

As the Universe cools, atomic nuclei form, followed by neutral atoms as it cools further. All of these atoms (practically) are hydrogen or helium, and the process that allows them to stably form neutral atoms takes hundreds of thousands of years to complete. (E. SIEGEL)

Yet these earliest atoms were only hydrogen and helium: insufficient for life. Heavier elements, such as carbon, nitrogen, oxygen and more, are required to build the molecules that all life processes rely on. For that, we need to form stars in great abundance, have them go through their life-and-death cycle, and return the products of their nuclear fusion to the interstellar medium.

It takes 50-to-100 million years to form the first stars, sure, which form in relatively large clusters. But in the densest regions of space, these star clusters will gravitationally pull in other matter, including material for additional stars and other star clusters, paving the way for the first galaxies. By time only ~200-to-250 million years have passed, not only will multiple generations of stars have lived-and-died, but the earliest star clusters will have grown into galaxies.

The distant galaxy MACS1149-JD1 is gravitationally lensed by a foreground cluster, allowing it to be imaged at high resolution and in multiple instruments, even without next-generation technology. This galaxy’s light comes to us from 530 million years after the Big Bang, but the stars within it are at least 280 million years old. (ALMA (ESO/NAOJ/NRAO), NASA/ESA HUBBLE SPACE TELESCOPE, W. ZHENG (JHU), M. POSTMAN (STSCI), THE CLASH TEAM, HASHIMOTO ET AL.)

This is important, because we don’t just need to create the heavy elements like carbon, nitrogen, and oxygen; we need to create enough of them — and all of the life-essential elements — to produce a wide diversity of organic molecules.

We need those molecules to stably exist in a location where they can experience an energy gradient, such as on a rocky moon or planet in the vicinity of a star, or with enough undersea hydrothermal activity to support certain chemical reactions.

And we need for those locations to be stable enough that whatever counts as a life process can self-sustain.

Some of the atoms and molecules found in space in the Magellanic cloud, as imaged by the Spitzer Space Telescope. The creation of heavy elements, organic molecules, water, and rocky planets were all necessary for us to have even a chance of coming about. (NASA/JPL-CALTECH/T. PYLE (SSC/CALTECH))

In astronomy, all of these conditions get lumped together by a single term: metals. When we look at a star, we can measure the strength of the different absorption lines coming from it, which tell us — in combination with the star’s temperature and ionization — what the abundances of the different elements are that went into creating it.

Add them all up, and that gives you the star’s metallicity, or the fraction of the elements within it that are heavier than either plain hydrogen or helium. Our Sun’s metallicity is somewhere between 1-and-2%, but that might be excessive for a requirement for life. Stars possessing just a fraction of that, perhaps as little as 10% the Sun’s heavy element content, might still have enough of the necessary ingredients, across-the-board, to make life possible.

The visible light spectrum of the Sun, which helps us understand not only its temperature and ionization, but the abundances of the elements present. The long, thick lines are hydrogen and helium, but every other line is from a heavy element that must have been created in a previous-generation star, rather than the hot Big Bang. (NIGEL SHARP, NOAO / NATIONAL SOLAR OBSERVATORY AT KITT PEAK / AURA / NSF)

This gets really interesting, nearby, when we look at globular clusters. Globular clusters contain some of the oldest stars in the Universe, with many of them forming when the Universe was less than 10% its current age. They formed when a very massive cloud of gas collapsed, leading to stars that are all of the same age. Since a star’s lifetime is determined by its mass, we can look at the stars remaining in a globular cluster and determine its age.

For the more than 100 globular clusters in our Milky Way, most of them formed 12-to-13.4 billion years ago, which is extremely impressive considering the Big Bang was just 13.8 billion years ago. Most of the oldest ones, as you might expect, have just 2% of the heavy elements that our Sun has; they’re metal-poor and unsuited for life. But a few globular clusters, like Messier 69, offer a tremendous possibility.

A map of the nearest globular clusters to the Milky Way’s center. The globular clusters closest to the galactic center have a higher metal content than the ones on the outskirts. (WILLIAM E. HARRIS / MCMASTER U., AND LARRY MCNISH / RASC CALGARY)

Like most globular clusters, Messier 69 is old. It has no O-stars, no B-stars, no A-stars and no F-stars; the most massive stars remaining are comparable in mass to our Sun. Based on our observations, it appears to be 13.1 billion years old, meaning its stars come from just 700 million years after the Big Bang.

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But its location is unusual. Most globular clusters are found in the halos of galaxies, but Messier 69 is a rare one found close to the galactic center: just 5,500 light-years away. (For comparison, our Sun is about 27,000 light-years from the galactic center.) This close proximity means that:

  • more generations of stars have lived-and-died here than on the galaxy’s outskirts,
  • more supernovae, neutron star mergers and gamma-ray bursts have occurred here than where we are,
  • and, therefore, these stars should have a much greater abundance of heavy elements than other globular clusters.
The globular cluster Messier 69 is highly unusual for being both incredibly old, at just 5% the Universe’s present age, but also having a very high metal content, at 22% the metallicity of our Sun. (HUBBLE LEGACY ARCHIVE (NASA / ESA / STSCI), VIA HST / WIKIMEDIA COMMONS USER FABIAN RRRR)

And boy, does this globular cluster ever deliver! Despite its stars forming when the Universe was just 5% its present age, the close proximity to the galactic center means that the material its stars formed from were already polluted, and filled with heavy elements. When we deduce its metallicity today, even though these stars formed just a few hundred million years after the Big Bang, we find they have 22% the heavy elements that the Sun does.

So that’s the recipe! Make many generations of stars quickly, form a planet resilient enough around one of the lower-mass, longer-lived stars (like a G-star or a K-star) to protect itself from whatever supernovae, gamma-ray bursts, or other cosmic catastrophes it may encounter, and let the ingredients do what they do. Whether we get lucky or not, there’s certainly an opportunity for life at the centers of the oldest galaxies we could ever hope to discover.

The most distant galaxy ever discovered in the known Universe, GN-z11, has its light come to us from 13.4 billion years ago: when the Universe was only 3% its current age: 407 million years old. But there are even more distant galaxies out there, and we all hope that the James Webb Space Telescope will discover them. (NASA, ESA, AND G. BACON (STSCI))

Wherever we look in space around the centers of galaxies, or around massive, newly forming stars, or in the environments where metal-rich gas is going to form future stars, we find a whole host of complex, organic molecules. These range from sugars to amino acids to ethyl formate (the molecule that gives raspberries their scent) to intricate aromatic hydrocarbons; we find molecules that are precursors to life. We only find them nearby, of course, but that’s because we don’t know how to look for individual molecular signatures much beyond our own galaxy.

But even when we look in our nearby neighborhood, we find some circumstantial evidence that life existed in the cosmos before Earth did. There’s even some interesting evidence that life on Earth didn’t even begin with Earth.

On this semilog plot, the complexity of organisms, as measured by the length of functional non-redundant DNA per genome counted by nucleotide base pairs (bp), increases linearly with time. Time is counted backwards in billions of years before the present (time 0). Note that, if we do this extrapolation, we might conclude that life on Earth began billions of years prior to Earth’s formation. (SHIROV & GORDON (2013), VIA ARXIV.ORG/ABS/1304.3381)

We still don’t know how life in the Universe got its start, or whether life as we know it is common, rare, or a once-in-a-Universe proposition. But we can be certain that life came about in our cosmos at least once, and that it was built out of the heavy elements made from previous generations of stars. If we look at how stars theoretically form in young star clusters and early galaxies, we could reach that abundance threshold after several hundred million years; all that remains is putting those atoms together in a favorable-to-life arrangement. If we form the molecules necessary for life and put them in an environment conducive to life arising from non-life, suddenly the emergence of biology could have come when the Universe was just a few percent of its current age. The earliest life in the Universe, we must conclude, could have been possible before it was even a billion years old.

Further reading on what the Universe was like when:

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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