5 coincidences that make our existence possible

Our Universe has grown up impressively since the Big Bang.

At the start of the hot Big Bang, the Universe was rapidly expanding and filled with high-energy, very densely packed, ultra-relativistic quanta. An early stage of radiation domination gave way to several later stages where radiation was sub-dominant, but never went away completely, while matter then clumped into gas clouds, stars, star clusters, galaxies, and even richer structures over time, all while the Universe continues expanding.

From a hot, dense, near-uniform initial state, stars, galaxies, and living planets emerged.

Our Universe, from the hot Big Bang until the present day, underwent a huge amount of growth and evolution, and continues to do so. Our entire observable Universe was approximately the size of a modest boulder some 13.8 billion years ago, but has expanded to be ~46 billion light-years in radius today. The complex structure that has arisen must have grown from seed imperfections of at least ~0.003% of the average density early on, and has gone through phases where atomic nuclei, neutral atoms, and stars first formed.

Without these five coincidences, our Universe would have inevitably been lifeless.

At the high temperatures achieved in the very young Universe, not only can particles and photons be spontaneously created, given enough energy, but also antiparticles and unstable particles as well, resulting in a primordial particle-and-antiparticle soup. Yet even with these conditions, only a few specific states, or particles, can emerge, and by the time a few seconds have passed, the Universe is much larger than it was in the earliest stages. As the Universe begins expanding, the density, temperature, and expansion rate of the Universe all rapidly drop as well.

1.) A photon-rich early Universe.

In the early Universe, it’s very easy for a free proton and a free neutron to form deuterium. But while energies are high enough, photons will come along and blast these deuterons apart, dissociating them back into individual protons and neutrons. This prevents heavier elements from forming at early times, dependent on a large photon abundance.

Large photon abundances make forming stable atomic nuclei difficult early on.

This plot shows the abundance of the light elements over time, as the Universe expands and cools during the various phases of Big Bang Nucleosynthesis. The ratios of hydrogen, deuterium, helium-3, helium-4, and lithium-7 all arise from these processes.

With too few photons, all hydrogen would fuse into helium and beyond.

The predicted abundances of helium-4, deuterium, helium-3, and lithium-7 as predicted by Big Bang Nucleosynthesis, with observations shown in the red circles. If there were many fewer photons per baryon (far to the right), everything would have become helium-or-heavier early on, with no free hydrogen remaining.

No Sun-like stars would ever be possible.

In the very early Universe, there were tremendous numbers of quarks, leptons, antiquarks, and antileptons of all species. After only a tiny fraction-of-a-second has elapsed since the hot Big Bang, most of these matter-antimatter pairs annihilate away, leaving a very tiny excess of matter over antimatter. How that excess came about is a puzzle known as baryogenesis, and it is one of the greatest unsolved problems in modern physics.

2.) A big enough matter-antimatter asymmetry.

As the Universe expands and cools, unstable particles and antiparticles decay, while matter-antimatter pairs annihilate and photons can no longer collide at high enough energies to create new particles. Antiprotons will collide with an equivalent number of protons, annihilating them away, as will antineutrons with neutrons. After all the carnage, somehow, more matter than antimatter remains: evidence for a large initial asymmetry.

We have about 1 proton (and no antiprotons) for every 1.6 billion photons.

Only approximately 1000 stars are present in the entirety of dwarf galaxies Segue 1 and Segue 3, the latter of which has a gravitational mass of an impressive 600,000 Suns. The stars making up the dwarf satellite Segue 1 are circled here. As we discover smaller, fainter galaxies with fewer numbers of stars, we begin to recognize just how common these small galaxies are as well as how elevated their dark matter-to-normal matter ratios can be; there may be as many as 100 for every galaxy similar to the Milky Way, with dark matter outmassing normal matter by factors of many hundreds or even more.

If that abundance was just ~1000 times smaller, stellar enrichment would be extraordinarily low.

This close-up view of Messier 82, the Cigar Galaxy, shows not only stars and gas, but also the superheated galactic winds and the distended shape induced by its interactions with its larger, more massive neighbor: M81. (M81 is located off-screen, to the upper right.) When star-formation actively occurs across an entire galaxy, it becomes what’s known as a starburst galaxy, characterized by violent, gas-expelling winds. If the galaxy is too low in mass, this enriched material will all get ejected, preventing the formation of later-generation stars with the potential for rocky planets.

Without sufficient heavy elements produced in stars, rocky worlds could never form.

While the web of dark matter (purple, left) might seem to determine cosmic structure formation on its own, the feedback from normal matter (red, at right) can severely impact the formation of structure on galactic and smaller scales. Both dark matter and normal matter, in the right ratios, are required to explain the Universe as we observe it. Structure formation is hierarchical within the Universe, with small star clusters forming first, early protogalaxies and galaxies forming next, followed by galaxy groups and clusters, and lastly by the large-scale cosmic web.

3.) The existence of dark matter.

This decade-long timelapse, from 2008 to 2017, shows incredibly detailed features in the gaseous and filamentary structures of the Crab Nebula expanding over time. Over the timescale of this animation, the nebula has further increased in size by about a tenth of a light-year, allowing us to visualize the passage of extremely large timescales in mere instants, and to comprehend the incredible speeds at which material is ejected from a supernova.

Without dark matter, supernova ejecta would escape their home galaxies.

According to models and simulations, all galaxies should be embedded in dark matter haloes, whose densities peak at the galactic centers. On long enough timescales, of perhaps a billion years, a single dark matter particle from the outskirts of the halo will complete one orbit. Without a massive dark matter halo, galaxies would be smaller, lower in mass, and unable to hold onto the ejecta from stellar cataclysms.

This “cosmic glue” holds galaxies together, a necessity for late-generation stars.

This conceptual image shows meteoroids delivering all five of the nucleobases found in life processes to ancient Earth. All the nucleobases used in life processes, A, C, G, T, and U, have now been found in meteorites, along with more than 80 species of amino acids as well: far more than the 22 that are known to be used in life processes here on Earth. Similar processes no doubt happened in stellar systems all throughout most galaxies over the course of cosmic history, bringing the raw ingredients for life to all sorts of young worlds.

4.) The excited Hoyle state of carbon.

The prediction of the Hoyle State and the discovery of the triple-alpha process is perhaps the most stunningly successful use of anthropic reasoning in scientific history. This process is what explains the creation of the majority of carbon that’s found in our modern-day Universe, and demonstrates that it was created in the process of stellar nucleosynthesis.

Inside red giants, three helium atoms undergo carbon fusion.

When main sequence stars evolve into subgiants, as illustrated here, they get larger, cooler, and much more luminous, as their cores contract and heat up, increasing the rate of fusion but also making the star itself a lot puffier in the process. The subgiant phase ends when, and if, helium fusion begins: where helium atoms fuse into an excited state of carbon in their cores; without this excited state, the production of carbon (and all heavy elements) would be too low to admit life in the Universe.

A carbon resonance of just the right mass is required to build up life-friendly elements.

A proton isn’t just three quarks and gluons but a sea of dense particles and antiparticles inside. The more precisely we look at a proton and the greater the energies that we perform deep inelastic scattering experiments at, the more substructure we find inside the proton itself. There appears to be no limit to the density of particles inside, but whether a proton is fundamentally stable or not is an unanswered question.

5.) A stable proton.

Two possible pathways for proton decay are spelled out in terms of the transformations of its fundamental constituent particles. These processes have never been observed, but are theoretically permitted in many extensions of the Standard Model, such as SU(5) Grand Unification Theories.

In theory, protons can decay to mesons plus leptons.

The particle content of the hypothetical grand unified group SU(5), which contains the entirety of the Standard Model plus additional particles. In particular, there are a series of (necessarily superheavy) bosons, labeled “X” in this diagram, that contain both properties of quarks and leptons, together, and would cause the proton to be fundamentally unstable. Their absence, and the proton’s observed stability, provide strong evidence against the validity of this theory in a scientific sense.

If these decays occurred too easily, normal matter would be unstable.

Neutrino detectors, like the one used in the BOREXINO collaboration here, generally have an enormous tank that serves as the target for the experiment, where a neutrino interaction will produce fast-moving charged particles that can then be detected by the surrounding photomultiplier tubes at the ends. These experiments are all sensitive to proton decays as well, and the lack of observed proton decay in BOREXINO, SNOLAB, Kamiokande (and successors) and others have placed very tight constraints on proton decay, as well as very long lifetimes for the proton.

The proton’s stability, still unexplained, enables all cosmic life.

If life began with a random peptide that could metabolize nutrients/energy from its environment, replication could then ensue from peptide-nucleic acid coevolution. Here, DNA-peptide coevolution is illustrated, but it could work with RNA or even PNA as the nucleic acid instead. Asserting that a “divine spark” is needed for life to arise is a classic “God-of-the-gaps” argument, but asserting that we know exactly how life arose from non-life is also a fallacy. These conditions, including rocky planets with these molecules present on their surfaces, likely existed within the first 1-2 billion years of the Big Bang.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.