According to modern cosmology, a momentous event occurred 13.8 billion years ago. Inexplicably, the universe began without any measurable dimension. If there was something to measure, we don’t know how. Space and time, as we understand them, did not yet exist. What came before remains utterly unknown.
According to theory and faith, there may have been no ‘before’ — perhaps time itself literally began with the ‘Big Bang’. Another theory is space and matter are by-products of three dimensional time. It is possible there was nothing, or something unknowable. All known matter and energy derives from one origin, which can be described as a violent eruption from within a small point or singularity. Scientifically, cosmic microwave background (CMB) radiation confirms we are adrift within a primordial plasmaball, continuously expanding over billions of years. Indeed, there was one awesome burst of energy as God said, “Let there be Light.”
All known energy, matter, and spacetime emerged from an extremely hot dense source, known as a singularity. Some models (more philosophic than scientific) attribute this eruption to a whitehole — a blackhole in reverse, from which matter and energy flow outward rather than inward. It is not clear what caused this, but one possibility is blackholes may explode (not unlike supernovae). It is plausible they detonate when they exceed a critical mass-density. Of course, there is a theory which posits blackholes dissolve over time, and we must consider the effects of Hawking radiation. However, this does not mean blackholes are inert, and we use the metaphor of a dormant volcano to illustrate this point. Furthermore, we hold the position that supermassive blackholes do not dissolve, and in fact (on the fringe of speculative astrophysics) we contend many blackholes may grow continuously over trillions of years.
The ‘Big Bang’ did not merely explode into a pre-existing void. Indeed, the fabric of space itself literally expanded (and has continued to expand), increasing the distance between all points in the universe. Indeed, the space between galaxies, and between each and every object, is steadily expanding at a measurable rate — currently estimated at 73.3 kilometres per second per megaparsec. This can be observed today through the redshift of distant light. As light from distant galaxies travels through expanding space, its wavelength becomes stretched. This stretching shifts the light toward the red end of the spectrum — hence the term redshift. The greater the redshift, the faster an object is receding, and the farther away it is. Redshift is one of the strongest lines of evidence that the universe is expanding, as distant galaxies are receding at speeds exceeding the speed of light. This is not because the galaxies are themselves moving so fast, but because the space in between is literally increasing. Apparent recession is due to the metric expansion of space itself, rather than mere motion. Indeed, spatial expansion is a counterintuitive concept.
Spatial expansion is caused by subatomic interactions. A vacuum is not truly empty, but filled with brief quantum fluctuations where ‘virtual’ particles momentarily appear and vanish. When these particles emerge, they also generate new space — and when they disappear, that space remains behind. This space is arranged like a grid or pixel field — a vast chessboard whose squares can displace. As new intervals of space emerge, the distance between points will thereby increase. This means that two galaxies separated by one megaparsec (3.26 million light-years) will move apart at 73.3 km/s due to spatial expansion. Galaxies twice as far (2 Mpc) recede at 146.6 km/s, and so on. With a diameter of more than 28500 megaparsecs, objects at opposite ends of the known universe will move apart at more than two million kilometres per second — seven times the speed of light!
For every metre of separation, space continuously expands by roughly 7.5 centimetres per billion years. This occurs everywhere, continuously. On a cosmic scale, gravitationally unbound structures are always moving apart. This is happening at the subatomic level. For example, an iron atom may find it occupies slightly more space, briefly enjoying a lower density before strong nuclear forces assert dominance. Over trillions of years, this internal expansion (or dilution) can erode objects entirely, and also serves to separate galaxies. When particles (or particle pairs) appear within space, they create space within, and these spaces accrete into a pattern resembling Swiss cheese. Today, we can view this across the cosmic macrostructure of our universe, which appears ‘bubbled’. These bubbles, spanning millions of light years, were once tiny subatomic holes within the dense plasma of the initial Big Bang. Over time, these bubbles contributed to form gravitational voids of low density, which continue stretching due to ongoing spatial expansion. For example, the Boötes Void is 300 million light-years in diameter, while the Eridanus Supervoid spans a billion light-years. Indeed, on average, the universe is essentially empty with a density approaching zero.
Following the ‘Big Bang’, the universe was far too hot and pressurized for fundamental particles to exist. A basic principle of thermodynamics, as understood by Planck (who was influential on Einstein), is that increasing temperature will cause particles to move more energetically. Higher energy environments alter how particles behave. We also know nuclear forces govern the structure and behavior of atoms, acting in a manner not unlike electromagnetism. While nuclear forces are distinct from electromagnetism, they share underlying characteristics: transferring energy, binding particles, and exhibiting both attraction and repulsion. The strong nuclear force holds quarks inside protons, while the weak force governs radioactive decay. Though these forces are mediated by different particles, physicists believe they may all be aspects of a unified interaction that dominated the early universe. We deduce theoretically that sufficiently high energy can be applied to overcome fundamental nuclear forces. This corresponds to a temperature of 10³² kelvin — a hundred million trillion trillion degrees. Beyond this Planck energy, we are unsure how the fundamental forces behave. Mathematically, we perceive a ‘probabilistic soup’ of infinite and undefined variables. This ‘quantum foam’ appears to be in a state of perpetual fluctuation.
Recognizing the entire universe erupted from a singularity, and considering the energy involved, we estimate the universe exceeded this Planck temperature within the first fraction of a second. For much less than a trillionth of a second, the universe was sufficiently hot (or ‘energy dense’) to ensure the four fundamental forces were indistinguishable, or unified. By this, we mean physicists have observed the relationship between energy and force. As energy increases to an extreme scale, all forces appear to converge mathematically into a singular force. The earliest universe existed in this state, within which gravity, electromagnetism, and the nuclear forces were equal in strength. Theoretical particles likely existed, but fundamental particles (quarks, photons, electrons, gluons, and neutrinos) were not yet able to form.
We thus describe the Planck Epoch (0-10⁻⁴³s), during which the four fundamental forces were unified: gravity, electromagnetism, the strong nuclear force (SNF), and the weak nuclear force (WNF). This concept is similar to phase transition: water, ice, and steam behave differently. At high temperatures, metals blend together indistinguishably, but separate as temperatures cool. There is some primordial connection between gravity and electromagnetism, but the collapse of symmetry revealed elementary differences. As temperature rapidly dropped below 10³² Kelvin (a hundred million trillion trillion degrees), gravity separated first, followed by the other fundamental forces.
Theoretical physicists have dissected the first second of the ‘Big Bang’. As discussed, the Planck lasted 10⁻⁴³ of a second, during which fundamental forces were unified. This was followed by the Grand Unification (10⁻⁴³ to 10⁻³⁶s), during which gravity was separated but the other forces remained unified. This epoch lasted a trillionth of a trillionth of a trillionth of a second. The term ‘Grand Unification’ is derived from theoretical physics. In the 1960s, it became clear electromagnetism and the weak nuclear force were simply two forms of the same force. During the 1970s, it likewise became clear the strong nuclear force was also part of this family. We might thus think of the Planck as an idealized ‘Supreme Unification’. The separation of gravity may have accelerated inflation, and some physicists describe a repulsive antigravity field, generated by the release of vacuum energy. This temporary rate of exponential expansion is almost incomprehensible: within a fraction of a second the universe expanded by a factor of 10²⁶ – a hundred trillion trillion times. For comparison, this is analogous to a grain of sand instantaneously expanding across 90 billion lightyears!
During the 1980s, it became clear deunification of the fundamental forces is associated with inflation. The Inflationary Epoch (10⁻³⁶ to 10⁻³²s) was triggered at the end of the Grand Unification, as the strong nuclear force (SNF) began separating. Once again, cause and effect remain unclear. Perhaps fundamental forces triggered expansion, or vice-versa. Regardless, the universe underwent an instantaneous phase transition, similar to how water boils and expands into steam, releasing vacuum energy which temporarily increased the rate of inflation. Vacuum energy is normally present across empty space, but can exist in a high-energy configuration (or ‘false vacuum’) which stores excess energy until it is released. Static electricity is a loose analogy.
During the 1990s, it became clear this was followed by the Electroweak Epoch (10⁻³⁶ to 10⁻¹²), during which the SNF was also fully separated, and the rate of expansion declined sharply (between 10⁻³⁶ and 10⁻³²s). This declining rate was related to the exponential decay of vacuum energy. It is no mystery where this energy came from. During the Supreme Unification, we might think of all particles and forces as equal. Since vacuums are never truly empty, always containing some particles and some forces, unification stored tremendous energy inside vacuums. As space expanded, there was more space and more vacuum energy. With the separation of gravity and the strong nuclear force, both governing the interaction of both particles and forces, this tremendous ‘false’ energy surged out of ’empty’ space and itself generated additional space. Eventually, the supply of excess vacuum energy was exhausted, and inflation slowed (but did not cease).
The strong nuclear force binds quarks together, allowing them to form neutrons, and allowing protons to bind with neutrons (overcoming electromagnetic repulsion between protons). Meanwhile, the electromagnetic and weak nuclear force remained unified. However, although the SNF was now separated, the temperature remained too hot for quarks to bind. At this time, the SNF acted like a weak magnetic field, merely affecting quarks without binding them. At the end of this epoch, there was likely another phase transition, as the weak nuclear force and electromagnetism separated. Of course, electromagnetism refers to both electricity and magnetism (which are mathematically unified), while the weak nuclear force governs nucleosynthesis (fusion), radioactive decay, and the flavour transmutation of quarks (for example, from down to up).
The Quark Epoch (10⁻¹² to 10⁻⁶s) began one nanosecond after the Big Bang, lasting for one microsecond. In other words, a trillion times longer than the Planck, a million times longer than the Grand Unification, and a thousand times longer than the Electroweak. At 10⁻¹² seconds, as the universe cooled below a quadrillion degrees, the electroweak force “broke” into separate electromagnetic and weak nuclear forces. Subsequently, temperatures remained above 2 trillion degrees, across a spherical quark-gluon plasma. Gluons, the SNF analog to electromagnetic photons (light), are massless particles that carry charge and move at the speed of light. However, unlike photons, they are dominated by the strong nuclear force. As temperatures continued to drop, these were drawn together with quarks and confined into neutrons and protons (each containing three quarks).
The Hadronic (10⁻⁶ to 1s) lasted nearly the entire first second, a million times longer than the Quark Epoch. Along with neutrons and protons, hadrons include mesons (composed of one quark and one antiquark). According to quantum field theory, when energy condenses into mass, it produces a particle-antiparticle pair. Likewise, electrons are paired with positrons, and there are antineutrons and antiprotons. These antimatter particles have equal mass, but opposite charge. When they encounter matter, both particles are annihilated with a burst of energy (E = mc²). This is similar to how a chemical bomb explodes, when two volatile chemicals mix together. Such reactions occurred continuously during earlier epochs, with the heat from proton-antiproton interactions maintaining high temperatures (triggering the creation of additional matter-antimatter pairs). However, this chain reaction ended because the ongoing inflation diluted energy density, causing temperatures to decline. Subsequently, once it became too cool for new pairs to generate, the remaining pairs destroyed one another. This would have led to the destruction of all matter, but there was slightly more matter than antimatter — about one matter particle in a billion. The remaining matter has survived to form all stars, planets, and galaxies. This implies the mass of the ‘Big Bang’ was 2 billion times greater than our current universe, which supports the notion that an especially supermassive blackhole was involved.
The Leptonic (1-10s) began as temperatures fell to ten billion degrees. Matter-antimatter reactions continued, but at a much lower rate, as the particle ‘soup’ composition of the universe continued changing. Hadrons (ie: neutrons and antineutrons) already completed their destructive interaction, but leptons (ie: electrons and positrons, neutrinos and antineutrinos) remained in abundance until they also annihilated each other. Previously, during the Hadronic, the strong nuclear force (SNF) was dominant, as the majority of particles (quarks) were affected by the SNF. However, the weak nuclear force became more significant as the number of hadrons declined due to matter-antimatter interaction. One consequence was the decoupling of neutrinos. As the number of particles declined, and space expanded, the density continued dropping. Neutrinos are not affected by the SNF, while the WNF only operates at short range. Reduced density made it less likely neutrinos would encounter matter at WNF range, reducing the overall exchange of energy and heat. With energy thus ‘locked’ inside neutrinos, temperatures continued declining.
The Photonic (10s-380’000y) lasted hundreds of thousands of years, more than a trillion times longer than the Leptonic. The first minutes constitute the Nucleosynthesis (10s-20m). As temperatures dropped below a billion degrees, nuclear fusion became possible, allowing neutrons and protons to form the first atomic nuclei — primarily helium-4, with smaller amounts of hydrogen-2 (deuterium), helium-3, lithium-7, and beryllium-7 (which decays into lithium-7). This determined the universe’s primordial composition: roughly 75% hydrogen and 25% helium by mass, with trace amounts of heavier elements. Temperatures continued dropping, becoming ‘too cool’ for nuclear fusion, and the universe remained a hot plasma dominated by radiation for hundreds of thousands of years.
With most antiparticles annihilated, radiation temporarily dominated energy density. This means, if we consider all the energy across the universe (including energy within matter), most of the energy budget was radiation (specifically, photons). However, radiative energy steadily declined as photons and neutrinos redshifted through expanding space. As space expands, the wavelength of light necessarily expands, while the photon maintains velocity (c). This redshift causes the photon to lose energy, as stretching means the peak of each wave is lower (a ‘Slinky’ is often used to illutrate such principles). This lost energy does not violate conservation of energy, as energy losses due to changes in the geometry of space are allowed by General Relativity. Indeed, photon energy is defined by wavelength: E = hc/λ. As space expands, the definition of distance itself stretches — thereby increasing the wavelength and by definition lowering energy.
In other words, the same photon observed at a later time carries less energy — not because of intrinsic change, but because it exists within expanded space. For this reason, we define spacetime as the unified fabric of space and time — a four-dimensional structure whose geometry evolves as the universe expands. As noted, some physicists believe time to be itself three-dimensional, meaning that spacetime is actually a six-dimensional strucutre. In other words, the definitions of time and space are mathematically dependent upon where we are located within spacetime. Regardless, although time itself is not expanding (as far as we can determine), the rate of spatial expansion does change over time, which means the relationship between time and space is constantly changing.
After fifty thousand years, the universe’s energy budget reached matter-radiation equality, with matter subsequently (but only temporarily) containing the majority of energy. Previously, radiative pressure resisted gravitational collapse. However, as matter dominated, gravity would increasingly amplify density fluctuations (enabling the continued formation of cosmic structures). Over subsequent millenia, the universe would remain a opaque plasma, gradually cooling and expanding. Radiation has continued to decline as a percentage of the energy budget, currently accounting for 0.01%. Meanwhile (over billions of years), the energy budget became increasingly dominated by matter, reaching a peak of 65-70%. However, around seven billion years ago, this ratio began to decline as dark energy increased. This energy is merely infinitesimal energy of a vacuum, which increases in total as the universe expands (because the area of this vacuum increases). Today, matter accounts for just 31% of the budget.
During the Recombination (380’000-400’000y), temperatures dropped below 3000 Kelvin. Previously, during the Hadronic, light nuclei such as Helium-4 were created, but they were not yet electrically neutral (because it was too hot for electrons to bond with nuclei). As temperatures dropped below 3000 Kelvin, electrons were able to slow down and bond with atomic nuclei (containing neutrons and protons), forming stable atoms of hydrogen and helium. Consequently, the number of free electrons in space declined. Since these free electrons interfered with the movement of photons, scattering light, the universe was initially opaque. However, as temperatures dropped, this ionised plasma transitioned into transparent gas because photons could now travel freely. This allowed the release of light which can still be seen today. Known as ‘cosmic microwave background’ (CMB) radiation, this energy has continued traveling through expanding space, and has been redshifted from visible light into 160 GHz microwaves. Discovered in 1965, CMB provide strong evidence for the ‘Big Bang’. This energy fills the entire sky, arriving from all directions, proving we are within an expanding space. In other words, we can literally look across spacetime, and clearly see that we are within what was once a massive fireball.
The Cosmic Dark Eon (400k-100my) lasted a hundred million years. This is often referred to as a ‘Dark Age’ reminiscent of the medieval ‘Dark Ages’, but eon is more appropriate given the timescale. Once photons were able to travel freely, light temporarily streamed across the universe. However, although the ‘Big Bang’ released tremendous energy, little new light was created because there were no stars. At the start of this Dark Era, the color of the universe was orange-red, fading to a foggy red glow. There was still visible radiation from hot cosmic gas, but after a million years temperature dropped below 2000K, and most of this emission energy became infrared and invisible. A faint red glow would still have been visible (due to the nature of ‘blackbody’ radiation), but the number of visible photons emitted from hydrogen and helium gas decrease exponentially as temperatures cool. Subsequently, it would take fifteen million years for existing photons to fully redshift into the infrared spectrum. At this point, the universe became pitch black.
Cosmic background radiation reveals that modern cosmic structures expanded from primordial regions which began forming during the Planck. Over tens of millions of years, gravity continued to amplify density variations within cosmic gas, as dense regions attracted matter and become increasingly dense. Meanwhile, voids became less dense. Gravitational wells continued developing during the Dark Era, gradually forming dark matter halos. This dark matter is a mysterious form of matter, formed from an unknown subatomic particle which does not interact via the strong nuclear force and does not carry an electric charge. We don’t know exactly what this is, but it has gravitational pull and exceeds the mass of ordinary matter. Indeed, 85% of all matter is dark matter, which has had enormous influence upon the gravitational structure of the universe.
The Reionisation Eon (100my-700my) lasted more than half a billion years. As matter continued accretion, dense regions began to collapse under increasing gravitational pressure. This inward pressure heated gas, and nuclear fusion began when temperatures exceeded ten million degrees. Although helium was initially inert, hydrogen nuclei (protons) began fusing into helium. This released energy as light and heat, igniting a stellar core. These first stars, known as Population III, emitted ultraviolet radiation which stripped electrons from nearby hydrogen gas. This reionised the universe, which once again resembled a diffuse glowing cloud, although this pervasive glow was fainter than during the Photonic. As expansion continued, cosmic gas remained ionised, but density dropped low enough that photons were no longer scattered by stray electrons (the universe thus became transparent again). Meanwhile, metals did not yet exist, so these stars contained no metal. Since metals radiate heat efficiently, modern Population I stars are much smaller and cooler. In contrast, Population III stars were massive, generally more than a hundred times the size of Sol (our star) and ten times hotter. Within a few million years, the stellar core reached a hundred million degrees (due to thermal runaway), allowing helium fusion to produce carbon-12. At 600 million degrees, carbon fused into oxygen-16 and neon-20. At one billion degrees, oxygen produced silicon-28 and sulfur-32. For perhaps a hundred thousand years, the core reached three billion degrees, allowing silicon to produce iron-56 (along with isotopes of nickel and cobalt). Subsequently, the core quickly exceeded ten billion degrees, and the star exploded.
As discussed, nucleosynthesis began during the Photonic, once temperatures dropped below ten billion degrees. Likewise, at this temperature, stellar cores stall, as photodisintegration occurs (high-energy gamma photons shatter heavy nuclei, reversing fusion). Meanwhile, as iron accumulates in the core (and does not fuse), gravity increases while the outward pressure of fusion declines. This eventually causes the iron core to collapse, with electrons squeezed into protons, producing neutrons and neutrinos. The collapse happens almost instantaneously, as a core the size of Luna (or larger) is crushed to a diameter of perhaps just a few kilometres, reaching the density of a neutron star (10¹⁴ grams/cm³). Such a core has been crushed into nothing but neutrons. Due to quantum mechanics (specifically the Pauli exclusion principle, which states neutrons cannot occupy the same quantum state), these resist compression, generating counterpressure which halts the inward collapse caused by gravity and triggering a rebound shockwave which bursts outward. This detonation, a smaller version of the ‘Big Bang’, is known as a supernova. During such an explosion, stray neutrons bombard iron and nickel atoms, producing gold, platinum, uranium, lead and other heavy elements.
Following a supernova, the neutron core remains intact, with temperatures approaching one trillion degrees (cooling to a million degrees, over a million years). Within this core, neutrons are compressed so densely that one cubic millimetre weighs as much as a mountain (400 million tons). However, a blackhole will develop if the remaining neutron star is significantly more than twice the size of Sol. This becomes likely when the original star exceeds 25 solarmasses, although high-metallicity Population I stars can reach 40 solarmasses before breaching this Oppenheimer-Volkoff limit. At this point, there is sufficient gravitational pressure to overcome the strong nuclear force which binds quarks, causing the neutron core to collapse into an even denser state. Subsequently, gravitational force draws additional matter into the growing blackhole, which can evolve into a supermassive blackhole.
We do not know what exists within blackholes, but they presumably contain matter and energy within some unknown state. We know the exterior of a blackhole is a region where gravity is so intense that not even light can escape. Deeper in, beyond the event horizon, it is likely that at least some of the fundamental forces begin to reunify. The most powerful blackholes contain billions of solarmasses and reside at the core of a galaxy. For example, TON 618 exceeds 40 billion solarmasses. Sagittarius A, at the centre of Galaxias, is a modest 4 million solarmasses. Even this is sufficient to rip stars apart, but gravitational force is relatively shortrange. Our Local Group of galaxies is gravitationally bound within the Laniakea Supercluster, but is not spiraling into a blackhole, nor are we. The Great Attractor, a gravitational anomaly pulling many nearby galaxies, is sometimes described as a supermassive blackhole. More likely, it is a dense region of galactic clusters (including dark matter and multiple blackholes) which we are orbiting. The nearest known blackhole is Gaia BH1, a ten solar-mass blackhole about 1500 lightyears away (in the constellation Ophiuchus), posing no more danger than any large star. Arguably less, since stars are more likely to explode.
Around 400 million years after the first Population III stars, enough metal had been seeded by supernovas, enabling the formation of nearby Population II stars. It is important to note that ‘metal’ has differing definitions, between chemistry and astrophysics. Here, we are specifically talking about the cooling effect of oxygen and carbon (which have properties of ‘metallicity’). As noted, this allows stars to be smaller and cooler. For example, a Population III star might exceed 100 solarmasses, with a surface temperature of 100’000 degrees, burning for a few million years. In contrast, a Population II star may form at 0.8 to 1.5 solarmasses, with surface temperatures around 5000 degrees, and a lifespan exceeding ten billion years.
Unlike Population III, which has long since disappeared, Population II continues to exist and new stars can still form in regions of sufficiently low metallicity (ie: globular clusters and galactic halos). For example, Kapteyn’s Star is only 12.8 lightyears away, and nearby clusters (such as Omega Centauri and Messier 4) contain thousands of Population II stars. These are generally located beyond the rotating spiral disk of our galaxy (within the halo), although they are also found in the galactic core. Most remain outside the disk because they formed while the galaxy was small (perhaps while it was merely a protogalaxy), long before the disk – consequently, their unaligned orbits follow random elliptical paths along the periphery.
The Galactic Eon (700my to 3by) lasted more than two billion years, describing a time when Population II was prevalent, and the first large galaxies were fully formed. Such gravitational accretions began forming around the same time as Population II. The first galaxies were relatively small irregular globs, containing just a few million stars, clustered around the most dense regions of dark matter (and blackholes created by Population III). Inevitably, supermassive blackholes formed, attracting nearby galaxies into clusters. Our Galaxias (also known as the Milky Way or Silver River) thus began forming at this time, evolving from a small Population III protogalaxy presumably centered upon Sagittarius A. This was a gradual process, as the galactic disk only began forming near the end of this eon, but was not fully developed for billions of years.
The Stellar ‘Stelliferous’ Eon (3by-9.2by) witnessed the formation of Population I stars, including Sol. As previous generations enriched the interstellar medium with metals, cosmic gas cooled more efficiently. Population I — rich in metals — thus appeared at an accelerating rate. These stars formed in greater numbers, across a wider range of mass, with peak formation at 3.5by. Meanwhile, supermassive blackholes became larger and more common. Whereas the smallest Population III stars were at least 30 solar masses (ranging to 300), and the smallest Population II were 0.5 solarmasses (ranging to 50), Population I can be a tenth the size of Sol. This is due to the much higher metallicity of Population I, which burns efficiently for tens of billions of years. In the future, newborn Population I will have increasingly high metallicity and efficiency, but no new ‘population’ is expected. Meanwhile, as metallicity increases, Population II will become increasingly less common (with formation ceasing 75 billion years from now).
At 6 billion years since the ‘Big Bang’, star formation began to decline, due to decreasing quantities of cosmic gas (burned within stars). Formation declined from a peak 15 times higher than today, to 7.5 times higher at 9 billion years. After another ten billion years, we can expect star formation to be one percent of peak. After one hundred billion years, star formation will cease. However, red dwarves, white dwarves, and neutron stars will continue burning for trillions of years. Indeed, red dwarves are the most common form of star. Population III did not produce these, and they are uncommon within Population II. As discussed, most (presumably all) Population III produced supernovas, leaving behind a dense neutron star or blackhole (as do Population II and I, when the star is much larger than Sol). However, since Population I is most prevalent and generally smaller than Sol, red dwarves are common.
Most Population II produce white dwarves, and most Population I produce red dwarves. Red dwarves are associated with high metallicity, being small cool red stars. Although Sol is part of Population I, its metallicity is low enough that it resembles Population II. White dwarfs form from such hot yellow-white stars, which remain small enough to avoid a supernova. Counterintuitively, white dwarves form from red giants, which themselves evolve from yellow dwarves. In five billion years, when the core of Sol runs out of hydrogen, the loss of fusion pressure will cause the core to contract and overheat from fifteen to one hundred million degrees. This will cause hydrogen fusion within internal layers surrounding the core, and the resulting heat and pressure will cause the outer layers to expand well beyond the orbit of Earth. Although the core itself would thus be much hotter, such stars appear cool (red) because the outer layers are pushed far away, with a surface temperature of four thousand degrees (compared to six thousand for Sol today). After another billion years, the outer layers of the red giant will dissipate, revealing a hot white inner core.
Red dwarves may burn for a sextillion (billion trillion, 10²¹) years, while hotter white dwarves and neutron stars may last a nonillion (quintillion trillion, 10³⁰) years. After red dwarfs burn out, the Degenerate Eon (10²¹-10⁴⁰y) will begin, during which only white dwarfs, neutron stars, and blackholes will remain. After ten duodecillion (ten thousand trillion trillion trillion, 10⁴⁰) years, the Black Hole Eon (10⁴⁰-?) will begin, as remaining stars are extinguished. According to theory, blackholes may dissipate over time, via Hawking radiation. If so, the universe will effectively cease to exist, after a googol (10¹⁰⁰) years. Thus will begin an endless Dark Eon, during which the universe will be filled with cold dissipated radiation. Debris may remains for trillions of years, but this will gradually erode at the quantum level. Indeed, there is evidence protons decay, meaning all atoms inevitably dissolve. However, at the same time, another theory describes the spontaneous generation of subatomic particles. These particles may seed the universe over trillions of years, perhaps achieving a steady state equilibrium, allowing new energy and mass to accumulate. If accumulation exceeds loss, blackholes may continue growing and merging, until a supermassive blackhole erupts into a whitehole. There is thus debate over whether the universe will expand from nothing into nothing, or whether it experiences cyclical whiteholes.
9.2 billion years after the ‘Big Bang’, our Solar system began to form from gas and dust, seeded by the detonation of Population III and II supernovas. At this point, we invert our counting of the years, as we do between 1 BC and 1 AD. Previously, we dated time since the ‘Big Bang’, but in subsequent chapters we define time in years before our ‘common era’. Technically, we date this with reference to an arbitrary moment two thousand years ago. According to tradition, we base this upon the birth of Jesus, although historical evidence suggests he was likely born several years earlier (in 5 or 6 BC). The discrepancy comes from a miscalculation by the Sixth Century monk Dionysius Exiguus (who attempted to define this date).
4.6 billion years ago, sufficient material condensed, allowing Sol to form within a nebula (dense gas cloud, or stellar nursery), much like the modern Orion Nebula. This was likely triggered by a nearby supernova, which exerted force upon a dense molecular cloud, pushing it into a critical density. The epicentre of the consequent gravitational storm became increasingly hot, surrounded by a rotating protoplanetary disk (not unlike a whirlpool or hurricane). Eventually, nuclear fusion began, while matter in the disk accreted into planets and moons. The outer Solar System retained more volatile gases, forming gas giants, while the inner planets condensed from heavier elements. Thus began the Hadean Eon (4.6b-4bya).
