Written by Graham | Created: Tuesday 9th July 2019 @ 1808hrs | Revised: Thursday 1st October 2020 @ 0038hrs
The very early universe
13.8 billion years ago | 1 picosecond (10-12 seconds).
The universe in the minute amounts of time immediately after the Big Bang was a very active place. The first picosecond saw the coming and going of the Planck epoch (tp, 10-43 seconds), a truly bizarre fraction of a moment during which the laws of physics as we know them seem not to have applied, at the end of which the first fundamental interaction, namely gravity, emerged.
This was followed in turn by the grand unification epoch, before the separation of the electronuclear force and emergence of the three remaining fundamental interactions in order: -
Strong nuclear force
Weak nuclear force
The early universe
13.8 billion-13.422 billion years ago | 377,000 years.
10-12 to 10-6 s.
10-6 to 1 s.
1 to 10 s.
10 s. to 377 k.y.
Shortly after this time, matter and antimatter appear in the form of the various types of subatomic particle-antiparticle pairs. The annihilation of much of the content of the universe in the form of matter-antimatter reactions left a residue of leftover matter, which laid the foundations for the universe we see today.
The most primitive and earliest of these were quarks and antiquarks, which formed a quark-gluon plasma, until the cooling of the universe enabled baryogenesis to take place, leading to the first baryons (subatomic particles usually made of three quarks, such as the proton and neutron) and mesons (made up of an even number of quarks) forming. Baryons and mesons are collectively referred to as hadrons, which were the dominant form of matter until about a second after the Big Bang. At this point, neutrino decoupling took place, leaving the cosmic neutrino background. Neutrinos are another species of elementary particle, one of the class of fermions which rarely interact with matter, hence most of the neutrinos which appeared at this time remain unchanged. Also possibly making a first appearace at this time are primordial black holes. The end of the hadron epoch is also marked by the mutual annihilation of most of the hadrons and antihadrons, leaving the nascent universe dominated by leptons, another group of elementary particles which include the electron. Again, the leptons and antileptons annihilate one another by the end of this period, leaving photons as the dominant particle for a considerable time span thereafter.
Indeed, photons dominated the universe during a period between 10 seconds and 377,000 years after the formation of the universe, during which the universe was predominantly filled with a photon-baryon fluid. The first nuclei form between three and 20 minutes after the Big Bang, with protons (hydrogen ions) combining by means of nuclear fusion to form helium nuclei, until the rapid cooling put a stop to this process. At this point, the photons are unable to travel far, thus filling the cosmos with a dense, opaque plasma.
Further cooling takes place until about 47,000 years after the universe forms, by which stage cold dark matter, in the form of the atomic nuclei, comes to dominate over relativistic radiation, comprised of photons.
100,000 years in, the first molecules - helium hydride - form. These would eventually react with hydrogen to form molecular hydrogen, which would in turn provide the fuel needed for the formation of stars.
The Dark Ages and structure formation
13.422-12.8 billion years ago | 377,000-1 billion years.
Recombination and decoupling resulted in the universe becoming filled with a brilliant pale orange glow, which lasted for about 3 million years, before this "light" redshifted into a non-visible spectrum (i.e. its wavelength became longer, moving it into the infrared band).
About 10 to 17 million years after its formation, the average temperature of the universe was somewhere between 0 and 100°C - the temperature at which H2O is in liquid form. This has led to some speculation that, in some portion of the universe, conditions may have been suitable for the formation of rocky planets, bathed in the warmth of the universe as a whole, which may in turn have provided the opportunity for life to emerge.
About 200-500 million years after the universe came to be, the universe again burst into light with the formation of the first stars and galaxies. By this stage, those ripples which formed during the first moments of the universe had developed into dark matter filaments, to which the earliest large structures became drawn, eventually leading to the clusters and superclusters of galaxies which fill the universe today.
These first stars were not, however, like our own sun: they were enormous, between 100 and 300 solar masses and, having formed from the combination of hydrogen and helium hydride, possessed none of the metallic spectra which most stars today possess. Additionally, due to their size, these stars were extremely unstable and, as such, had much shorter lifespans than their successors, exploding after only a few million years as pair-instability supernovae, which in turn yielded the heavier elements which are found in the universe - as well as in you and I - today.
The universe we know
From 12.8 billion years ago | 1-13.8 billion years.
After about a billion years, reionization had completed its work and the universe took on an appearance roughly the same as it is today. This is known as the "Stelliferous Era," as it is the combined luminosity of the stars which shine like a beacon in the darkness of space. Thus far three distinct populations of stars have been identified. The first of these, known as Population III, is represented by the very first stars, the supermassive metal-free stars which appeared during the Dark Ages.
These gave rise to Population II, which includes the oldest stars visible via astronomy. HE 1523-0901, which is, at some 13.2 billion years old, the oldest star in the Milky Way; Cayrel's Star (alias the less-snappy BPS CS31082-0001), around 12.5 billion years old; Sneden's Star (BPS CS22892-0052); and BD +17° 3248, perhaps even older than HE 1523-0901. These stars are defined by the paucity of metals contained within. 2MASS J18082002−5104378 B, part of a binary system, is probably the oldest star yet discovered, coming in at a venerable 13.53 billion years of age.
By about 4 billion years after the Big Bang, Population I - or metal-rich - stars begin to form. Our sun is an intermediate member of this group: some stars, such as μ Arae, possess a greater proportion of metallic elements. Between Population II and Population I can be placed stars located among the intermediary disc population.
Within the Orion Spur of the Milky Way galaxy, some 4.6 billion years ago, material within a molecular cloud or "stellar nursery" began a process of gravitational collapse. Much of the mass gravitated to the centre, with a circular disk forming in orbit around it. This would give rise in relatively short order to the sun and its system of planets, asteroids and sundry other objects. You can find out more about these developments here.