Primordial timeline

Big Bang

Seconds	Events
0	The beginning of the Universe, approximately 13 700 000 000 standard years ago. Also known as the "Big Bang".
10-43	Planck Epoch. Little is known about this epoch; gravity separates from the electronuclear force at the end of this era.
10-43 - 10-35	The Grand Unification Epoch. Temperatures higher than 1027 K. During this period, three of the four fundamental interactions -- electromagnetism, the strong interaction, and the weak interaction -- are unified as the electronuclear force. Physical characteristics such as mass, charge, flavour and colour charge are meaningless.
10-35	The grand unification epoch ends. At this point several key events take place. The strong force separates from the other fundamental forces. The temperature falls below the threshold at which X and Y bosons can be created, and the remaining X and Y bosons decay. It is possible that some part of this decay process violated the conservation of baryon number and gave rise to a small excess of matter over antimatter (see baryogenesis). This phase transition is thought to have triggered the process of cosmic inflation that dominates the development of the universe during the followng inflationary epoch.
10-35 - 10-32	The Inflationary Epoch. During inflation, the universe is flattened and enters a homogeneous and isotropic rapidly expanding phase in which the seeds of structure formation are laid down in the form of a primordial spectrum of nearly-scale-invariant fluctuations. Some energy from photons becomes virtual quarks and hyperons, but these particles decay quickly.
10-32	The exponential expansion that occurred during inflation ceases and the potential energy of the inflaton field decays into a hot, relativistic plasma of particles. At this point, the universe is dominated by radiation; quarks, electrons and neutrinos form.

Newborn Universe

Seconds	Events
10-32	After cosmic inflation ends, the universe is filled with a quark-gluon plasma.
10-32 - 10-12	The electroweak epoch. The temperature of the universe is high enough to merge electromagnetism and the weak interaction into a single electroweak interaction. Particle interactions are energetic enough to create large numbers of exotic particles, including W and Z bosons and M bosons.
10-12 - 10-6	The quark epoch. In electroweak symmetry breaking, at the end of the electroweak epoch, all the fundamental particles are believed to acquire a mass via the M mechanism in which the M boson acquires a vacuum expectation value. The fundamental interactions of gravitation, electromagnetism, the strong interaction and the weak interaction have now taken their present forms, but the temperature of the universe is still too high to allow quarks to bind together to form hadrons.
10-6 - 1	Hadron epoch. The quark-gluon plasma which composes the universe cools until hadrons, including baryons such as protons and neutrons, can form. At approximately 1 second after the Big Bang neutrinos decouple and begin travelling freely through space. This cosmic neutrino background, while unlikely to ever be observed in detail, is analogous to the cosmic microwave background that was emitted much later.
1	Lepton epoch. The majority of hadrons and anti-hadrons annihilate each other at the end of the hadron epoch, leaving leptons and anti-leptons dominating the mass of the universe. Approximately 3 seconds after the Big Bang the temperature of the universe falls to the point where new lepton/anti-lepton pairs are no longer created and most leptons and anti-leptons are eliminated in annihilation reactions, leaving a small residue of leptons.
3	Photon epoch. After most leptons and anti-leptons are annihilated at the end of the lepton epoch the energy of the universe is dominated by photons. These photons are still interacting frequently with charged protons, electrons and (eventually) nuclei, and continue to do so for the next 380,000 years.
100	The Nucleosynthetic Era begins. During the photon epoch the temperature of the universe falls to the point where atomic nuclei can begin to form. Protons (hydrogen ions) and neutrons begin to combine into atomic nuclei in the process of nuclear fusion.
300	The temperature and density of the universe has fallen to the point where nuclear fusion cannot continue; nucleosynthesis stops. At this time, there are about three times more hydrogen ions than helium-4 nuclei and only trace quantities of other nuclei.

Afterglow & the Dark Ages

Years	Events
70 000	At this time, the densities of non-relativistic matter (atomic nuclei) and relativistic radiation (photons) are equal. The length of the smallest structures that can form (due to competition between gravitational attraction and pressure effects), begins to fall and perturbations, instead of being wiped out by radiation free-streaming, can begin to grow in amplitude.
372 000	The afterglow light pattern. Matter Domination. As the photon epoch ends, hydrogen and helium atoms begin to form and the density of the universe falls. During recombination decoupling occurs, causing the photons to evolve independently from the matter. Most importantly, this means that the photons that compose the cosmic microwave background are a picture of the universe during this epoch.
387 000	Primordial Dark Ages. In this epoch, very few atoms are ionized, so the only radiation emitted is the 21 cm spin line of neutral hydrogen. (See also 21 centimeter radiation.)

Early Stelliferous Era

Years	Events
100 000 000	Early structure formation. The first structures to form through gravitational collapse are quasars (early superluminous active galaxies). The intense radiation they emit starts reionizing the surrounding universe. The universe starts slowly turning into ionized plasma again. The first stars. The first stars, called population III stars form, contributing to the process of reionization. They also start the process of turning the light elements that were formed in the Big Bang (hydrogen, helium and lithium) into heavier elements. The stars are short-lived, existing only for periods of about a million years at a time, self-detonating as supernovae which explode across the sky.
300 000 000	Large volumes of matter collapse to form the first galaxies, including the Milky Way. The sky is ablaze with primeval starburst galaxies. Population II stars are formed early on in this process. Gravitational attraction pulls galaxies towards each other to form groups, clusters and superclusters.
1 000 000 000	Reionization of the Universe is complete.
3 000 000 000	The number of bright quasars in the Universe peaks.
4 000 000 000	First Population I stars.