From the Big Bang to Today's Universe

t < 10^-43 s : The Big Bang
The universe is considered to have expanded from a single point with an infinitely high energy density (infinite temperature). Is there a meaning to the question "What existed before the big bang?"

t ≈ 10^-43 s, 10³² K (10¹⁹ GeV, 10^-34m): Gravity “freezes” out All particle types (quarks, leptons, gauge bosons, and undiscovered particles e.g. Higgs, sparticles, gravitons) and their anti-particles are in thermal equilibrium, meaning they are created and annihilated at an equal rate. They coexist with radiation (photons). Through a phase transition, gravity "freezes" out and becomes distinct in its action from the weak, electromagnetic, and strong forces. At this time, the other three forces cannot be distinguished from one another in their action on quarks and leptons. This is the first instance of the breaking of symmetry amongst the forces.

t ≈ 10^-35 s, 10²⁷ K (10¹⁶ GeV, 10^-32 m) : Inflation The rate of expansion increases exponentially for a short period of time. The universe doubles in size every 10^-34 s. Inflation stops at around 10^-32 s, by which time the universe has increased in size by a factor of 10⁵⁰. This is equivalent to an object the size of a proton swelling to 10¹⁹ light years across!
The whole universe is estimated to have had a size of ~10²³ m at the end of the period of inflation. The universe we can see today, however, was only 3 m in size after inflation. This solves the problems of ‘horizon’ (how is it possible for two opposing parts of the present universe to be at the same temperature when they cannot have interacted with each other before recombination?) and ‘flatness’ (density of matter is close to the critical density).

t ≈ 10^-32 s : Strong forces freezes out Through another phase transition the strong force "freezes" out and a slight excess of matter over anti-matter develops. This excess, at a level of 1 part in a billion, is sufficient to give the presently observed dominance of matter over anti-matter. The temperature is too high for quarks to remain clumped to form neutrons or protons and so they exist in the form of a quark-gluon plasma. The LHC can study this by colliding together high-energy nuclei.

t ≈ 10^-10 s, 10¹⁵ K (100 GeV, 10^-18 m) : Electromagnetic and Weak Forces separate The energy density corresponds to that at LEP. As the temperature falls, the weak force "freezes" out and all four forces become distinct in their actions. The antiquarks annihilate with the quarks, leaving a residual excess of matter. W and Z bosons decay. In general, unstable massive particles disappear when the temperture falls to a value at which photons from the black-body radiation do not have sufficient energy to create a particle-antiparticle pair.

t ≈ 10^-4 s, 10¹³  K (1 GeV, 10^-16 m) : Protons and Neutrons form The universe has grown to the size of our solar system. As the temperature drops, quark-antiquark annihilation stops and the remaining quarks combine to make protons and neutrons.

t = 1 s, 10¹⁰ K (1 MeV, 10^-15 m) : Neutrinos decouple The neutrinos become inactive (essentially, they do not participate further in interactions). The electrons and positrons annihilate and are not recreated. An excess of electrons is left. The neutron-proton ratio shifts from 50:50 to 25:75.

t = 3 minutes, 10⁹ K (0.1 MeV, 10^-12 m) : Nuclei are formed The temperature is low enough to allow nuclei to be formed. Conditions are similar to those that exist in stars today or in thermonuclear bombs. Heavier nuclei such as deuterium, helium, and lithium soak up the neutrons that are present. Any remaining neutrons decay with a time constant of ~ 1000 seconds. The neutron-proton ratio is now 13:87. The bulk constitution of the universe is now in place, consisting essentially of protons (75%) and helium nuclei. The temperature is still too high to form any atoms. Electrons form a gas of free particles.

t = 300 000 years, 6000 K (0.5 eV, 10^-10 m) : Atoms are created Electrons begin to stick to nuclei. Atoms of hydrogen, helium, and lithium are created. Radiation is no longer energetic enough to break atoms. The universe becomes transparent. Matter density dominates. Astronomy can study the evolution of the Universe back to this time.

t = 10⁹ years, 18 K : Galaxy Formation Local mass density fluctuations act as seeds for stellar and galaxy formation (the exact mechanism is still not understood). Nucleosynthesis, the synthesis of heavier nuclei such as carbon up to iron, starts occurring in the thermonuclear reactors that are stars. Heavier elements are synthesized and dispersed in the moments when stars collapse and supernova explosions occur.

t = 15 x 10⁹ years, 3 K : Humans The present day. Chemical processes have linked atoms to form molecules. From the dust of stars and through coded messages (DNA) humans emerge and begin to observe the universe around them.