Big Bang : The Big Theory is the prevailing cosmological model for the observable universe. The model describes how the universe expanded from a very high-density and high-temperature state and offers a comprehensive explanation for a broad range of observed phenomenon

George Lemaitre, in his paper, proposed a model of the universe that is expanding throughout. Edwin Hubble later observed that galaxies move away from each other. Thus if the whole universe is expanding, it implied that at some point in the past it was closer together, denser and hotter.

Extrapolation of the expansion of the universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past. This primordial singularity is itself sometimes called “the Big Bang”.

At the beginning after the big bang, there was only radiation. However over time ranging from 300 seconds to 1 million years, the particles combined to form atoms (matter).

Physicists rely on Particle physics theory, General Theory of Relativity and experimental evidence from Large colliders to figure out what happened in the early stages of the universe.

• The earliest stages of Big bang are difficult to estimate.
  • It is believed that at the earliest stage , the universe was homogenous and uniformly distributed, with very high energy density and extremely high temperature and pressure.
  • As it started cooling and expanding very fast, the four fundamental forces disintegrated into its current form and particles began to form.
• The earliest particles were formed from radiation, through a process called pair production.
  • radiation under suitable condition gave birth to particle-antiparticle pairs.

• • The particle-antiparticle pairs were also annihilated, and thus radiation and matter were at equilibrium.

• At some point this equilibrium breaks, and there is a slight bias towards formation of particles over anti-particles.
  • This is generally attributed to a hypothetical phenomena called baryogenesis.
• Elementary particles like quark come together with gluons to form protons and neutrons (particles known as Baryons)
  • Because there is slight bias of particles over antiparticles, more baryons are formed than antibaryons.
• As the temperature drops, formation of proton-antiproton stops (or neutron-antineutron).
  • Annihilation dominates and most particles and antiparticles are annihilated to radiation.
  • Because there were more particles, than antiparticles, thus particles like protons, neutrons, and electrons survive the mass annihilation.
  • Neutrons and protons were almost at 1:1 ratio and could change between each other by combining with electrons/positrons or neutrinos/antineutrinos.

• • But this ratio broke in favour of protons due to slightly lower mass of proton
• When temperature was about a billion kelvin, a process called Big-Bang Nucleosynthesis happened.
  • Neutrons combined with protons to form the first Deuterium and Helium nuclei.
  • Most Protons largely remained as isolated Hydrogen atoms.
• It took a lot longer before electrons combines with the deuterium, helium and hydrogen nuclei to form atoms.

Extract from Physics Stage 6 Syllabus © 2017 NSW Education Standards Authority (NESA)