M8-S1: The Big Bang Model of The Origin of The Universe

  • investigate the processes that led to the transformation of radiation into matter that followed the ‘Big Bang’

The Big Bang Theory

The Big Bang Theory or model is the most popular understanding of the origin of the universe. It is supported by various experimental evidence and astronomical observations.

The model describes the universe as being in an extremely hot and dense state which subsequently expanded to create time and space. Throughout its creation, several things happened as discussed below in chronological order.


  1. Creation and separation of fundamental forces
    • There are four fundamental forces in the current universe: gravitational force, electromagnetic force, strong and weak nuclear force. All four forces were created at the beginning of the Big Bang
    • Gravitational force first separated from strong-electro-weak force (this group of three forces is termed Grand Unified force) during which the temperature cools to 1032
    • Shortly after (less than one second), strong nuclear force separated from electro-weak force during which the temperature cools to 1027
    • Radiation is more abundant than matter (besides fundamental particles, no other matter exists at this stage).
    • Formation of fundamental particles - quarks, leptons and antiparticles were created.


  1. Inflation of the universe occurred shortly after the formation and distinction of fundamental forces.
  • The expansion led to the release of energy which allows the universe to cool rapidly but more importantly create matter. The initial phase of expansion occurred over a very short interval of time (magnitude of 10-43 seconds), during which the size of the universe expanded by a factor of 1025.
  • Scientists believe that this rapid inflation is critical to prevent the universe from collapsing back into a black hole (due to its immense density and high level of energy).


  1. Formation of particles and antiparticles (1013 – 1015 K)
    • Leptons separate into electrons, neutrinos and antiparticles e.g. positron.
    • Quarks and antiquarks form protons, neutrons and antiparticles e.g. antiproton.
    • Particles and anti-particles have the mass but are always opposite in one of the fundamental characteristics of particles such as charge and magnetic moment.
    • Initially, the production of particles and antiparticles are always in pairs such that they come in equal ratios. As such, they would spontaneously undergo annihilation to convert back into gamma photons.
    • Formation of matter
      • Continuous outward expansion of the universe eventually reduced the likelihood of annihilation as a pair of particle and antiparticle becomes separated.
      • At the same time, for reasons unknown to physicists, the production of particle and antiparticle eventually became unbalanced; particles started to form more often than their antiparticles.
      • This increased particle to antiparticle ratio created subatomic particles such as protons and neutrons. The composition of subatomic particles will be covered in the standard model of matter.

  1. Big Bang Nucleosynthesis starts to occur after 3 minutes.
    • As the universe continues to cool (109 K), the lower temperature brought protons and neutrons to a less energetic state. As a result, their collision led to the formation of deuterium (hydrogen-2).
      • The formation of deuterium is a reversible reaction in the presence of gamma photons as their energy is sufficient to disintegrate deuterium nuclei.
    • Deuterium nuclei collide with neutrons to form tritium (hydrogen-3).
    • Tritium nuclei collide with protons to form helium-4.


  • Each stage of nucleosynthesis involves the emission of energy in the form of gamma photon. This process is understood and analysed using Einstein’s energy and mass equivalence equation.
  • Subsequently, as the universe continues to cool, electrons start to orbit around nuclei to form atoms.


  1. Matter decouples from radiation
    • As nucleosynthesis continues, more atoms are formed and the interaction between radiation and matter becomes less frequent.
    • Matter becomes more abundant than radiation.
    • The universe cools down to 3000 K.
    • As a result, high energy gamma photons are able to travel longer distances without being interfered by particles.
    • The transmission of these photons can be observed today in the form of low energy microwave photons (Cosmological Background Microwaves)


  1. Formation of stars and galaxies
    • After a billion years, stars and galaxies form.
    • Stellar fusion creates larger and heavier elements.
    • Supernova explosions can also create heavier elements.
    • Around 8 billion years after the Big Bang, the Solar system and the Sun are created.
    • The temperature of the universe today is around 3 K.


Change in matter, radiation and temperature

  • Decrease in temperature is modelled by exponential decay.


Figure shows the decrease in density of matter and radiation as the Universe expands and cools. As matter becomes more dominant than radiation, its density eventually 'crosses over' to become greater than that of radiation. Knowledge of dark energy is not required for HSC Physics.
  • investigate the evidence that led to the discovery of the expansion of the Universe by Hubble (ACSPH138)

The prominence and popularity of the Big Band theory and more importantly the expansion of the universe is due to a diversity of experimental evidence and astronomical observations. The primary one is the work done by Edwin Hubble.


Edwin Hubble’s contribution to astrophysics

  • Hubble used the largest telescope at the time to obtain long spectral exposures of nearby and distant galaxies. For most galaxies, Hubble observed red-shifting spectral lines indicating that most galaxies are moving away from Earth. Some galaxies that are nearby displayed blue-shift, indicating their movement towards Earth.


  • By measuring the degree of Doppler’s Effect, Hubble was able to determine the relative velocities of these galaxies.
  • Hubble noticed that the size of galaxies had an effect on their relative velocity. Smaller galaxies (an indication of distance away from Earth) produced much greater red-shift effect which in turn corresponded to greater velocity.

  • Hubble plotted the relationship between distance of galaxies and their velocity and observed a correlation. Therefore, he concluded that the more distant a galaxy is from Earth, the faster it is receding. This is known as Hubble’s Law.


where v is the velocity of a galaxy relative to Earth (or The Milky Way), H0 is Hubble’s constant and d is the distance of a galaxy away from Earth.


  • Since, velocity is dependent on distance, Hubble’s constant increases with the age of the universe. As a result, it is used to estimate the time since the Big Bang. Currently, the estimation derived from Hubble’s law is around 13.7 billion years with an error of 1%.


  • Hubble’s Law and along with his observations support the expansion model of the universe. More importantly, his work strongly suggests that the universe would have expanded from a much smaller space which supports the Big Bang model.


Cosmic background microwaves

  • Arno Penzias and Robert Wilson were adapting radio-antennae for astronomy research whereby they encountered background noise in the form of radiation. After troubleshooting they figured this was not a systematic error but resulted from radiation of primitive cosmic origin.
  • As mentioned previously, nucleosynthesis created gamma photons which were able to travel long distances. As the universe continues to cool, the energy of these photons also decreases in frequency and become longer in wavelength.
  • The background temperature of the universe is about 2.7 K which, when substituted into Wien’s displacement law, corresponds to a peak wavelength of about 0.2 cm. This wavelength lies within the microwave spectrum.


  • This observation is consistent with the process of nucleosynthesis which occurred shortly after the Big Bang.


 Analysis of primordial elements

  • The proposed process for nucleosynthesis resulted in a predictable ratio (by mass) of hydrogen and helium nuclei.
  • It is estimated that the current universe, by mass, consists of 23-26% helium (exact number is controversial), 73-75% hydrogen and the rest is heavier elements.
  • This discovery provided evidence against the theory that the production of nuclides mostly occurred inside stars. The outcome of this theory predicts that helium would make up a small amount of the universe – which obviously contradicts contemporary evidence.
  • Furthermore, this greater composition of helium validates the theory that during early stages of expansion, the temperature was sufficient to facilitate nucleosynthesis and thus create a vast abundance of helium nuclei.


 Practice Question 1

(a) Explain how radiation is transformed into matter in The Big Bang model of the origin of the Universe. (3 marks)

(b) Explain how Hubble’s work influenced the scientific community’s view on the Big Bang theory. (2 marks)


Next section: Spectra of Stars