The Big Bang – Learn

Big Bang Theory – BBT

The Big Bang Theory (BBT) is used to explain the beginning of the Universe as we best understand it. It provides an outline of the major events that have occurred since the time of the Big Bang including the interactions between energy and matter and the creation of the fundamental forces.

Radiation Transforms into Matter – Big Bang Timeline

At the instant before the Big Bang started the Universe as we know it, nothing existed – no time or matter. According to BBT energy was all that existed at this time. This energy spread out from a single point known as a singularity. The initial temperature at this single point was extremely high and as the energy spread further from the singularity the temperature began to cool. As the energy spread out and the temperature cooled energy was converted into matter. Particles that formed early on were rapidly destroyed by the high temperatures. At lower temperatures the particles became more stable and began to combine, forming atoms and molecules. 

Significant Events in the Early Universe:

  • (0 sec) – Universe is born
  • (10−36 to 10−32 sec) Inflation occurs
  • (10−12 to 10−6 sec, temp = 1016 K) Elementary particles including quarks and leptons form
  • (10−6 to 100 sec, temp = 1012  K) Annihilation of antimatter and matter leave a relatively small amount of matter 
  • (102 sec, temp = 109  K) Commencement of nuclear fusion
  • (103 sec) Cessation of fusion
  • (380,000 years, temp = 3000  K) The formation of atoms (recombination), CMB produced
  • (380,000 to 800,000,000 years) The Dark Ages (stars yet to form)
  • (800,000,000 years) The first stars and galaxies form; most atoms re-ionised
  • (9.3 billion years) The Earth and solar system form
  • (13.82 billion years, temp = 2.7  K) Today


For a more extensive explanation click here

Elementary Particles

Elementary particles are particles which can not be separated. The search for the elementary, or indivisible particle has seen scientists move from the atom to the particles which make up atoms – electrons, protons and neutrons. These three particles were the basis for understanding the periodic table. Later discoveries found that protons and neutrons are divisible but electrons are not, and electrons are therefore considered an elementary particle.

There are two other types of elementary particles – quarks and leptons. Different types of quarks can combine to produce other particles such as neutrons and protons. There are six types of quarks – up, down, strange, charm, bottom and top. Each quark has its own antiquark equivalent. Leptons are small elementary particles: electron, electron neutrino, muon, muon neutrino, tau and tau neutrino. Leptons interact via the gravitational force, electromagnetic force and weak force, but not the strong force. 

Matter and Antimatter

Particles are always created in pairs. For every matter particle there is an antimatter particle. At the instant they form and because they are so close together, they immediately annihilate each other and convert to pure energy. During the rapid expansion and cooling of the universe slightly more matter was created than antimatter. This is why most of the Universe that we detect and observe now is matter.

Formation of Particles

During the first few microseconds after the Big Bang the four forces separated and quarks and leptons came into existence. Over the next few minutes, the temperature cooled rapidly and neutrons and protons were formed. During the following 380,000 years the nuclei of hydrogen, helium and some lithium were able to form. At this stage atoms were not able to form and these nuclei existed as charged ions. Following this 380,000 year period, atoms and molecules were able to form and matter clumped together under gravity and gas clouds, stars and galaxies were able to form.

The Fundamental Forces

According to BBT the four fundamental forces separated in the first microsecond after the Big Bang. The four fundamental forces are the gravitational force, the electromagnetic force and the strong and weak nuclear force. The particles which mediate these forces are known as field particles.

Evidence for the Big Bang

Cosmic Background Radiation

In the 1960s physicists were investigating the ideas of the Steady State theory versus the Big Bang theory. It was assumed and concluded correctly that it must have been extremely hot at the beginning. This would have resulted in large amounts of radiation being emitted in all directions. As the universe rapidly expanded, this radiation would have been stretched and it was calculated that the wavelength would have increased to approximately a millimetre. If this was correct, the consequence would be that the apparent temperature of the radiation would have fallen from billions of degrees to just a few degrees above absolute zero.

Two other physicists using satellites designed to pick up radio signals were observing what they thought to be ‘interference’ from space. This radiation turned out to be the exact radiation that had been predicted to remain after the Big Bang. It also had the characteristics of heat radiation of just 2.7K (that is, 2.7 degrees above absolute zero) as predicted.

This evidence strongly supported the Big Bang Theory as there was no explanation as to how the Steady State Theory could predict or produce this sort of radiation. This leftover radiation came to be called the cosmic microwave background radiation, or CMB radiation for short. Special satellites have carefully mapped this radio ‘fingerprint’ of the early Universe.

Whilst the radiation that remains as a result of the Big Bang is very uniform, the CMB radiation image below illustrates that the radiation is not perfectly uniform. This indicates very small differences in temperature. This is very significant because it indicates that the early universe was not perfectly uniform. This is important because it allowed matter to clump together with other nearby pieces of matter under their own gravity resulting in the formation of early stars and galaxies.


The Helium-Hydrogen Ratio

Calculations have demonstrated that if the Universe did begin with a Big Bang then the ratio of H-He would be in the ratio 3:1. The composition of the Universe is 75% hydrogen and 25% helium which supports BBT.

The Red Shift of Light from Distant Galaxies

No matter which direction we look, the Universe is expanding. Galaxies observed in all directions from our galaxy are moving away from us and those which are further away are travelling at greater speeds.

Blue Stars in Deep Space

The light from galaxies has taken billions of years to reach Earth, hence, we are observing these galaxies as they were billions of years ago. These galaxies contain many more blue stars which are very hot. Very hot blue stars have much shorter life spans than stars like our Sun. This higher ration of blue stars in distant galaxies supports the BBT and the ageing of the Universe.

Particle Physics and Cosmology

The study of cosmology focuses on the time and events after the Big Bang. With the advancement in technology of particle accelerators, physicists have been able to recreate the early conditions after the Big Bang and explore the structure of matter at this time. Particle Physicists have worked backward to arrive at the same conclusions as cosmologists with respect to the early conditions and events after the Big Bang

The Expanding Universe

Edwin Hubble developed a method that allowed the distance to galaxies outside of our own to be determined. Further to this, careful observation by Hubble found that the spectra of the observed stars in these galaxies showed the familiar spectra of elements observed on Earth – particularly hydrogen and helium. He also noted that the characteristic wavelengths of these spectra had increased – or moved toward the red end of the spectrum. This became known as red shifted and was evidence that the Universe was expanding in all directions. 

Edwin Hubble – Hubble’s Law/constant

Hubble combined his data on the distances to galaxies and their red shift to conclude that the further away a galaxy, the faster it was travelling relative to Earth.

When he graphed his data he found that the speed of recession was proportional to the distance a galaxy was from Earth. This is known as Hubble’s Law and is stated as:

v={ H }_{ 0 }d


v is the recessional velocity (in km/s)

{ H }_{ 0 } is Hubble’s constant (73 kms-1 Mpc-1)

d is the distance away (in Mpc)


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