5.12.9 Evolution of the Universe

• There are many different stages in the evolution of the universe
  • As the time from the Big Bang increases the temperature of the universe decreases
• The key stages of evolution are:
  • Stage 0: The Big Bang
  • Stage 1: Big Bang → 10^–35 s after the Big Bang
  • Stage 2: 10^–35 s after the Big Bang → 10^–6 s after the Big Bang
  • Stage 3: 10^–6 s after the Big Bang → 225 s after the Big Bang
  • Stage 4: 225 s after the Big Bang → 1000 years after the Big Bang
  • Stage 5: 1000 years after the Big Bang → 3000 years after the Big Bang
  • Stage 6: 3000 years after the Big Bang → 300 000 years after the Big Bang
  • Stage 7: 300 000 years after the Big Bang → Present

Stage 0 

• This is when the Big Bang occured
• At this point, time and space are created
• The universe is infinitely dense, hot and small, a hot singularity

Stage 1

• Just after the Big Bang → 10^–35 s after the Big Bang
• The universe expands rapidly
  • This is known as inflation
• There is no matter, only high energy gamma photons and electromagnetic radiation

Stage 2

• This is from 10^–35 s after the Big Bang → 10^–6 s after the Big Bang
• Building block particles come into existence (quarks, leptons, photons, and their antiparticles)
• These particles cannot form heavier particles (protons and neutrons) because of the high temperatures present
• There is slightly more matter than antimatter
  • As matter and antimatter annihilate, they leave a matter-dominated universe made from particles and not antiparticles

Stage 3

• This is from 10^–6 s after the Big Bang → 225 s after the Big Bang
• As the universe cools protons and neutrons begin to form from quarks
• Matter and antimatter continue to collide and annihilate
  • Producing enormous quantities of high-energy photons
  • These are continually absorbed and re-emitted as they interact with charged particles

Stage 4

• This is from 225 s after the Big Bang → 1000 years after the Big Bang
• As the universe continues to cool it behaves in the same way as the core of a star
• Nuclear fusion begins
  • Protons and neutrons fuse to form light nuclei like deuterium, helium and lithium
• Matter is in plasma form
  • A state in which protons and electrons are not bound to one another because of high temperatures
• Rapid expansion of the universe continues until 25% of matter is helium nuclei

Stage 5

• This is from 1000 years after the Big Bang → 3000 years after the Big Bang
• At this time, nuclear fusion ends
• Electrons are formed

Stage 6

• This is from 3000 years after the Big Bang → 300 000 years after the Big Bang
• The universe continues to cool and electrons combine with nuclei to form hydrogen and helium atoms
• In decoupling more electrons become attached to protons
  • Radiation and matter separate from each other
  • Photons travel freely through space
  • The universe becomes transparent
  • Photons now become the microwave background radiation that we detect today

Stage 7

• This is from 300 000 years after the Big Bang → Present
• After about 30 million years, the first stars form
• Galaxies begin to form from tiny density fluctuations because of gravitational forces pulling together clouds of hydrogen and existing stars
• Billions of years later, heavy elements form from the gravitational collapse of stars
• After approximately 9 billion years the solar system forms from a supernova nebula
  • Our Sun is formed at the centre of the nebula
  • Earth is formed almost 1 billion years later
• Approximately 11 billion years after the Big Bang, primitive life begins on Earth
• 7 billion years after the Big Bang, the first modern humans evolve