Evolution of the Universe (OCR A Level Physics): Revision Note

The 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: Inflation ( → )

  • Stage 2: Gamma photons ( → )

  • Stage 3: Quarks and leptons form ( → )

  • Stage 4: Hadrons form ( → )

  • Stage 5: Nuclei form ( → )

  • Stage 6: Atoms form ( → )

  • Stage 7: Stars and galaxies form ( → Present)

Stage 0: The Big Bang

• At time , about 13.7 billion years ago, the Big Bang occurred

• At this point, time and space are created

• The universe is infinitely dense, hot and small, a hot singularity

Stage 1: Inflation

• Time after Big Bang: to

• The universe expands rapidly

  • This is known as inflation

Stage 2: Gamma photons

• Time after Big Bang: to

• There is no matter, only high-energy gamma photons

Stage 3: Quarks and leptons form

• Time after Big Bang: to

• 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 4: Hadrons form

• Time after Big Bang: to

• As the universe cools, the first hadrons, such as protons and neutrons, begin to form from quarks

• Matter and antimatter continue to collide and annihilate

  • This produces enormous quantities of high-energy photons

  • These are continually absorbed and re-emitted as they interact with charged particles

Stage 5: Nuclei form

• Time after Big Bang: to

• When the universe reaches temperatures similar to the core of a star, nucleosynthesis begins

• The first nuclei form through electron capture and nuclear fusion

  • Protons and neutrons fuse to form light nuclei like deuterium, helium, lithium, and beryllium

• 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 6: Atoms form

• Time after Big Bang: to

• The universe continues to cool, and electrons combine with nuclei to form the first 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: Stars and galaxies form

• Time after Big Bang: to 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

• 13.7 billion years after the Big Bang, the first modern humans evolve

• The final temperature of the universe becomes 2.7 K