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The Big Bang Theory (Oxford AQA IGCSE Physics)

Revision Note

Dan Mitchell-Garnett

Expertise

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Cosmic Microwave Background Radiation

  • Cosmic microwave background radiation (CMBR) is a form of electromagnetic radiation filling the universe

    • It comes from radiation that was present shortly after the beginning of the universe

  • It is detected everywhere throughout the Universe

    • The CMBR map is the closest image that exists to a map of the Universe

  • It shows that the temperature of the Universe, and therefore the objects in it, are more or less uniformly spread out

CMBR map of the universe

An oval shape showing an evenly distributed pattern of orange and blue, representing higher and lower temperature, for GCSE & IGCSE physics revision notes.
The CMBR map shows areas of higher and lower temperatures in the Universe. Regions with higher temperatures have a higher concentration of galaxies.

  • The CMBR comes from high-energy radiation which originated just after the Big Bang

  • As the Universe expanded, the wavelength of this radiation increased

    • It therefore lost energy and produced the CMBR which can be observed today

The Big Bang Theory

  • Scientists believe the universe began with a ‘big bang’, 14 thousand million years ago

  • The Big Bang theory is currently the only theory that can explain the existence of CMBR

  • A giant explosion, known as the Big Bang, caused the Universe to expand outwards

    • As each point moved away from the others, the Universe began to cool

    • As a result of the initial explosion, the Universe continues to expand

The expansion of the Universe over time

The Universe was initially very small and very hot. It has since expanded and cooled over time. GCSE & IGCSE physics revision notes
Tracing the expansion of the Universe back to the beginning of time leads to the idea it must have begun with a “Big Bang”

  • The Big Bang theory is very well supported by two main pieces of evidence:

    1. Cosmological red-shift
      This provides strong evidence for the expansion of the Universe

    2. Cosmic Microwave Background Radiation (CMBR)
      This provides evidence for the expansion of the Universe and that it had a hot beginning

Worked Example

Describe and explain what can be deduced about the history of the Universe from the CMBR.

Answer:

Step 1: Recall the features of the CMBR

  • Microwave radiation is detected from all directions at a similar intensity

Step 2: State the source of this radiation

  • This is the radiation produced just after the formation of the Universe

Step 3: Describe how the wavelength has changed and explain why

  • When the Universe was formed, the radiation was high in energy and short in wavelength

  • Now the wavelength is longer

  • This is because the Universe has expanded, causing the wavelength to increase

Step 4: Suggest what this infers about the Universe at its beginning

  • This suggests the Universe was initially very small and very energetic and has been expanding since

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    Dan Mitchell-Garnett

    Author: Dan Mitchell-Garnett

    Dan graduated with a First-class Masters degree in Physics at Durham University, specialising in cell membrane biophysics. After being awarded an Institute of Physics Teacher Training Scholarship, Dan taught physics in secondary schools in the North of England before moving to SME. Here, he carries on his passion for writing enjoyable physics questions and helping young people to love physics.