OCR A Level Physics

Revision Notes

5.12.7 The Big Bang

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The Big Bang Theory

  • Around 13.7 billion years ago the universe was created from a hot singularity (a single point) which was infinitely dense, hot and small
  • There was a giant explosion, which is known as the Big Bang
    • Both space and time were created at this instant
  • This caused the universe to expand and cool from a single point, to form the universe today
  • Each point expands away from the others
    • This is seen from galaxies moving away from each other
    • The further away they are, the faster they are moving
  • As a result of the initial explosion, the Universe continues to expand

universe-expansion, IGCSE & GCSE Physics revision notes

All galaxies are moving away from each other, indicating that the universe is expanding

  • There are 2 key pieces of evidence to support the Big Bang theory:
    1. Hubble’s Law shows the universe is expanding, through the red shift of light from distant galaxies
    2. Microwave background radiation provides evidence that the universe has expanded from a single point and cooled significantly during the time it has been expanding

Microwave Background Radiation

  • After space flight was developed astronomers were able to send telescopes into orbit above the atmosphere
  • In 1964, this led to the discovery of radiation in the microwave region of the electromagnetic spectrum
    • A microwave has a wavelength of about 1 mm


Electromagnetic spectrum, IGCSE & GCSE Physics revision notes

Microwaves have the second-longest wavelength in the electromagnetic spectrum

  • Astronomers were unable to detect these microwaves before the development of space flight since microwaves are absorbed by the atmosphere
  • The microwave radiation detected came from all directions and at a generally uniform temperature of 2.73 K
  • Microwave background radiation is a type of electromagnetic radiation which is a remnant from the early stages of the Universe
  • According to the Big Bang theory, the early Universe was an extremely hot and dense environment
    • As a result of this, it must have emitted thermal radiation
  • This radiation has expanded with the expansion of the universe and is now in the microwave region of the EM spectrum
    • Initially, this would have been high energy radiation, towards the gamma end of the spectrum
    • This is because over the past 13.7 billion years the radiation initially from the Big Bang has become redshifted as the Universe has expanded
    • As the Universe expanded, the wavelength of the radiation increased
    • It has increased so much that it is now in the microwave region of the spectrum

CMB Red Shift, downloadable IGCSE & GCSE Physics revision notes

The Microwave Background Radiation is a result of high energy radiation being redshifted over billions of years

  • Microwave background radiation is uniform and has the exact profile expected to be emitted from a hot body that has cooled down over a very long time
    • This phenomenon is something that other theories (such as the Steady State Theory) cannot explain
  • Microwave background radiation is represented by a map of the universe

CMB, downloadable IGCSE & GCSE Physics revision notes

The Microwave Background Radiation map with areas of higher and lower temperature. 

  • This is the closest image that exists to a map of the Universe
  • The different colours represent different temperatures
    • The red/orange/brown regions represent warmer temperatures indicating a higher density of galaxies
    • The blue regions represent cooler temperatures indicating a lower density of galaxies
  • The temperature of the microwave background radiation is mostly uniform
    • It has a value of 2.7 ± 0.00001 K
    • This implies that all objects in the Universe are uniformly spread out
  • The discovery of the microwave background radiation led to the Big Bang theory becoming the currently accepted model
    • If the universe had not started in a Big Bang then there would be no microwave background radiation
    • If the universe was younger than 13.7 billion years then the temperature would be higher than 2.7 K

The Big Bang & Space Time

  • Albert Einstein helped develop the idea of space-time as part of his theory of relativity
  • General relativity states that space and time are connected by a property known as space–time
    • Space–time connects the three dimensions of space (the x, y and z–axis) to a fourth dimension, which is time
  • The evidence suggests that the Big Bang gave rise to the expansion of space–time about 13.7 billion years ago
  • An analogy of this is to draw points on a balloon
    • The balloon represents space and the points represent galaxies
  • When the balloon is deflated, all the points are close together and an equal distance apart
  • As the balloon expands, all the points become further apart by the same amount
  • This is because the space between the galaxies has expanded 
    • The galaxies are not actually moving through space but being carried along as space itself expands

Big Bang Balloon Analogy, downloadable IGCSE & GCSE Physics revision notes

A balloon inflating is similar to the stretching of the space between galaxies

  • Regardless of which galaxy you observe the universe from, the other galaxies all appear to be moving away from one another by the same amount
  • This agrees with the cosmological principle which states that the Universe is
    • Homogeneous (i.e. matter is uniformly distributed)
    • Isotropic (i.e. the Universe is the same in all directions to every observer)

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