An Expanding Universe (OCR A Level Physics)

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Katie M


Katie M



An Expanding Universe

  • Hubble provided evidence from galactic redshift data that the universe is expanding
  • Astronomers have concluded that:
    • All galaxies are moving away from the Earth
    • Galaxies are moving away from each other
    • Distant galaxies are receding faster than closer galaxies
  • The diagram below shows:
    • Light coming to the Earth from a close object, such as the Sun,
    • Light coming to the Earth from a supernovae in a distant galaxy

Comparing the light spectrum produced from the Sun and a distant galaxy, downloadable IGCSE & GCSE Physics revision notes

Comparing the light spectrum produced from the Sun and a distant galaxy

  • Red-shift is observed when the spectral lines from the distant galaxy move closer to the red end of the spectrum
    • This is because light waves are stretched by the expansion of the universe so the wavelength increases (or frequency decreases)
    • This indicates that the galaxies are receding (moving away) from us
  • Light spectra produced from distant galaxies are red-shifted more than nearby galaxies
    • This shows that the greater the distance to the galaxy the greater the redshift
    • This means, that the further away a galaxy is, the faster it is receding (moving away) from the Earth

Hubbles-law, IGCSE & GCSE Physics revision notes

Graph showing the greater the distance to a galaxy, the greater the redshift

  • The fact the galaxies are moving further apart is what we would expect after an explosion
    • Matter is first densely packed and as it explodes it moves out in all directions getting further and further from the source of the explosion
    • Some matter will be lighter and travel at a greater speed, further from the source of the explosion
    • Some matter will be heavier and travel at a slower speed, closer to the source of the explosion
  • If someone were to travel back in time and compare the separation distance of the galaxies:
    • It would be seen that galaxies would become closer and closer together until the entire universe was a single point


  • If galaxies were originally all grouped together at a single point and then there was an explosion a similar effect would be observed:
    • The galaxies that are the furthest are moving the fastest
    • Their distance, d, is proportional to their speed, v, as shown by Hubble’s law, v = H0d
    • The galaxies that are closer are moving slower

Big Bang, downloadable AS & A Level Physics revision notes

Tracing the expansion of the universe back to the beginning of time leads to the idea the universe began with a “big bang”

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Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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