An Expanding Universe (OCR A Level Physics): Revision Note

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

• 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

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

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