An Expanding Universe (OCR A Level Physics)

• 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”