The Universe (Cambridge O Level Physics)

• Astronomical distances, such as the distances between stars and galaxies, are so large that physicists use a special unit to measure them called the light-year
• One light-year is defined as:

The distance travelled by light through (the vacuum of) space in one year

• The speed of light is the universal speed limit, nothing can travel faster than the speed of light
• But over astronomical distances, light actually travels pretty slowly
• The diameter of the Milky Way is approximately 100 000 light-years
  • This means that light would take 100 000 years to travel across it

• One light year is equal to 9.5 × 10¹² km, or 9.5 × 10¹⁵ m

• Usually, when an object emits waves, the wavefronts spread out symmetrically
• If the wave source moves, the waves can become squashed together or stretched out

The Doppler Effect

Diagram showing the wavefronts produced from a stationary object and a moving object

• A moving object will cause the wavelength, λ, (and frequency) of the waves to change:
  • The wavelength of the waves in front of the source decreases and the frequency increases
  • The wavelength behind the source increases and the frequency decreases
  • This effect is known as the Doppler effect

• The Doppler effect also affects light
  • If an object moves away from an observer the wavelength of light increases
  • This is known as redshift as the light moves towards the red end of the spectrum

• Redshift is defined as:

An increase in the observed wavelength of electromagnet radiation emitted from receding stars and galaxies

Redshift & Blueshift

Light from a star that is moving towards an observer will be blueshifted and light from a star moving away from an observer will be redshifted

The observer behind the source observes redshift

• The Milky Way is just one of billions of galaxies that make up the Universe
• Light emitted from distant galaxies appears redshifted when compared with light emitted on Earth

• The diagram below shows the light coming to us from a close object, such as the Sun, and the light coming to us from a distant galaxy

Comparison of Light Spectra

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

• The diagram also shows that the light coming to us from distant galaxies is redshifted
  • The lines on the spectrum are shifted towards the red end

• This indicates that the galaxies are moving away from us
• If the galaxies are moving away from us it means that the universe is expanding
• The observation of redshift from distant galaxies supports the Big Bang theory
• Another observation from looking at the light spectrums produced from distant galaxies is 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 moving away from us

Redshift & Distance from Earth

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

• Around 14 billion years ago, the Universe began from a very small region that was extremely hot and dense
• Then there was a giant explosion, which is known as the Big Bang
• This caused the universe to expand from a single point, cooling as it does so, to form the universe today
• Each point expands away from the others
  • This is seen from galaxies moving away from each other, and the further away they are the faster they move

• Redshift in the light from distant galaxies is evidence that the Universe is expanding and supports the Big Bang Theory
  • As a result of the initial explosion, the Universe continues to expand

Expansion of the Universe

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

• An analogy of this is points drawn on a balloon where the balloon represents space and the points as 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 itself has expanded between the galaxies

Analogy for the Expanding Universe

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