The Universe (Cambridge (CIE) O Level Physics): Revision Note

Astronomical Distances

• 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 even light takes many years to cross astronomical distances

• 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

Redshift

• If a star or galaxy is moving away from us (receding), the light we observe from it has a longer wavelength than the light it emitted

  • 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 electromagnetic radiation emitted from receding stars and galaxies

• 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 spectra 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

The Big Bang

• 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 are 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