• The?Doppler effect?is important in many key areas of science including:
  • Radar?readings for moving objects
  • Measuring the?rate of blood flow?within a patient
  • Finding planet?orbits?around distant stars
  • Mapping the?expansion?of the universe

Redshift of EM Radiation

• On Earth, the Doppler effect of sound can be easily observed when sound waves move past an observer at a notable speed
• In space, the Doppler effect of light can be observed when spectra of distant stars and galaxies are observed, this is known as:
  • Redshift?if the object is moving away from the Earth, or
  • Blueshift?if the object is moving towards the Earth

• Redshift is defined as:

• For?non-relativistic?galaxies, Doppler redshift can be calculated using:

• Where:
  • Δλ = shift in wavelength (m)
  • λ = wavelength emitted from the source (m)
  • Δf = shift in frequency (Hz)
  • f?= frequency emitted from the source (Hz)
  • v?= speed of recession (m s^-1)
  • c?=?speed of light?in?a vacuum (m s^-1)

• • Received frequency,?f_r?= 4.547 × 10¹⁴?Hz
  • Original frequency,?f₀?=?4.570 × 10¹⁴?Hz
  • Shift in frequency, Δf?= (4.547 – 4.570) × 10¹⁴?= –2.3 × 10¹²?Hz
  • Speed of light,?c?=?3.0 × 10⁸?m s^–1

Step 4:?Write a concluding sentence

The observed frequency is?less?than the emitted frequency (the light from a laboratory source), therefore, the source is?receding, or moving away, from the Earth at?1.5 × 10⁶?m s^–1

An Expanding Universe

• After the discovery of Doppler redshift, astronomers began to realize that almost all the galaxies in the universe are receding
• This led to the idea that the space between the Earth and the galaxies must be?expanding
• This expansion stretches out the light waves as they travel through space, shifting them towards the red end of the spectrum
• The more red-shifted the light from a galaxy is, the?faster?the galaxy is moving away from Earth

• The expansion of the universe can be compared to dots on an inflating balloon
  • As the balloon is inflated, the dots all move away from each other
  • In the same way as the rubber stretches when the balloon is inflated, space itself is?stretching out between galaxies
  • Just like the dots, the galaxies move away from each other, however, they themselves do not move