Galactic Redshift (HL IB Physics)

• 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
    • The wavelength is increasing but the frequency is decreasing
  • Blueshift if the object is moving towards the Earth
    • The wavelength is decreasing but the frequency is increasing

Stars and galaxies can be red or blueshifted to an observer on Earth

• Redshift is defined as:

The fractional increase in wavelength (or decrease in frequency) due to the source and observer receding from each other

• Redshift can be observed by comparing the light spectrum produced from a close object, such as our Sun, with that of a distant galaxy
  • The light from the distant galaxy is shifted towards the red end of the spectrum (compared to the Sun's spectra)
  • This provides evidence that the universe is expanding

Comparing the light spectrum produced by the Sun with light from a distant galaxy

 

Positive and Negative Velocities

• If the speed of the galaxy relative to Earth is positive then the galaxy is moving towards the Earth
  
  • This is the case when the observed frequency f₀ is greater than the reference frequency f

• If the speed of the galaxy relative to Earth is negative then the galaxy is moving away from the Earth
  • This is the case when the observed frequency f₀ is less than the reference frequency f

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 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

• Another observation from looking at the light spectra produced by distant galaxies is that the greater the distance to the galaxy, the greater the redshift
  
  • This means that the greater the degree of redshift, the faster the galaxy is moving away from Earth

The further a galaxy is from Earth, the greater its redshift tends to be

The furthest galaxies appear to be redshifted the most and are receding the fastest