Galactic red-shift

• Galactic Red-shift

  • 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
  • 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 (λ – Δλ) and the frequency increases
  • The wavelength behind the source increases (λ + Δλ) and the frequency decreases
  • This effect is known as the Doppler effect

   
• • The Doppler effect also affects light
  • If an object moves towards an observer the wavelength of light decreases
  • This is known as blueshift as the light moves towards the blue end of the spectrum
  • 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
• The Expanding Universe

  The Big Bang

  • 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
• • The main pieces of evidence for the Big Bang are
  • Galactic red-shift
  • Cosmic Microwave Background (CMB) radiation
• Evidence from Galactic Red-Shift

  • 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

  A) close

  B) distant

  
• Comparing the light spectrum produced from the Sun and a distant galaxy

  • 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
  • Greater the distance to the galaxy, the greater the redshift
  • This means that the further away a galaxy, the faster it is moving away from us
• • 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 the galaxies were originally all grouped together at a single point and were then exploded a similar effect would be observed
  • The galaxies that are the furthest are moving the fastest - their distance is proportional to their speed
  • The galaxies that are closer are moving slower
• Evidence from CMB Radiation

  • The discovery of the CMB (Cosmic Microwave Background) led to the Big Bang theory becoming the currently accepted model
  • The CMB is a type of electromagnetic radiation which is a remnant from the early stages of the Universe
  • It has a wavelength of around 1 mm making it a microwave, hence the name Cosmic Microwave Background
• In 1964, Astronomers discovered radiation in the microwave region of the electromagnetic spectrum coming from all directions and at a generally uniform temperature of 2.73 K

  • They were unable to do this any earlier since microwaves are absorbed by the atmosphere
  • Around this time, space flight was developed which enabled astronomers to send telescopes into orbit above the atmosphere
  • According to the Big Bang theory, the early Universe was an extremely hot and dense environment
  • As a result of this, it must have emitted thermal radiation
• • The radiation is in the microwave region
  • This is because over the past 14 billion years or so, the radiation initially from the Big Bang has become redshifted as the Universe has expanded
  • Initially, this would have been high energy radiation, towards the gamma end of the spectrum
  • As the Universe expanded, the wavelength of the radiation increased
  • Over time, it has increased so much that it is now in the microwave region of the spectrum
• RED SHIFT:

  A) Redshift

  B) The Big Bang

  
• • The CMB radiation is very uniform and has the exact profile expected to be emitted from a hot body that has cooled down over a very long time
  • The different colours represent different temperatures
  • The red / orange / brown regions represent warmer temperature indicating a higher density of galaxies
  • The blue regions represents cooler temperature indicating a lower density of galaxies
  • The temperature of the CMB is mostly uniform, however, there are minuscule temperature fluctuations
  • This implies that all objects in the Universe are more or less uniformly spread out
• Cosmic background radiation

  A) higher density

  B) lower density