Galactic Red-shift (OCR GCSE Physics A (Gateway))

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

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 (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

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 in front observes a blue shift, the observer behind observes a red shift

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

• As a result of the initial explosion, the Universe continues to expand

• 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

Comparing the light spectrum produced from 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, the faster it is moving away from us

    

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

• 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

Tracing the expansion of the universe back to the beginning of time leads to the idea the universe began at a single point

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

The CMB is a result of high energy radiation being redshifted over billions of years

• 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 CMB map with areas of higher and lower temperature. Places with higher temperature have a higher concentration of galaxies, Suns and planets

• This is the closest image to a map of the Universe

• 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 (on the order of 0.00001 K)

  • This implies that all objects in the Universe are more or less uniformly spread out