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

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Katie M

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Katie M

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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

doppler-effect, IGCSE & GCSE Physics revision notes

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

red-&-blue-shift, IGCSE & GCSE Physics revision notes

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

Light Doppler Shift, downloadable AS & A Level Physics revision notes

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

Examiner Tips and Tricks

You need to know that in the visible light spectrum red light has the longest wavelength and the smallest frequency.

To help you to remember what happens to the wavelength and the frequency of an object as it moves further away, it is useful to think about how the sound of a motorbike would change as it travels past and then away from you. As the motorbike moves away from you the pitch of the sound will become lower. This means the frequency of the sound is decreasing. If the frequency has decreased, the wavelength must also have increased.

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

universe-expansion, IGCSE & GCSE Physics revision notes
  • 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, downloadable IGCSE & GCSE Physics revision notes

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

      Hubbles-law, IGCSE & GCSE Physics revision notes

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

Big Bang, downloadable AS & A Level Physics revision notes

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

CMB Red Shift, downloadable IGCSE & GCSE Physics revision notes

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

CMB, downloadable IGCSE & GCSE Physics revision notes

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

Examiner Tips and Tricks

An analogy for the big bang can be to think of a balloon. This can be helpful for understanding, but it is not expected for you to compare the universe to a balloon in your exam!

  • The balloon represents space and the points as 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

    • Therefore, the density of galaxies falls as the Universe expands

Big Bang Balloon Analogy, downloadable IGCSE & GCSE Physics revision notes

A balloon inflating is similar to the stretching of the space between galaxies

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Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.