Galactic Red-shift (OCR GCSE Physics A (Gateway))
Revision Note
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
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
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
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
A balloon inflating is similar to the stretching of the space between galaxies
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