AQA Physics 4.8.2
Red-shift and the Big Bang
Light from distant galaxies arrives stretched. Line up their spectra, find the pattern, and follow how scientists got from that observation to a theory of how the universe began.
Triple Science · Foundation tier
Start here
A siren going past.
An ambulance races towards you with its siren on, passes, and drives away. As it moves away, the siren sounds lower in pitch: the sound waves reaching you are stretched out.
If you had to guess, would light from something moving away from you change too?
Observation · line up the spectra
The further the galaxy, the bigger the shift.
The dark lines in a galaxy’s spectrum sit at known wavelengths when measured in a lab. Compare each galaxy’s lines with the lab’s. Predict the furthest galaxy before you look.
A small shift: every line has moved a little towards the red.
There is an observed increase in the wavelength of light from most distant galaxies. Every wavelength shifts towards the red end of the spectrum: red-shift. The further away the galaxies, the faster they are moving and the bigger the observed increase in wavelength.
Observation, pattern, theory
From stretched light to the Big Bang.
Tap the four links in order.
No centre · pick where you stand
From any galaxy, every other galaxy is moving away, and the further ones faster. Galaxies are moving away because space itself is expanding, and we are not at the centre. The Big Bang was not an explosion into empty space.
Since 1998
Receding ever faster, and not yet understood.
Since 1998, observations of supernovae suggest that distant galaxies are receding ever faster. There is still much about the universe that is not understood, for example dark mass and dark energy. These have their own lesson.
Think again · what “red” means here
“Red-shifted galaxies look red because they are very hot.”
Your call
ChooseWhat does it mean when a galaxy’s light is red-shifted?
Sort
Observation, theory, or not true?
Place each statement.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Describe
- Say what is observed: wavelength increased, more for further galaxies.
- Explain
- Observation, then what it shows, then the theory it supports.
- Interpret
- Compare the line positions in two spectra.
Key fact
Red-shift is evidence that space itself is expanding, and supports the Big Bang theory: the universe began from a very small region that was extremely hot and dense.
Key note · AQA 4.8.2
Red-shift and the Big Bang
- There is an observed increase in the wavelength of light from most distant galaxies: red-shift.
- The further away the galaxies, the faster they are moving and the bigger the observed increase in wavelength.
- Red-shift is evidence that space itself (the universe) is expanding, and supports the Big Bang theory.
- The Big Bang theory: the universe began from a very small region that was extremely hot and dense.
- Since 1998, observations of supernovae suggest that distant galaxies are receding ever faster.
- Much is still not understood, for example dark mass and dark energy.
Photograph this card. Tomorrow, cover it and say each line first.
Previous and next
Connects to
Ask MrBadmus
Not sure what red-shift tells us?
Spectra and the galaxies figure are schematic: line positions and distances are not to scale.