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When astronomers spread out the light from a distant galaxy into its spectrum, they find something remarkable: the whole pattern of lines is shifted towards the red end, and the further away the galaxy is, the bigger the shift. This single observation — red-shift — is the strongest evidence that the entire Universe is expanding, and by imagining that expansion running backwards, scientists arrive at the idea that everything began, billions of years ago, from a single tiny, hot, dense point: the Big Bang. This lesson explains what red-shift is, how it shows the Universe is expanding, and how it provides the key evidence for the Big Bang theory, together with a brief look at the cosmic microwave background radiation.
Higher tier: Red-shift and the Big Bang are Higher-tier content in OCR Gateway Science A. The core idea — that distant galaxies show red-shift and the Universe is expanding — is worth knowing for all students.
By the end of this lesson you should be able to explain what red-shift is, describe how the red-shift of distant galaxies shows the Universe is expanding, explain how this is evidence for the Big Bang, and state briefly the role of the cosmic microwave background radiation.
Light is a wave, and a wave has a wavelength. Visible light of a longer wavelength looks red, and light of a shorter wavelength looks blue. When we look at the light from distant galaxies, we find that the light has been shifted towards longer wavelengths — towards the red end of the spectrum. This is called red-shift.
More precisely, astronomers look at the pattern of dark lines in a galaxy's spectrum (produced by particular elements). In light from distant galaxies, this whole pattern is shifted towards the red — the lines appear at longer wavelengths than they do in a laboratory on Earth. The observed wavelength is increased.
Red-shift happens because the distant galaxies are moving away from us. As a source of light moves away, the light waves reaching us are, in effect, stretched to a longer wavelength — much as the pitch (a sound "wavelength" effect) of a siren drops as an ambulance speeds away. The faster a galaxy is receding, the greater its red-shift.
The everyday sound analogy is worth dwelling on because it makes the idea concrete. When an ambulance races towards you the siren sounds higher-pitched, and as it passes and speeds away the pitch drops noticeably. This happens because the motion of the source bunches up the sound waves in front of it (shorter wavelength, higher pitch) and stretches them out behind it (longer wavelength, lower pitch). Light behaves in a comparable way: a source moving away from us has its light waves stretched to a longer wavelength, shifting the light towards the red end of the spectrum. A source moving towards us would instead show a blue-shift (shorter wavelength). The fact that the light from distant galaxies is red-shifted, not blue-shifted, tells us they are moving away, not towards us.
How do astronomers actually measure such a tiny effect reliably? Every chemical element absorbs and emits light at its own precise set of wavelengths, producing a characteristic pattern of dark or bright lines in a spectrum — a kind of "barcode" for that element. Hydrogen, for example, always produces its lines at exactly the same wavelengths when measured in a laboratory. When astronomers see that same recognisable barcode pattern in the light from a distant galaxy but shifted bodily towards longer wavelengths, they can measure precisely how much it has shifted, and from that work out how fast the galaxy is receding. This is far more reliable than trying to judge whether the light simply "looks redder".
Exam Tip: Red-shift means the light is shifted to longer wavelengths (towards red). It shows the galaxy is moving away from us. Be precise: it is the increase in observed wavelength that defines red-shift.
The crucial observation is not just that galaxies are red-shifted, but how the red-shift depends on distance:
The more distant a galaxy is, the greater its red-shift — meaning the more distant galaxies are moving away faster.
This is exactly what you would expect if the whole Universe is expanding — if space itself is stretching, carrying the galaxies apart. In an expanding Universe, every galaxy moves away from every other one, and the further apart two galaxies are, the faster they separate. Our own galaxy is not at any special centre; an observer in any galaxy would see the same pattern of everything rushing away.
A common way to picture this is dots on the surface of a balloon being inflated: as the balloon grows, every dot moves away from every other dot, and dots that are further apart move apart faster. The dots themselves are not moving across the surface — it is the surface (space) between them that is stretching.
Exam Tip: The key fact is that more distant galaxies have greater red-shifts (recede faster). This shows the whole Universe is expanding — not that Earth is at the centre. Every observer sees everything moving away.
The Big Bang theory states that the Universe began from a single, extremely hot, dense point and has been expanding ever since, cooling as it grows, over about 13.8 billion years.
Red-shift is the central evidence:
So the observed expansion, deduced from red-shift, points back to a hot dense beginning.
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