Revision notes · Space physics (physics only)
Red-shift (physics only)
Red-shift4.8.2
Definition: Red-shift: light observed from most distant galaxies has longer wavelengths (shifted towards the red end of the visible spectrum) than the same light would have if it came from a source at rest relative to us. Red-shift happens because those galaxies are moving away from us — a moving light source stretches the wavelength of the light it emits, in the same way a receding sound source is heard at a lower pitch.
The further away a galaxy is, the greater its observed red-shift, meaning more distant galaxies are moving away from us faster. This relationship between a galaxy's distance and its speed is itself evidence that the universe is expanding.
🔗 Think of it like: Imagine galaxies as dots on the surface of a balloon that starts almost completely deflated (representing the very start of the universe) and is then inflated. As the balloon expands, every dot moves further away from every other dot — and the further apart two dots already are, the faster the distance between them grows as the balloon inflates. This is why more distant galaxies show greater red-shift: the space between us and them has been stretching for longer, or more, than the space between us and closer galaxies.
If the universe is expanding now, then tracing that expansion backward in time implies the universe must once have been much smaller and more compressed — this is the basis of the Big Bang model: the theory that the universe began expanding rapidly from an extremely small, hot, and dense single point.
Definition: Cosmic Microwave Background (CMB) radiation (HT only) is a second major piece of evidence for the Big Bang. In the very early, hot, dense universe, matter would have emitted large amounts of short-wavelength radiation. As the universe has expanded ever since, this radiation has been stretched to much longer wavelengths, and is observed today as a faint microwave signal detectable in every direction across the sky. Its presence everywhere is strong evidence that the universe was once much hotter and denser, and has been expanding and cooling ever since.
⚠️ Common mistake: The Big Bang model is currently the model best supported by the available observational evidence (red-shift and the CMB), but significant aspects of the universe — including dark matter and dark energy — remain poorly understood, and current models continue to be refined as new evidence emerges.
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