P8.4 Red-shift and the expanding universe
The Doppler effect
When a wave source moves relative to an observer, the observed wavelength changes. If the source moves away, the wavelength appears stretched (longer). If the source approaches, it appears compressed (shorter).
For light:
- Source moving away → wavelength increases → light shifts towards the red end of the spectrum → red-shift.
- Source moving towards us → wavelength decreases → blue-shift.
Observations of distant galaxies
When astronomers examine the spectrum of light from distant galaxies, they find that the spectral lines (produced by specific elements like hydrogen) are all shifted to longer wavelengths (red-shifted) compared to the same element measured in a lab on Earth. The amount of red-shift tells us the recession speed (speed at which the galaxy is moving away).
Key finding: all distant galaxies are red-shifted. The further the galaxy, the greater the red-shift (and the faster it is receding).
Hubble's Law
Edwin Hubble (1929) found: recession speed ∝ distance. This is Hubble's Law: v = H₀d (where H₀ is the Hubble constant). The implication is profound — the universe is expanding. All galaxies are moving away from each other, not because they are moving through space, but because space itself is stretching.
Evidence for the expanding universe
- Red-shift of galaxies: All distant galaxies recede; further = faster recession.
- Cosmic Microwave Background CMB: Discovered in 1965; uniform microwave radiation from every direction — the remnant heat from the early hot universe, now cooled and stretched to microwave wavelengths.
Implications — the Big Bang
If the universe is currently expanding, then winding time backwards, everything was once in one place. The universe began in an enormously hot, dense event — the Big Bang — approximately 13.8 billion years ago.
Exam focus
- Red-shift = light from galaxy has longer wavelength than expected.
- Greater red-shift → greater recession speed → further away (Hubble's Law).
- CMB: evidence for Big Bang — universe was once much hotter.
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