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This lesson covers the key evidence for the expansion of the universe — red shift, Hubble's observations and cosmic microwave background radiation (CMBR) — and the Big Bang theory, as required by the Edexcel GCSE Combined Science specification (1SC0).
The Doppler effect is the change in the observed frequency (or wavelength) of a wave when the source and observer are moving relative to each other.
| Situation | Effect on Frequency | Effect on Wavelength |
|---|---|---|
| Source moving towards observer | Frequency increases | Wavelength decreases (blue shift) |
| Source moving away from observer | Frequency decreases | Wavelength increases (red shift) |
You can hear the Doppler effect when an ambulance siren sounds higher-pitched as it approaches and lower-pitched as it moves away.
When scientists examine the light from distant galaxies, they find that the absorption lines in the spectrum are shifted towards the red end (longer wavelength). This is called red shift.
Red shift tells us that distant galaxies are moving away from us — the space between us and the galaxies is expanding.
| Observation | Meaning |
|---|---|
| Light from a galaxy is red-shifted | The galaxy is moving away from us |
| Light from a galaxy is blue-shifted | The galaxy is moving towards us (very rare — only nearby galaxies like Andromeda) |
Exam Tip: Red shift does not mean the galaxies are actually red. It means the entire spectrum of light is shifted towards longer (redder) wavelengths compared to a stationary source.
In 1929, Edwin Hubble made a ground-breaking discovery by studying the red shift of many galaxies:
The further away a galaxy is, the greater its red shift — and therefore the faster it is moving away from us.
This relationship is known as Hubble's law:
v=H0×d
| Symbol | Quantity | Unit |
|---|---|---|
| v | Recession speed of the galaxy | km/s |
| H0 | Hubble constant | km/s per Mpc (megaparsec) |
| d | Distance to the galaxy | Mpc |
It is not that galaxies are flying through space; rather, the space between galaxies is expanding, carrying them apart.
The observation that the universe is expanding leads to a powerful conclusion: if we could rewind time, all the matter and energy in the universe would converge to a single point. This is the basis of the Big Bang theory.
| Point | Detail |
|---|---|
| The universe began from an extremely hot, dense point | All matter, energy, space and time originated here |
| The Big Bang occurred approximately 13.8 billion years ago | Estimated using Hubble's constant |
| Space has been expanding ever since | Galaxies are carried apart by the expansion of space |
| The Big Bang was not an explosion in space | It was the rapid expansion of space itself |
Exam Tip: A very common misconception is that the Big Bang was an explosion that happened at a particular point in existing space. In fact, space itself began with the Big Bang. There is no "centre" of the universe — every point is moving away from every other point.
Cosmic microwave background radiation is low-energy microwave radiation that fills the entire universe in every direction. It was first detected in 1965 by Arno Penzias and Robert Wilson.
| Explanation | Detail |
|---|---|
| Shortly after the Big Bang, the universe was extremely hot | It was filled with very high-energy, short-wavelength radiation |
| As the universe expanded, space stretched | The radiation's wavelength was stretched (increased) too |
| Today, this radiation has cooled to about 2.7 K (−270.5 °C) | It is now in the microwave region of the EM spectrum |
CMBR is the afterglow of the Big Bang, stretched by the expansion of space over 13.8 billion years. It is detected uniformly in all directions, which is exactly what the Big Bang theory predicts.
In 1998, observations of distant Type Ia supernovae revealed a surprising result: the expansion of the universe is accelerating — it is getting faster, not slowing down.
The cause of this accelerating expansion is unknown and has been named dark energy. Scientists estimate that dark energy makes up approximately 68% of the total energy content of the universe.
| Component | Approximate Proportion |
|---|---|
| Dark energy | ~68% |
| Dark matter | ~27% |
| Ordinary (visible) matter | ~5% |
Exam Tip: You do not need to explain dark energy in detail for Combined Science. Just know that the expansion of the universe is accelerating and that the cause is attributed to an unknown dark energy.
| Evidence | What It Shows |
|---|---|
| Red shift of galaxies | Galaxies are moving away from us; the universe is expanding |
| Hubble's law | Further galaxies recede faster; consistent with expansion from a single point |
| CMBR | Remnant radiation from the hot, early universe; fills all of space uniformly |
| Accelerating expansion | The expansion rate is increasing, attributed to dark energy |
Explain how the observation of red shift in the light from distant galaxies provides evidence for the Big Bang theory.
Light from distant galaxies is red-shifted — the wavelengths are longer than expected. This means the galaxies are moving away from us. Furthermore, more distant galaxies show greater red shift, meaning they are moving away faster (Hubble's law). If all galaxies are moving apart, then in the past they must have been closer together. Extrapolating back suggests that all matter was once concentrated at a single, extremely hot and dense point — this is the Big Bang. The universe has been expanding ever since.
| Misconception | Correction |
|---|---|
| The Big Bang happened at a specific location in space | The Big Bang was the expansion of space itself; there is no centre |
| Only galaxies far away are moving; nearby ones are stationary | Almost all galaxies are moving apart due to the expansion of space (a few very nearby ones, like Andromeda, are blue-shifted due to local gravitational attraction) |
| Red shift means galaxies are red | Red shift means the entire spectrum is shifted to longer wavelengths; the galaxies themselves are not red |
| CMBR comes from a single direction | CMBR is detected in all directions uniformly, consistent with the Big Bang |
The clue that galaxies are moving away from us comes from comparing the absorption spectrum of a distant galaxy with the known absorption spectrum of elements in the laboratory. Hydrogen, for example, absorbs light at very specific, well-measured wavelengths. When we look at a distant galaxy, we see those same patterns of dark lines — but they are shifted to longer wavelengths (toward the red end of the spectrum).
| Source | Observed Absorption Lines | Interpretation |
|---|---|---|
| Laboratory hydrogen | Normal wavelengths (reference) | At rest relative to us |
| Distant galaxy A | Slight red shift | Moving away slowly |
| Distant galaxy B (further) | Larger red shift | Moving away faster |
| Very distant galaxy | Huge red shift | Moving away very fast |
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