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This lesson explores the concept of the expanding universe in greater depth, including Hubble's observations, the implications of expansion, and the ultimate fate of the universe, as required by the AQA GCSE Physics specification (4.8.2). This is a Physics-only topic. You need to understand how the universe is expanding, what this means for the distances between galaxies, and the current scientific understanding of the future of the universe.
In the 1920s, the American astronomer Edwin Hubble made observations that fundamentally changed our understanding of the universe. By studying the light from distant galaxies, Hubble found two crucial things:
The universe contains many galaxies beyond our own Milky Way. Before Hubble, many astronomers thought the Milky Way was the entire universe. Hubble showed that "spiral nebulae" were actually separate galaxies, each containing billions of stars.
The light from nearly all distant galaxies is red-shifted. This meant that nearly all galaxies are moving away from us. Crucially, Hubble found that the speed at which a galaxy recedes is proportional to its distance from us.
This relationship — known as Hubble's Law — can be expressed as:
v = H x d
where:
Exam Tip: You do not need to memorise the value of the Hubble constant for the exam, but you should be able to use the equation v = H x d if given values. Remember that the Hubble constant tells us the rate at which the universe is expanding.
When we say the universe is expanding, we mean that the space between galaxies is increasing over time. The galaxies themselves are not moving through space like bullets through air — instead, the fabric of space itself is stretching.
A commonly used analogy is a balloon with dots drawn on its surface. As the balloon inflates:
This analogy illustrates key features of the expanding universe:
| Feature | Balloon Analogy | Real Universe |
|---|---|---|
| Dots | Drawn on surface | Galaxies |
| Surface | Rubber of the balloon | Fabric of space |
| Inflation | Balloon being blown up | Space expanding |
| Centre of expansion | No dot is at the centre | No galaxy is at the centre |
| Distance between dots | Increases as balloon inflates | Distance between galaxies increases |
Exam Tip: The expanding universe does not mean there is a centre or an edge. Every point in space sees other galaxies receding. A common misconception is that the Big Bang happened at a specific location in space — in fact, the Big Bang happened everywhere in the universe simultaneously, because the universe was the Big Bang.
To appreciate the vastness of the expanding universe, consider these scales:
| Object / Distance | Size / Distance |
|---|---|
| Earth to Moon | 384,400 km (1.3 light-seconds) |
| Earth to Sun | 150 million km (8.3 light-minutes, 1 AU) |
| Sun to nearest star (Proxima Centauri) | 4.2 light-years |
| Diameter of Milky Way | About 100,000 light-years |
| Distance to nearest large galaxy (Andromeda) | About 2.5 million light-years |
| Diameter of observable universe | About 93 billion light-years |
| Age of the universe | About 13.8 billion years |
A light-year is the distance that light travels in one year. Since light travels at approximately 3 x 10 to the power 8 m/s, one light-year is approximately 9.46 x 10 to the power 12 km (about 9.5 trillion km).
Light-years are used because distances in space are so vast that using kilometres would give unmanageably large numbers.
The observable universe is the part of the universe from which light has had time to reach us since the Big Bang (13.8 billion years ago). Because the universe is expanding, the edge of the observable universe is now about 46.5 billion light-years away in any direction, giving an observable universe diameter of about 93 billion light-years.
There may be more universe beyond what we can observe, but we cannot see it because the light from those regions has not yet had enough time to reach us.
Astronomers use several methods to measure distances to objects at different scales:
| Method | Range | Principle |
|---|---|---|
| Parallax | Nearby stars (up to about 1,000 light-years) | Measuring the apparent shift of a star as Earth orbits the Sun |
| Standard candles (Cepheid variables) | Nearby galaxies | Cepheid variable stars have a known relationship between brightness and period; comparing observed brightness to actual brightness gives distance |
| Red-shift (Hubble's Law) | Distant galaxies | Using v = H x d, measuring red-shift gives velocity, from which distance can be calculated |
| Type Ia supernovae | Very distant galaxies | All Type Ia supernovae have the same peak brightness, so their observed brightness indicates distance |
Exam Tip: You are not expected to know the details of each distance measurement method, but you should understand that red-shift and Hubble's Law allow astronomers to estimate the distances to very distant galaxies. The greater the red-shift, the more distant the galaxy.
If the universe is expanding and we know the rate of expansion (the Hubble constant), we can estimate the age of the universe by imagining running the expansion backwards until all galaxies are at the same point.
A rough estimate of the age of the universe is:
Age of the universe is approximately 1 / H
Using the current best estimate of the Hubble constant (about 70 km/s/Mpc), this gives an age of approximately 13.8 billion years.
This estimate has been confirmed by other methods, including:
The ultimate fate of the universe depends on the rate of expansion and the density of matter (and energy) in the universe. There are several possible scenarios:
If the density of the universe is below a critical value, the gravitational attraction between galaxies is insufficient to stop the expansion. The universe will expand forever, growing colder and darker as stars run out of fuel. This is sometimes called the "Big Freeze" or "Heat Death" of the universe.
If the density is exactly at the critical value, the expansion will slow down over time, approaching zero but never quite stopping. The universe is said to be "flat" in this scenario.
If the density exceeds the critical value, gravitational attraction will eventually halt the expansion and cause the universe to contract, potentially ending in a "Big Crunch" — the reverse of the Big Bang.
Current observations suggest that the expansion of the universe is accelerating, not slowing down. This was a surprising discovery in the late 1990s (from observations of distant Type Ia supernovae) and implies the existence of a mysterious repulsive force called dark energy. The leading model is that the universe will continue to expand forever at an accelerating rate.
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