The Expanding Universe and Hubble's Law
This lesson covers the discovery and evidence for the expanding universe and Hubble's law — as required by the Edexcel GCSE Physics specification (1PH0), Topic 7: Astronomy. This is a Paper 2 topic. You need to understand Hubble's observations, what they tell us about the universe, and the concept of the accelerating expansion.
Hubble's Discovery
In the 1920s, the American astronomer Edwin Hubble made a groundbreaking observation. By studying the light from distant galaxies, he found:
- Almost all galaxies are red-shifted — they are moving away from us.
- The further away a galaxy is, the greater its red-shift — meaning the faster it is receding.
What This Means
- If almost all galaxies are moving away from us, the universe is expanding.
- The further away a galaxy is, the faster it is moving away — this is a systematic pattern, not random motion.
- This was the first observational evidence that the universe is not static — it is getting bigger over time.
Exam Tip: Hubble's key observation was not just that galaxies are red-shifted, but that there is a proportional relationship between distance and speed of recession. This relationship is what tells us the universe is expanding uniformly.
Hubble's Law
Hubble's observation can be expressed as a simple mathematical relationship known as Hubble's law:
The Equation
v = H × d
Where:
- v = speed of recession of the galaxy (in km/s)
- H = the Hubble constant (in km/s/Mpc — kilometres per second per megaparsec)
- d = distance of the galaxy from Earth (in Mpc — megaparsecs)
What Hubble's Law Tells Us
- The speed of recession (v) is directly proportional to the distance (d).
- A galaxy twice as far away moves away twice as fast.
- A galaxy three times as far away moves away three times as fast.
- The Hubble constant (H) is the gradient of the line on a graph of recession speed vs distance.
The Hubble Constant
- The current best estimate of the Hubble constant is approximately H ≈ 70 km/s/Mpc.
- This means that for every megaparsec (Mpc) of distance, a galaxy's recession speed increases by about 70 km/s.
- 1 megaparsec ≈ 3.26 million light-years ≈ 3.09 × 10¹⁹ km.
Worked Example
Question: A galaxy is observed to be 200 Mpc from Earth. Using the Hubble constant H = 70 km/s/Mpc, calculate the speed at which this galaxy is moving away from us.
Answer:
v = H × d
v = 70 × 200
v = 14,000 km/s
The galaxy is receding at approximately 14,000 km/s.
Exam Tip: You may be asked to use the equation v = Hd or to rearrange it to find d (d = v/H). Make sure you can do both. Also ensure your units are consistent — v in km/s, d in Mpc, and H in km/s/Mpc.
Earth Is NOT at the Centre
A very important point: the expansion of the universe does not mean that Earth (or the Milky Way) is at the centre of the universe.
The Balloon Analogy
Imagine drawing dots on the surface of a balloon and then inflating it:
- As the balloon expands, every dot moves away from every other dot.
- An observer on any dot would see all the other dots moving away.
- An observer on any dot would also notice that more distant dots move away faster.
- No dot is at the "centre" — the expansion is happening everywhere equally.
This is exactly what is happening with the universe:
- The space itself is expanding.
- Galaxies are not moving through space (like objects flying apart from an explosion).
- Instead, the space between galaxies is stretching.
- Every galaxy sees other galaxies moving away — it does not matter which galaxy you observe from.
Exam Tip: A common misconception is that the Big Bang was an explosion at a point in space, and everything is flying away from that point. This is WRONG. The expansion is of space itself — there is no centre. Use the balloon analogy to explain this if asked.
The Accelerating Expansion
In 1998, two independent teams of astronomers made a surprising discovery while studying distant supernovae (Type Ia supernovae, which have a known brightness):
- The expansion of the universe is not slowing down as expected.
- It is actually speeding up — the expansion is accelerating.
Dark Energy
- The mysterious force causing this acceleration is called dark energy.
- Dark energy makes up approximately 68% of the total energy content of the universe.
- We do not yet understand what dark energy is — it is one of the biggest unsolved problems in physics.
- Dark energy acts like a repulsive force that opposes gravity and pushes the expansion to accelerate.
| Component | Approximate Percentage of Universe |
|---|
| Dark energy | ~68% |
| Dark matter | ~27% |
| Ordinary matter (atoms, stars, planets) | ~5% |
Exam Tip: At GCSE level, you need to know that the expansion of the universe is accelerating and that this is attributed to dark energy. You do not need to explain what dark energy is (nobody fully can yet!), but you should know it was discovered in 1998 through observations of distant supernovae.
Implications of the Expanding Universe
The fact that the universe is expanding has profound implications:
Looking Backwards in Time
- If the universe is expanding now, then in the past it must have been smaller.
- Going further back, it was smaller still.
- If we rewind all the way back, the universe must have started from an incredibly small, hot, dense point.
- This is the basis of the Big Bang theory (covered in the next lesson).
Estimating the Age of the Universe
- Using Hubble's law, we can estimate when the expansion began.
- The age of the universe is approximately 1/H (the reciprocal of the Hubble constant, with appropriate unit conversions).
- Using H ≈ 70 km/s/Mpc, this gives an age of approximately 13.8 billion years — consistent with other measurements.