Big Bang theory · CMB radiation · Hubble's law · Evidence · Fate of the universe
AQA GCSE Physics 4.8 | Year 11 | Higher Tier
🌌 Describe the Big Bang theory as the origin of space, time and matter
📡 Explain cosmic microwave background (CMB) radiation as evidence for the Big Bang
🔭 State Hubble's law and use it to calculate recessional velocity and distance
🔴 Explain how redshift of galaxies provides evidence for an expanding universe
⏳ Estimate the age of the universe from Hubble's constant
🌠 Describe possible fates of the universe depending on its density
🌌 The Big Bang Theory
The Big Bang theory is the leading scientific model for the origin of the universe. It proposes that approximately 13.8 billion years ago, all matter, energy, space and time began from an extremely hot, dense singularity. This was not an explosion into space — it was an expansion of space itself.
Big Bang Theory: The universe began from a single, infinitely hot and dense point (a singularity) and has been expanding and cooling ever since.
In the very early universe (fractions of a second after the Big Bang), the temperature was so high that only fundamental particles such as quarks and leptons existed. As the universe expanded, it cooled, allowing protons and neutrons to form, then simple nuclei (hydrogen and helium), and eventually — after about 380,000 years — neutral atoms formed. This is when the universe became transparent to radiation for the first time.
Over hundreds of millions of years, gravity pulled hydrogen and helium gas clouds together to form the first stars and galaxies. Heavier elements were forged inside stars through nuclear fusion, and scattered into space when stars exploded as supernovae. This is why everything — including us — is made of "stardust".
The Big Bang was not an explosion in space. It was an explosion of space — every point in the universe was part of the original singularity.
📡 Cosmic Microwave Background (CMB) Radiation
One of the most powerful pieces of evidence for the Big Bang is the Cosmic Microwave Background (CMB) radiation. About 380,000 years after the Big Bang, the universe had cooled enough for electrons to combine with nuclei to form neutral atoms. Before this, the universe was opaque — photons were constantly scattered by free electrons. When atoms formed, photons could travel freely for the first time. This release of radiation is called the surface of last scattering.
CMB Radiation: Thermal radiation left over from the early universe, now detected as microwave radiation coming uniformly from all directions in space, with a temperature of approximately 2.7 K.
As the universe expanded, the wavelength of this radiation stretched — it was originally in the infrared/visible range but has been redshifted all the way to microwave wavelengths. It was accidentally discovered in 1965 by Arno Penzias and Robert Wilson using a microwave antenna, who initially thought the noise was caused by pigeon droppings on their detector!
The CMB is almost perfectly uniform — the same intensity in every direction — which supports the idea that the early universe was extremely smooth and uniform (homogeneous and isotropic). Tiny fluctuations in the CMB (about 1 part in 100,000) correspond to slight density variations that eventually grew into galaxies and galaxy clusters.
The CMB is the "afterglow" of the Big Bang. Its discovery is considered the strongest evidence for the Big Bang model, alongside the redshift of distant galaxies.
🔴 Redshift & Evidence for an Expanding Universe
When we observe light from distant galaxies, we find that the spectral lines (dark absorption lines) are shifted towards longer wavelengths — towards the red end of the spectrum. This is called redshift.
Redshift: The increase in the observed wavelength of electromagnetic radiation from a source that is moving away from the observer. It is analogous to the Doppler effect for sound.
The greater the redshift of a galaxy, the faster it is moving away from us. Observations show that almost all galaxies are redshifted — they are moving away from us. Crucially, more distant galaxies are moving away faster. This tells us the universe is expanding in all directions, and there is no special centre — every galaxy sees every other galaxy moving away from it.
This discovery was first made by Edwin Hubble in 1929. He plotted the recessional speed of galaxies against their distance and found a straight-line relationship, now known as Hubble's Law.
If we run the expansion of the universe backwards in time, all galaxies converge to a single point — supporting the idea that the universe began as a singularity.
🔭 Hubble's Law
Hubble's Law states that the recessional velocity of a galaxy is directly proportional to its distance from us:
v = H₀ × d
Symbol
Quantity
SI Unit
v
Recessional velocity of galaxy
km/s
H₀
Hubble's constant
km/s/Mpc ≈ 65–70 km/s/Mpc
d
Distance to the galaxy
Mpc (megaparsecs) or light-years
The currently accepted value of Hubble's constant is approximately H₀ ≈ 65–70 km/s/Mpc (there is ongoing scientific debate about the precise value). In SI units, H₀ ≈ 2.2 × 10⁻¹⁸ s⁻¹.
Hubble's constant also allows us to estimate the age of the universe. If we assume the universe has been expanding at a constant rate, the age is approximately:
t ≈ 1 ÷ H₀ ≈ 14 billion years (using H₀ in SI units: t ≈ 1 ÷ (2.2 × 10⁻¹⁸) ≈ 4.5 × 10¹⁷ s)
The age of the universe estimated from Hubble's constant (≈ 13–14 billion years) agrees well with other independent methods such as the ages of the oldest stars.
🌠 Fate of the Universe
The ultimate fate of the universe depends on the total mass-energy density of the universe and the nature of dark energy (a mysterious form of energy causing the expansion to accelerate).
There are several possible scenarios:
Big Freeze (Heat Death): If dark energy continues to drive expansion, galaxies will drift further apart, stars will burn out, and the universe will approach absolute zero. This is currently the most favoured outcome.
Big Crunch: If gravity is strong enough to overcome expansion, the universe could eventually stop expanding and collapse back in on itself — the reverse of the Big Bang.
Big Rip: If dark energy grows stronger over time, it could eventually overcome all forces — tearing apart galaxies, solar systems, atoms, and even subatomic particles.
Modern observations suggest the expansion of the universe is actually accelerating — driven by dark energy. Dark energy makes up approximately 68% of the total energy content of the universe, while dark matter accounts for about 27%, and ordinary (baryonic) matter makes up only about 5%.
The Big Freeze (continued expansion leading to a cold, dark, empty universe) is currently the most scientifically supported fate, due to the observed accelerating expansion driven by dark energy.
Example 1: A distant galaxy has a recessional velocity of 14,000 km/s. Using Hubble's constant H₀ = 70 km/s/Mpc, calculate the distance to the galaxy in megaparsecs (Mpc).
1Write down Hubble's Law: v = H₀ × d
2Rearrange for distance d: d = v ÷ H₀
3Substitute values: d = 14,000 km/s ÷ 70 km/s/Mpc
4Calculate: d = 200 Mpc
✅ Distance to the galaxy = 200 Mpc
Example 2: A galaxy is observed to be 350 Mpc away. Use Hubble's constant H₀ = 70 km/s/Mpc to calculate its recessional velocity. What does this tell us about the galaxy?
1Write down Hubble's Law: v = H₀ × d
2Substitute values: v = 70 km/s/Mpc × 350 Mpc
3Calculate: v = 24,500 km/s
4Interpret: The galaxy is moving away from us at 24,500 km/s — approximately 8% of the speed of light. This is consistent with the universe expanding in all directions.
✅ Recessional velocity = 24,500 km/s. The galaxy is moving away rapidly — evidence for an expanding universe.
Example 3: Using H₀ = 2.2 × 10⁻¹⁸ s⁻¹, estimate the age of the universe in seconds and in years. (1 year ≈ 3.15 × 10⁷ s)
1Use the relationship: t ≈ 1 ÷ H₀
2Substitute: t = 1 ÷ (2.2 × 10⁻¹⁸ s⁻¹)
3Calculate in seconds: t = 4.55 × 10¹⁷ s
4Convert to years: t = (4.55 × 10¹⁷) ÷ (3.15 × 10⁷) ≈ 1.44 × 10¹⁰ years ≈ 14.4 billion years
✅ Estimated age of the universe ≈ 4.55 × 10¹⁷ s ≈ 14 billion years
Example 4: An astronomer observes a hydrogen spectral line in a distant galaxy at a wavelength of 486.2 nm. In the laboratory, this line is measured at 434.0 nm. Explain what this tells us and identify the phenomenon observed.
4Interpret: Redshift means the galaxy is moving away from us (Doppler effect). The larger the redshift, the faster the recession. This is evidence that the universe is expanding.
✅ The spectral line is redshifted by 52.2 nm — the galaxy is moving away from Earth, consistent with an expanding universe.
Question 1: What is the current temperature of the Cosmic Microwave Background (CMB) radiation?
Question 2: A galaxy has a recessional velocity of 2,800 km/s. Using H₀ = 70 km/s/Mpc, what is its distance?
Question 3: What type of electromagnetic radiation is the CMB? (one word)
Question 4: What does the redshift of light from distant galaxies tell us?
Question 5: Using H₀ = 2.2 × 10⁻¹⁸ s⁻¹, calculate the estimated age of the universe in seconds. Give your answer in standard form to 2 significant figures.
Challenge 1 [6 marks]: A student states: "The Big Bang was a massive explosion that happened at a specific location in space." Evaluate this statement and explain what the Big Bang theory actually proposes, including reference to two pieces of observational evidence that support it.
Mark scheme points (any 6):
• The statement is incorrect — the Big Bang was not an explosion at a specific location [1]
• The Big Bang was an expansion of space itself from an extremely hot, dense singularity [1]
• All matter, energy, space and time originated from this event approximately 13.8 billion years ago [1]
• Evidence 1 — Redshift of galaxies: almost all galaxies show redshift, meaning they move away from us; more distant galaxies recede faster (Hubble's Law), consistent with expanding space [1]
• Evidence 2 — CMB radiation: uniform microwave radiation detected from all directions, temperature ≈ 2.7 K, is the remnant "afterglow" of radiation released ~380,000 years after the Big Bang [1]
• Running the expansion backwards in time implies all matter was once concentrated in a single point [1]
Challenge 2 [5 marks]: Galaxy NGC 4889 has a recessional velocity of 6,500 km/s.
(a) Using H₀ = 65 km/s/Mpc, calculate its distance in Mpc. [2 marks]
(b) A second galaxy is twice as far away. Calculate its recessional velocity. [1 mark]
(c) What assumption does Hubble's Law make about the expansion of the universe? [1 mark]
(d) Why might the actual age of the universe differ from the estimate t ≈ 1/H₀? [1 mark]
(a) d = v ÷ H₀ = 6,500 ÷ 65 = 100 Mpc ✓✓
(b) v = H₀ × 2d = 65 × 200 = 13,000 km/s ✓
(c) Hubble's Law assumes the expansion rate (H₀) has been constant throughout the universe's history ✓
(d) The expansion rate has not been constant — the universe's expansion is now accelerating (due to dark energy), so the simple estimate t = 1/H₀ is an approximation only ✓
Challenge 3 [4 marks]: Describe and explain the three possible fates of the universe. For each, state the condition that would lead to that outcome and whether it is currently favoured by observations.
Big Freeze / Heat Death: If dark energy continues to drive accelerating expansion [1] — the universe expands forever, galaxies drift apart, stars burn out, temperature approaches absolute zero. Currently the most favoured outcome based on observations of accelerating expansion.
Big Crunch: If total mass-energy density is high enough for gravity to overcome expansion [1] — the universe halts, reverses, and collapses back to a singularity. Not currently favoured — expansion appears to be accelerating, not decelerating.
Big Rip: If dark energy grows increasingly strong over time [1] — eventually overcomes gravity, electromagnetic force, and nuclear forces, tearing apart all structures including atoms. Possible but not strongly supported by current data.
Award 1 mark for clearly linking at least two fates to observational evidence (accelerating expansion / CMB data) [1]
Challenge 4 [5 marks]: The Hubble constant is sometimes quoted as H₀ = 70 km/s/Mpc where 1 Mpc = 3.09 × 10¹⁹ km.
(a) Show that H₀ in SI units (s⁻¹) is approximately 2.3 × 10⁻¹⁸ s⁻¹. [3 marks]
(b) Hence calculate the estimated age of the universe in years. (1 year = 3.15 × 10⁷ s) [2 marks]