The Universe, Redshift and the Big Bang

Redshift is the stretching of light from distant galaxies towards longer wavelengths, and it is the key evidence that the Universe is expanding from a Big Bang about 14 billion years ago. This chapter is Extended (Supplement) only. Examiners test it because it rewards precise wording: most students know the story but lose marks telling it loosely.

What is redshift and why does it matter?

The Universe contains billions of galaxies, each with billions of stars. When astronomers analyse light from distant galaxies, the wavelengths are longer than expected, shifted towards the red end of the spectrum. This is redshift.

Redshift shows that distant galaxies are moving away from us. The further away a galaxy is, the greater its redshift, so the faster it is receding. Speed away is proportional to distance. Working backwards, everything was once together at a single point. The Universe began expanding from that point in the Big Bang, roughly 14 billion years ago, and is still expanding.

A second piece of evidence is cosmic microwave background radiation (CMBR). Microwave radiation of a specific frequency arrives from every direction in space. It is light produced shortly after the Big Bang, stretched by the expansion of the Universe into the microwave region. No other model explains it.

How do you use the Hubble constant?

In words: the Hubble constant equals the speed at which a galaxy moves away divided by its distance from Earth. In symbols: $H_0 = \dfrac{v}{d}$. The 0625 syllabus value is $H_0 = 2.2 \times 10^{-18}$ per second.

Quantity	Symbol	Unit
Hubble constant	$H_0$	$\text{s}^{-1}$ (per second)
Speed of recession	$v$	m/s
Distance to galaxy	$d$	m

Rearranged, $d = \dfrac{v}{H_0}$ gives a galaxy’s distance. The equation also estimates the age of the Universe: if expansion has been steady, age $\approx \dfrac{1}{H_0} \approx 4.5 \times 10^{17}\ \text{s}$, about 14 billion years. You should also know $d = \dfrac{v}{H_0}$ represents the idea that all matter started at the same point.

Worked Exam Question

Light from a distant galaxy shows redshift. Measurements give the galaxy a speed of recession of $2.0 \times 10^{6}\ \text{m/s}$. (a) Calculate the distance of the galaxy from Earth. Use $H_0 = 2.2 \times 10^{-18}\ \text{s}^{-1}$. [3] (b) State what the redshift of light from distant galaxies tells us about the Universe. [1]

Solution, set out the way we teach it:

1. Equation: $H_0 = \dfrac{v}{d}$
2. Rearrange: $d = \dfrac{v}{H_0}$
3. Substitute: $d = \dfrac{2.0 \times 10^{6}}{2.2 \times 10^{-18}}$
4. Answer (a): $d = 9.1 \times 10^{23}\ \text{m}$ (2 significant figures)

(b) Distant galaxies are moving away from us, so the Universe is expanding.

Mark scheme:

• M1: $H_0 = \dfrac{v}{d}$ stated or used
• M1: correct rearrangement and substitution $d = \dfrac{2.0 \times 10^{6}}{2.2 \times 10^{-18}}$
• A1: $9.1 \times 10^{23}\ \text{m}$ (unit required)
• B1: galaxies receding / Universe expanding

Common Mistakes

• “Light turns red.” Redshift means the wavelength increases (frequency decreases). The light shifts towards the red end; it does not become red.
• Vague Big Bang answers. “Everything exploded” scores nothing. Write: all matter was once at a single point, then the Universe began expanding about 14 billion years ago.
• Forgetting CMBR. “State two pieces of evidence” needs redshift and microwave background radiation. One answer repeated is one mark.
• Powers-of-ten slips with $H_0$. Dividing by $2.2 \times 10^{-18}$ gives a huge number. If your distance is small, you multiplied instead of dividing.
• Saying Earth is the centre. Every galaxy sees every other galaxy receding. Expansion has no centre; do not claim a special position for Earth.

Exam Technique Tip

For the 4-mark “describe the evidence for the Big Bang” question, write four separate sentences: light from distant galaxies is redshifted; this shows they move away from us; further galaxies recede faster, so the Universe is expanding; CMBR is radiation from shortly after the Big Bang, observed in all directions. Each sentence maps to one B1 mark.

How This Is Examined

This subtopic appears only on Extended papers. Paper 2 sets multiple-choice items on what redshift implies or a one-step $H_0$ calculation. Paper 4 combines a calculation using $H_0 = 2.2 \times 10^{-18}\ \text{s}^{-1}$ with written evidence questions, typically 5-7 marks in total. The value of $H_0$ is printed on the paper, so memorise the equation, not the number. No Core or practical paper touches it. It pairs naturally with the electromagnetic spectrum from Topic 3: examiners sometimes ask where microwaves sit relative to visible light, so revise the two together.