Red-shift and the Big Bang

Watch first

CognitoRed-shift and the Big Bang

Key facts & equations

Red-shift

When a light source moves away from an observer, the wavelength of light appears longer (shifted toward red end of spectrum).
This is the Doppler effect applied to light.
Galaxies moving away: spectral lines shifted to longer wavelengths — red-shifted.
Distant galaxies are more red-shifted than nearer ones → they are moving away faster.
Hubble's law: recession speed ∝ distance. v = H₀d (H₀ = Hubble constant ≈ 68 km/s/Mpc).
This means the universe is expanding — all galaxies moving away from each other.

Evidence for the Big Bang

Red-shift of galaxies: all distant galaxies are red-shifted → moving away → universe expanding.
Hubble's law: more distant = faster recession → consistent with expansion from a single point.
Cosmic microwave background (CMB) radiation: low-level microwave radiation detected from all directions in space.
CMB is the afterglow of the Big Bang — radiation from when universe cooled enough for atoms to form (~380,000 years after Big Bang).
Its wavelength has been stretched (red-shifted) as universe expanded — originally gamma/X-rays, now microwaves.

Big Bang theory

Universe began ~13.8 billion years ago from an extremely hot, dense point.
Space itself has been expanding since — not explosion into space, but space expanding.
As universe expanded it cooled. First hydrogen and helium nuclei formed. Then atoms. Then stars, galaxies.
All matter and energy, space and time, created in the Big Bang.
No "before" the Big Bang — time itself began with it.

Blue-shift (Higher)

Objects moving TOWARD us: wavelength decreases → blue-shifted.
Andromeda galaxy is blue-shifted — it is moving toward the Milky Way.
Galaxies in our local group (gravitationally bound) can be blue-shifted.
Most distant galaxies are red-shifted — expansion of universe dominates.
Red/blue-shift can also occur with sound (Doppler effect — ambulance siren).

Exam questions — 4 questions · 10 marks · AQA 8463 style

1 markRed-shift observationAQA 8463 style

What does the red-shift of distant galaxies tell us?

A Distant galaxies are cooler than nearby galaxies

B Distant galaxies are moving away from us, and the further away they are, the faster they recede

C Distant galaxies are moving toward us

D The speed of light decreases in deep space

3 marksCMB radiationAQA 8463 P2 style

Explain what the cosmic microwave background (CMB) radiation is and why it provides evidence for the Big Bang. (3 marks)

Hint: What it is (uniform microwave radiation from all directions). Where it came from (early hot universe). Why it supports Big Bang (predicted and explained by the theory, wavelength stretched as universe expanded).

+30 XP

3 marksExpansion evidenceAQA 8463 style

Describe two pieces of evidence that the universe is expanding, and explain how each piece of evidence supports the Big Bang theory. (4 marks)

Hint: Two separate pieces of evidence. For each: state the observation, then explain how it leads to the conclusion that the universe began at a point and has been expanding.

+30 XP

1 markHubble's lawAQA 8463 style

Hubble's law states that the recession speed of a galaxy is proportional to its distance from us. What does this imply?

A The Milky Way is at the centre of the universe

B All galaxies are moving away from each other — the universe is expanding

C Only nearby galaxies are moving away from us

D The universe is contracting toward the Milky Way

Red-shift and the Big Bang

Red-shift

Evidence for the Big Bang

Big Bang theory

Blue-shift (Higher)

Module complete! 🎉