Red-shift and the Big Bang theory

What you'll learn

This guide covers the observational evidence for the expanding universe and the Big Bang theory, as required by the AQA GCSE Physics specification. You'll understand how astronomers detect red-shift in light from distant galaxies, what this tells us about the motion of galaxies, and how this evidence supports the Big Bang model of the universe's origin.

Key terms and definitions

Red-shift — the observed increase in wavelength (and decrease in frequency) of electromagnetic radiation from a source moving away from an observer

Doppler effect — the change in observed wavelength and frequency of waves when the source and observer are moving relative to each other

Galaxy — a collection of billions of stars held together by gravitational attraction

Big Bang theory — the scientific theory that the universe began from a very small, extremely hot and dense initial point approximately 13.8 billion years ago and has been expanding ever since

Cosmic microwave background radiation (CMBR) — electromagnetic radiation filling the universe, which is the remnant thermal energy from the Big Bang

Wavelength — the distance between corresponding points on adjacent waves, measured in metres

Recession velocity — the speed at which a galaxy is moving away from Earth

Hubble's law — the observation that the further away a galaxy is from Earth, the faster it is moving away from us

Core concepts

The electromagnetic spectrum and wavelength

Light and other electromagnetic radiation can be described as waves. When we observe light from distant objects in space, we can measure its wavelength. Visible light has wavelengths from approximately 400 nm (violet) to 700 nm (red).

Different wavelengths correspond to different colours:

• Shorter wavelengths: violet, blue
• Medium wavelengths: green, yellow
• Longer wavelengths: orange, red

When light is shifted towards longer wavelengths, it moves towards the red end of the visible spectrum — hence red-shift. This shift can occur across the entire electromagnetic spectrum, not just visible light.

The Doppler effect and red-shift

The Doppler effect explains why waves from a moving source appear to have a different wavelength to a stationary observer. You may have experienced this with sound waves when an emergency vehicle passes you — the siren sounds higher-pitched as it approaches and lower-pitched as it moves away.

The same principle applies to light waves:

When a light source moves away from an observer:

• The wavelength increases (becomes stretched)
• The frequency decreases
• The light is red-shifted

When a light source moves towards an observer:

• The wavelength decreases (becomes compressed)
• The frequency increases
• The light is blue-shifted

For GCSE, you need to understand that red-shift indicates movement away from Earth. The greater the red-shift observed, the faster the object is moving away.

How astronomers detect red-shift

Astronomers use spectroscopy to detect red-shift. When light from a star or galaxy passes through a spectrometer, it produces a spectrum showing dark absorption lines at specific wavelengths. These lines are created when elements in the star's atmosphere absorb particular wavelengths of light.

Each element produces a unique pattern of absorption lines — like a fingerprint. Astronomers know what wavelengths these lines should have from laboratory measurements on Earth.

The detection process:

1. Measure the wavelengths of absorption lines in light from a distant galaxy
2. Compare these wavelengths to the same absorption lines measured in a laboratory on Earth
3. Calculate the shift in wavelength
4. A shift towards longer wavelengths indicates red-shift

The change in wavelength (Δλ) can be used to calculate the recession velocity of the galaxy. For GCSE purposes, you need to know that a larger red-shift means a greater recession velocity, though the exact calculation is beyond GCSE requirements.

Observations of distant galaxies

When astronomers examined light from distant galaxies, they made a crucial discovery:

• Light from almost all distant galaxies shows red-shift
• The further away a galaxy is from Earth, the greater its red-shift
• This means more distant galaxies are moving away from us faster than closer galaxies

This relationship between distance and recession velocity is known as Hubble's law, named after astronomer Edwin Hubble who first documented this pattern in 1929.

Key observations:

• Galaxies in all directions show red-shift (with very few exceptions of nearby galaxies)
• The red-shift is proportional to distance
• No special location is implied — the same pattern would be observed from any galaxy

The expanding universe model

The red-shift of galaxies provides strong evidence that the universe is expanding. However, it's important to understand what "expanding universe" actually means:

• Space itself is expanding
• Galaxies are not simply moving through static space
• The expansion carries galaxies apart from each other
• Think of galaxies as dots on the surface of an inflating balloon — as the balloon inflates, all dots move apart from each other

This expansion explains:

• Why we observe red-shift in all directions
• Why more distant galaxies have greater red-shift (they've been carried apart by more expanding space)
• Why there's no central point of expansion observable from Earth

The expanding universe model can be run backwards in time. If galaxies are moving apart now, they must have been closer together in the past. Tracing this backwards leads to the Big Bang theory.

The Big Bang theory

The Big Bang theory proposes that:

• The universe began approximately 13.8 billion years ago
• Initially, all matter and energy in the universe was concentrated in an extremely small, hot, and dense point
• The universe has been expanding and cooling ever since
• Space, time, matter, and energy all originated in this event

Evidence supporting the Big Bang theory:

1. Red-shift of galaxies

• Provides evidence for universal expansion
• Consistent with the universe expanding from an initial point

2. Cosmic microwave background radiation (CMBR)

• Detected in 1964 by Penzias and Wilson
• Fills the entire universe uniformly
• Has a temperature of approximately 2.7 K (-270°C)
• Represents cooled radiation from the hot early universe
• Provides a "snapshot" of the universe when it was about 380,000 years old

The CMBR is particularly compelling evidence because:

• Its existence was predicted by the Big Bang theory before it was discovered
• Its uniform temperature matches theoretical predictions
• Its spectrum precisely matches that of thermal radiation from a cooling source

3. Relative abundances of light elements

• The proportions of hydrogen, helium, and other light elements in the universe match predictions from Big Bang nucleosynthesis
• This is A-level content but may appear in Higher Tier questions as given information

Age and scale of the universe

Based on observations of red-shift and other astronomical data, scientists have determined:

• The universe is approximately 13.8 billion years old
• The observable universe has a radius of approximately 46 billion light-years (due to expansion)
• The universe contains hundreds of billions of galaxies
• Each galaxy contains hundreds of billions of stars

For GCSE, you should know the age of the universe to the nearest billion years (approximately 14 billion years is acceptable).

Worked examples

Example 1: Interpreting red-shift data

Question: An astronomer observes light from a distant galaxy. The calcium absorption line, which should have a wavelength of 393 nm when measured in a laboratory, is observed to have a wavelength of 402 nm in the galaxy's spectrum.

(a) State whether the galaxy is moving towards or away from Earth. [1 mark]

(b) Explain your answer. [2 marks]

(c) Describe what this observation tells us about the universe. [2 marks]

Mark scheme answer:

(a) Away from Earth [1]

(b) The observed wavelength (402 nm) is longer than the laboratory wavelength (393 nm) [1]. This is red-shift, which occurs when a source is moving away [1].

(c) This observation (along with similar observations of many galaxies) shows that the universe is expanding [1]. Galaxies are moving apart from each other as space itself expands [1].

Examiner note: Part (b) requires you to reference both the data given and the physical principle. Simply stating "red-shift means moving away" would only earn 1 mark.

Example 2: Evaluating evidence for the Big Bang

Question: Scientists use several pieces of evidence to support the Big Bang theory.

(a) Name two pieces of observational evidence that support the Big Bang theory. [2 marks]

(b) Explain how red-shift of galaxies supports the Big Bang theory. [3 marks]

Mark scheme answer:

(a) Any two from:

• Red-shift of distant galaxies [1]
• Cosmic microwave background radiation [1]
• Abundance of light elements (if this appears in the question stem) [1]

(b) Red-shift shows galaxies are moving away from us [1]. More distant galaxies have greater red-shift, showing the universe is expanding [1]. If we trace this expansion backwards, all matter must have been together at a single point in the past / this supports the idea of the universe beginning from a very small initial point [1].

Examiner note: Three-mark "explain" questions require a logical chain of reasoning. Each mark point builds on the previous one.

Example 3: CMBR and the Big Bang

Question: The cosmic microwave background radiation (CMBR) was discovered in 1964.

(a) State what the CMBR is. [1 mark]

(b) Explain why the discovery of CMBR supports the Big Bang theory. [2 marks]

Mark scheme answer:

(a) Electromagnetic radiation that fills the universe / thermal radiation left over from the Big Bang [1]

(b) The Big Bang theory predicts that the early universe was very hot [1]. The CMBR represents cooled radiation from this hot early state / is evidence the universe has been cooling as it expands [1].

Examiner note: Alternative valid points for part (b) include: CMBR's existence was predicted before it was discovered; its uniform distribution matches Big Bang predictions.

Common mistakes and how to avoid them

• Confusing red-shift with the red colour of stars. Red-shift is about wavelength change, not the actual colour of the object. Even blue light can be red-shifted (it becomes longer wavelength blue or green light). Always describe red-shift in terms of wavelength increasing or shifting towards the red end of the spectrum.

• Stating galaxies are moving through space rather than space expanding. The expansion of the universe means space itself is expanding, carrying galaxies apart. This is different from objects simply moving through existing space. Use phrases like "space is expanding" rather than "galaxies are flying apart."

• Claiming red-shift proves the Big Bang. Red-shift provides evidence for an expanding universe, which supports the Big Bang theory. Combined with CMBR and other evidence, it provides very strong support, but scientists don't claim absolute proof. Use words like "supports" or "provides evidence for."

• Writing vague statements about the Big Bang. Avoid phrases like "everything exploded" or "there was nothing before the Big Bang." State clearly that the universe began from an extremely small, hot, dense point and has been expanding since. The Big Bang describes the beginning of space and time, not an explosion into existing space.

• Forgetting to reference the data given in questions. When questions provide wavelength values or other data, you must reference these specific values in your answer to earn full marks. Don't give generic answers when data is provided.

• Mixing up wavelength and frequency changes. Remember: red-shift means wavelength increases (becomes longer) and frequency decreases. Keep these inversely related changes clear.

Exam technique for "Red-shift and the Big Bang theory"

• Command word awareness: "Explain" questions (common in this topic) require you to give reasons or link cause and effect. For 2-3 marks, provide a logical sequence of linked points. "Describe" questions require you to state characteristics or features without necessarily explaining why. "State" or "Name" questions need only the answer with no explanation.

• Use correct scientific terminology: Gain marks by using precise terms like "wavelength increases," "recession velocity," "expanding universe," and "cosmic microwave background radiation." Avoid colloquial phrases like "stretching light" or "galaxies flying apart."

• Structure longer answers logically: For 3-4 mark questions, plan a brief sequence: observation → interpretation → conclusion. For example: red-shift observed → galaxies moving away → universe expanding → supports Big Bang.

• Six-mark questions may ask you to evaluate evidence: If asked to "discuss" or "evaluate" the Big Bang theory, present multiple pieces of evidence (red-shift, CMBR) and explain how each supports the theory. A conclusion that acknowledges strong supporting evidence earns top marks.

Quick revision summary

Red-shift is the observed increase in wavelength of light from objects moving away from us. Observations show almost all distant galaxies are red-shifted, with greater red-shift at larger distances, providing evidence the universe is expanding. This expansion supports the Big Bang theory — that the universe began approximately 13.8 billion years ago from an extremely small, hot, dense point. The cosmic microwave background radiation, thermal radiation filling the universe at 2.7 K, provides additional strong evidence for the Big Bang by confirming the universe has cooled as predicted.