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Red-shift and the Universe

Use red-shift as evidence for an expanding Universe.

Start here

The key idea

Light from distant galaxies is shifted towards longer wavelengths when the galaxies move away from us.

Red Shift And The Universe
laboratorydistant galaxy: red-shift

Use the labels to explain the scientific relationship shown.

Revision notes

The bit that matters

Short notes first. Learn the idea, then use the worked example and questions to check it properly.

1

Red-shift

When we observe light from distant galaxies, the wavelengths of the lines in their spectra are increased (shifted towards the red end of the spectrum).This is called red-shift.It happens because the galaxies are moving away from us, stretching the light waves to longer wavelengths.Red-shift is similar in idea to how the pitpitch of a siren drops as it moves away.

2

Evidence for an expanding universe

Observations show that the further away a galaxy is, the greater its red-shift and so the faster it is moving away from us.This provides strong evidence that the whole universe is expanding.Because galaxies in all directions are moving apart, it suggests that space itself is expanding rather than galaxies moving through fixed space.These observations are central to our model of the universe.

3

The Big Bang theory

8 billion years ago and has been expanding ever since.Red-shift evidence supports this idea because, if the universe is expanding now, it must have been smaller in the past.The discovery of cosmic microwave background radiation, left over from the early universe, is further strong evidence for the Big Bang.The Big Bang is currently the best accepted scientific model for the origin of the universe.

4

Dark matter and dark energy

Since 1998, observations of supernovae in distant galaxies suggest that the expansion of the universe is actually accelerating.Scientists cannot fully explain this with known matter and forces, so they have proposed dark energy as a cause of the accelerating expansion.Dark matter is also proposed to explain how galaxies hold together, as there appears to be more mass than we can see.There is still much about the universe that is not understood, and scientific ideas may change as new evidence is found.

Key terms

Definitions to learn

Red-shift

The increase in wavelength of light from galaxies moving away from us.

Big Bang theory

The model that the universe began from a hot, dense point and has expanded since.

Cosmic microwave background

Leftover radiation from the early universe, evidence for the Big Bang.

Dark energy

A proposed cause of the accelerating expansion of the universe.

Dark matter

Proposed unseen mass that explains how galaxies hold together.

Worked example

Explain how red-shift supports the Big Bang model.

1

Describe what red-shift shows.

2

Link expansion back in time.

Final answer

Most galaxies are moving away, so the Universe is expanding. Extrapolating backwards suggests it began from a much smaller, hotter state.

Exam habit

Describe red-shift as an increase in observed wavelength (shift towards longer wavelengths).Link the observation to recession speed: more distant galaxies → greater red-shift → faster recession.State this as evidence for an expanding Universe.

Watch out

Do not say red-shift proves every detail of the Big Bang model.

Examiner tips

How to score full marks

  • 1Link red-shift to motion: greater red-shift means a galaxy is moving away faster.
  • 2Use red-shift as evidence for expansion, and expansion as evidence for the Big Bang.
  • 3Mention that the cosmic microwave background radiation is key supporting evidence for the Big Bang.
Practice questions

Try these yourself

Start with the core skill, then open the answer only after you have attempted the full question.

1What happens to the wavelength of light during red-shift?
Mark scheme
  1. 1.Use the meaning of shifted towards red.
The wavelength increases.
2What pattern is observed for more distant galaxies?
Mark scheme
  1. 1.Recall the relationship between distance and recession speed.
More distant galaxies generally show greater red-shift and move away faster.
3State one other piepiece of evidence for the Big Bang model.
Mark scheme
  1. 1.Recall background radiation.
Cosmic microwave background radiation.
4State what is meant by red-shift.[1 mark]
Mark scheme
  1. 1.Define red-shift.
An increase in the wavelength of light from a galaxy because it is moving away from us, shifting its spectrum towards red (1).
5Describe the relationship between a galaxy's distance and its red-shift.[2 marks]
Mark scheme
  1. 1.Further galaxies have larger red-shift.
  2. 2.So move away faster.
The further away a galaxy is, the larger its red-shift (1), meaning more distant galaxies are moving away faster (1).
6Explain how red-shift provides evidence that the universe is expanding.[3 marks]
Mark scheme
  1. 1.Galaxies moving away in all directions.
  2. 2.Greater distance, greater speed.
  3. 3.Implies expanding space.
Light from galaxies in all directions is red-shifted, showing they are moving away from us (1), and more distant galaxies move away faster (1), which is what we expect if the whole universe (space) is expanding (1).
7State what the Big Bang theory says about the origin of the universe and name one piepiece of supporting evidence other than red-shift.[3 marks]
Mark scheme
  1. 1.Universe began small, hot, dense.
  2. 2.Expanding since.
  3. 3.Evidence: CMB.
The universe began from a very small, hot, dense region about 13.8 billion years ago and has been expanding ever since (1) (1 for hot/dense start, 1 for expansion). Cosmic microwave background radiation is supporting evidence (1).
8Recent observations of distant supernovae suggest the expansion of the universe is accelerating. Explain why this was surprising and how scientists have responded, referring to dark energy and the nature of scientific models.[4 marks]
Mark scheme
  1. 1.Expected gravity to slow expansion.
  2. 2.Accelerating expansion unexpected.
  3. 3.Dark energy proposed.
  4. 4.Models change with new evidence.
Scientists expected gravity to gradually slow the expansion, so finding that it is accelerating was surprising (1). To explain this, they proposed dark energy as an unknown cause driving the acceleration (1). Because this cannot yet be fully explained by known physics, it shows our understanding is incomplete (1), and scientific models such as our model of the universe are revised as new evidence is gathered (1).
9Explain what cosmic microwave background radiation (CMBR) is, how it was discovered, and why it is considered strong evidence for the Big Bang.[4 marks]
Mark scheme
  1. 1.CMBR is low-level microwave radiation coming uniformly from all directions.
  2. 2.Discovered accidentally by Penzias and Wilson in 1965.
  3. 3.Predicted by Big Bang theory as afterglow of early hot universe.
  4. 4.Supports Big Bang: exactly what the model predicts.
The cosmic microwave background radiation is a faint, uniform microwave signal that comes from all directions in the sky (1); it was discovered accidentally in 1965 by Penzias and Wilson, who initially thought the signal was noise from their antenna (1); the Big Bang theory predicts that after the initial explosion the universe was extremely hot; as it expanded and cooled, the radiation released in the early universe would have stretched to longer (microwave) wavelengths and would now fill all of space uniformly — this is exactly what is observed (1); the near-perfect uniformity and the measured temperature of 2.7 K match the Big Bang predictions very closely, making it one of the strongest piepieces of evidence for the theory (1)
10A galaxy shows spectral lines shifted to longer wavelengths. Explain what this tells us about the galaxy's motion. If a particular spectral line is normally at 580 nm but is observed at 592 nm, calculate the fractional red-shift and comment on what it implies.[3 marks]
Mark scheme
  1. 1.Longer wavelength = moving away from us (red-shift).
  2. 2.Fractional red-shift = (observed - rest) / rest wavelength.
  3. 3.Calculate (592 - 580) / 580.
  4. 4.Interpret as recession speed as a fraction of c.
The shift to longer wavelengths shows the galaxy is moving away from us (1); fractional red-shift = (592 - 580) / 580 = 12 / 580 = 0.0207 (about 2.1%) (1); this fractional shift is proportional to the galaxy's recession speed as a fraction of the speed of light — the galaxy is receding at about 2.1% of the speed of light, which is consistent with an expanding universe (1)
11Describe what dark matter is, why scientists have proposed its existence, and how it differs from ordinary matter.[4 marks]
Mark scheme
  1. 1.Dark matter: matter that does not emit or reflect light.
  2. 2.Evidence: galaxies rotate faster at edges than visible mass alone explains.
  3. 3.Also: gravitational lensing stronger than visible matter predicts.
  4. 4.Differs from ordinary matter: cannot be detected by EM radiation.
Dark matter is a form of matter that does not emit, absorb or reflect any electromagnetic radiation, so it cannot be directly observed with any telescope (1); its existence is proposed because galaxies rotate at speeds that are too high to be explained by the gravitational pull of the visible stars and gas alone — the outer regions of galaxies rotate almost as fast as the inner regions, suggesting there is additional unseen mass (1); gravitational lensing observations also show that clusters of galaxies bend light from more distant objects more strongly than the visible mass alone would predict, again suggesting hidden mass (1); unlike ordinary (baryonic) matter, dark matter does not interact with photons at all, meaning it is completely transparent to all forms of electromagnetic radiation; its nature is not yet understood (1)
12Discuss the evidence for the Big Bang theory, including red-shift, cosmic microwave background radiation and the existence of light elements. Evaluate how well the theory explains the observations and comment on the status of the theory in science today.[6 marks]
Mark scheme
  1. 1.Red-shift: all galaxies moving away, more distant ones faster — implies expansion.
  2. 2.CMB: uniform microwave background at 2.7 K — predicted by Big Bang.
  3. 3.Light element abundances: Big Bang nucleosynthesis predicts correct ratio of H, He.
  4. 4.Theory well supported but some aspects unexplained (dark matter, dark energy).
  5. 5.Scientific status: best-supported current model; may be revised with new evidence.
The red-shift of light from distant galaxies shows that virtually all galaxies are receding from us, with more distant ones receding faster; this is precisely the pattern expected if space has been expanding from a single point — the Big Bang (1); the cosmic microwave background radiation is a faint, uniform microwave glow filling all space; the Big Bang theory predicted this radiation as the stretched remnant of the intense light from the early hot universe, and the observed temperature of 2.7 K matches the prediction well (1); the Big Bang also predicts the abundance of light elements (about 75% hydrogen and 25% helium by mass) formed in the first few minutes by nucleosynthesis; this matches the observed composition of very old stars that formed before heavier elements were made by later stellar fusion (1); together, these three independent lines of evidence strongly support the Big Bang model; however, the observations of accelerating expansion (explained by proposed dark energy) and galaxy rotation curves (explained by dark matter) show that our understanding is incomplete (1); the Big Bang is the current scientific consensus — the best-supported and most comprehensive model for the origin and evolution of the universe, but scientists recognise it may need revision as new observations are made, as with all scientific models (1) — award max 6
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