Learning outcomes
- Describe redshift as an increase in observed wavelength.
- Interpret shifted spectral lines.
- Relate greater redshift to faster recession.
- Relate galaxy distance to redshift.
- Distinguish redshift from a simple change in colour.
11.1 Spectral lines as identifiers
Atoms and ions emit or absorb electromagnetic radiation at characteristic wavelengths. A laboratory spectrum therefore provides a reference pattern of lines for particular elements.
When the same pattern is observed from a star or galaxy but all lines are displaced, the shift can be measured. Matching the pattern is important because a single red line alone does not prove redshift.
11.2 Meaning of redshift
Redshift is an increase in the observed wavelength of electromagnetic radiation from a receding star or galaxy. Spectral lines move toward the red, longer-wavelength end of the visible spectrum, although redshift can be measured using radiation outside the visible range as well.
The object need not literally appear red. The term describes a systematic wavelength increase in its spectrum.
11.3 Recession speed
A greater redshift indicates a greater speed away from Earth. At this level, the relationship is qualitative unless a calculation formula is supplied in a question.
If spectral lines shift toward shorter wavelengths, this is blueshift and indicates approach. The space-physics syllabus emphasises redshift from distant galaxies because most are receding.
11.4 Distance–redshift relationship
Observations show that more distant galaxies generally have greater redshifts and are receding faster. This pattern is often represented by a graph with recession speed or redshift on the vertical axis and distance on the horizontal axis.
Data points do not lie perfectly on one line because measurements have uncertainty and galaxies also have local motions. The overall trend, not exact perfection, is the evidence being interpreted.
11.5 What is actually expanding
The observation is best described as galaxies becoming more widely separated as the Universe expands. It is not necessary to imagine every galaxy travelling from one central explosion point through pre-existing empty space.
On smaller scales, gravity holds galaxies and planetary systems together, so expansion is mainly observed between distant galaxies rather than within the Solar System.
Worked examples
Reading a spectrum
If every identifiable absorption line is shifted to a longer wavelength compared with its laboratory value, the source is redshifted and is receding.
Comparing two galaxies
Galaxy B is farther away and has a larger redshift than galaxy A. The data indicate that B is moving away faster.
Practical focus
Investigation or modelling activity
Print a reference spectrum and several shifted spectra. Measure line positions with a ruler, calculate wavelength changes if scales are provided, and rank galaxies by recession speed. Discuss measurement uncertainty caused by broad or faint lines.
Examination guidance
- Define redshift using increased observed wavelength.
- Do not say only that light becomes red.
- Connect larger redshift with faster recession.
- Use the overall data trend when interpreting a graph.
- Do not claim every individual data point must lie exactly on a straight line.
Check your understanding
- What is redshift?
- What does a larger redshift indicate?
- How does redshift generally change with galaxy distance?
- Why are spectral patterns compared rather than single colours?
Answers
- An increase in the observed wavelength of radiation from a receding source.
- A greater recession speed.
- More distant galaxies generally show greater redshift.
- The pattern identifies the same atomic transitions and shows that all wavelengths have shifted systematically.
