Example 1
A spectral line has reference wavelength 500 nm and observed wavelength 550 nm. Identify the shift.
The observed wavelength is longer by 50 nm, so it is redshifted.
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Year 10
Galaxy redshift and expansion patterns
Interpret shifted spectral lines and supplied distance–recession data.
Method and worked examples
Light can be analysed into a spectrum containing identifiable lines. When the observed wavelengths of a distant galaxy’s lines are longer than their reference wavelengths, the spectrum is redshifted. On large cosmic scales, increasing redshift is evidence associated with the expansion of space. Most distant galaxies show this pattern, although nearby gravitational motions can produce exceptions such as blueshift.
A broad relationship connects distance and recession speed: more distant galaxies generally recede faster in the expanding-universe model. A graph shows the overall pattern, not an exact claim about every nearby object. Observers in other typical galaxies would also see large-scale recession patterns; this does not put Earth at a unique centre. Redshift alone is one line of evidence, used alongside background radiation and light-element abundance.
Example 1
Example 2
In supplied data, galaxies at distances 1,2 and 3 units recede at 70,140 and 210 speed units. Predict at distance 4 using this pattern.
The supplied proportional model has 70 speed units per distance unit, so distance 4 predicts 4×70=280 speed units.
Watch out: An expanding distribution produces comparable large-scale patterns for observers in other typical locations.