Physics A2 · Astronomy and cosmology. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The lines in the spectra of distant galaxies are at longer wavelengths than the same lines measured in a laboratory. This is redshift. It happens when a source moves away from the observer, and a bigger speed gives a bigger shift.
Light from almost all distant galaxies is redshifted, so they are all moving away from us. This leads to the idea that the Universe is expanding.
Hubble's law says the speed of a galaxy moving away is proportional to its distance from us. If everything is moving apart now, then in the past everything was closer together. Followed back far enough, all matter started from one very small, hot, dense point: the Big Bang.
Students divide Δλ by the observed wavelength. Divide by the laboratory (emitted) wavelength, and remember that Δλ is the change, not the observed value itself.
Answer: About 4.1 × 1024 m.
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