Figure 1 shows the absorption spectra of a specific element obtained from a laboratory reference source and from four distant galaxy clusters: Cluster Alpha, Cluster Beta, Cluster Gamma, and Cluster Delta.
All spectra are drawn to the same scale and aligned vertically.
a.[2]
Explain, with reference to the shift in spectral lines shown in Figure 1, which galaxy cluster is moving away from Earth at the greatest velocity.
b.[1]
Hence, deduce which galaxy cluster is closest to Earth, justifying your answer using your understanding of the expanding Universe and Hubble's Law.