Fig. 3.1 shows the absorption line spectra from two galaxies, A and B, compared with a reference spectrum from a laboratory source. (a) Describe what has ha...

(a) The absorption lines in both galaxy spectra have shifted toward the red end of the spectrum (toward longer wavelengths) compared with the laboratory reference spectrum. [1] The shift is greater for galaxy B than for galaxy A. [1]

In the diagram, the laboratory lines sit at their standard positions. Galaxy A's lines are displaced a small amount to the right (longer wavelength), while galaxy B's lines are displaced further still. Both galaxies therefore show red-shift, but to different degrees.

(b) Galaxy B shows the greater red-shift. [1]

Looking at Fig. 3.1, the absorption lines for galaxy B are shifted further toward the red end than those of galaxy A, confirming a larger red-shift.

(c) Galaxy B is further from the Earth. [1] A greater red-shift corresponds to a higher recession speed, and by Hubble's law the recession speed of a galaxy is proportional to its distance. Since galaxy B has the greater red-shift, it must be receding faster and therefore must be at a greater distance. [1]

Hubble's law: \(v = H_0 d\). If \(v\) is larger, \(d\) must be larger for the same constant \(H_0\).

(d) Red-shift evidence tells us that the universe is expanding. [1]

Because virtually all distant galaxies show red-shift (they are all moving away from us), and more distant ones move away faster, space itself must be stretching in every direction.

(e) Cosmic microwave background radiation (CMBR). [1]

This is faint microwave radiation detected from all directions in space, interpreted as the afterglow of the extremely hot early universe. Its existence and properties match predictions of the Big Bang theory.