Outline what is meant by redshift.

Determine the distance to this galaxy assuming a Hubble constant of H₀ = 72 km s^–1 Mpc^–1.

The cosmic microwave background (CMB) radiation provides strong evidence for the Big Bang model. State the two main pieces of this evidence.

The graph shows the variation of the intensity I of the CMB with wavelength λ.

Determine, using the graph, the temperature of the CMB.

«the received» wavelength is longer than that emitted ✔

Allow context of Doppler redshift as well as cosmological redshift.

v = zc = 0.15 × 3.0 × 10⁵ = 4.5 × 10⁴ «km s⁻¹» ✔

d = $\frac{v}{H_0}=\frac{4.5\times {10}^4}{72}$ = 625 «Mpc» ✔

Award [2] for bald correct answer.

Accept in other units, eg, 1.95 x 10²⁵m.

the radiation has a black body spectrum/it is black body radiation ✔

the radiation is highly isotropic/uniform ✔

matched the «predicted» wavelength/temperature if the Big Bang had increased/cooled by expansion ✔

peak wavelength read off graph as (1.1±0.05)«mm» ✔

substitution into Wien’s law to get T = (2.5 to 2.8)«K» ✔

Redshift. Surprisingly many candidates had problems outlining the meaning of redshift.

The distance to the galaxy was well calculated by most of the candidates, but some wrote unrealistic answers, such as 650 m. Also, many candidates made obvious POT errors, especially when working between metres and kilometres.

In (b)(i), most of the candidates were focused only on one piece of evidence – generally “isotopic or uniform”, or “discussing wavelength or temperature in light from an expanding universe”. Very few mentioned CBM as having black-body radiation characteristics.