| Date | May 2019 | Marks available | 2 | Reference code | 19M.3.SL.TZ2.14 |
| Level | Standard level | Paper | Paper 3 | Time zone | 2 |
| Command term | Calculate | Question number | 14 | Adapted from | N/A |
Question
The Hubble constant is 2.3 × 10-18 s-1.
A galaxy is 1.6 × 108 ly from Earth. Show that its recessional speed as measured from Earth is about 3.5 × 106 m s-1.
A line in the hydrogen spectrum when measured on Earth has a wavelength of 486 nm. Calculate, in nm, the wavelength of the same hydrogen line when observed in the galaxy’s emission spectrum.
Outline how observations of spectra from distant galaxies provide evidence that the universe is expanding.
Markscheme
d = «1.6 × 108 × 9.46 × 1015 =» 1.51 × 1024 «m»✔
v = «H0d = 2.3 × 10−18 ×1.5 × 1024 =» 3.48 × 106 «m s–1» ✔
Answer given, correct working required or at least 3sf needed for MP2.
» 5.64«nm» ✔
observed λ = «486 + 5.64 =» 492 «nm»✔
all distant galaxies exhibit red-shift ✔
OWTTE
Examiners report
This very simple application of Hubble’s law was answered correctly by the vast majority of candidates.
Many candidates subtracted the change in wavelength and obtained a blue shift. Others were unsure which wavelength λo is in the data book equation. But correct answers were common.
Nearly all candidates were able to mention redshift as the evidence for galaxy recession and the universe expansion.
