| Date | November 2020 | Marks available | 1 | Reference code | 20N.3.HL.TZ0.21 |
| Level | Higher level | Paper | Paper 3 | Time zone | 0 - no time zone |
| Command term | Calculate | Question number | 21 | Adapted from | N/A |
Question
The light from a distant galaxy shows that .
Calculate the ratio .
Outline how Hubble’s law is related to .
Hubble originally linked galactic redshift to a Doppler effect arising from galactic recession. Hubble’s law is now regarded as being due to cosmological redshift, not the Doppler effect. Explain the observed galactic redshift in cosmological terms.
Markscheme
«»
OR OR ✓
«Hubble’s » measure of v/recessional speed uses redshift which is
OR
redshift () of galaxies is proportional to distance «from earth»
OR
combines AND into one expression, e.g. ✓
OWTTE
reference to «redshift due to» expansion of the universe, «not recessional speed» ✓
expansion of universe stretches spacetime / increases distance between objects ✓
«so» wavelength stretches / increases leading to observed redshift ✓
Examiners report
Many candidates got the ratio upside down and ended up with R/R0 as 1.11.
This was not accepted as it would have required an identification of the variables. Perhaps candidates need to look more carefully at which R is which here. R is the current value of the scale factor in the data book, so R0/R = 0.9 was required.
To show the link between z and Hubble's law many rearranged formulae to obtain zc = Hd or similar. Others stated that Hubble used redshift z to determine that v was proportional to distance. Either approach was allowed.
The galactic redshift was successfully explained by many in terms of the stretching of spacetime.
