| Date | May 2015 | Marks available | 4 | Reference code | 15M.3.SL.TZ2.15 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 2 |
| Command term | Explain and State | Question number | 15 | Adapted from | N/A |
Question
This question is about the expanding universe.
Since 1929 it has been thought that the universe is expanding.
State what is meant by the expansion of the universe.
Red-shift of light from distant galaxies provides evidence for an expanding universe.
(i) State one other piece of evidence in support of an expanding universe.
(ii) Explain how your answer in (b)(i) is evidence for the Big Bang model of the universe.
Markscheme
(distant) galaxies are all moving away from each other/Earth;
the distance between galaxies is increasing;
the volume/diameter/radius/scale factor of the universe is increasing;
space itself is stretching with time;
Do not accept answers such as “everything is moving away from everything else” as this is clearly not true.
(i) cosmic microwave background/CMB/CBR;
helium/hydrogen ratio/abundance;
darkness of night sky (Olbers’ paradox);
Do not accept answers that refer to Hubble’s law/red-shift of galaxies.
radiation present in the early universe was at a high temperature/short wavelength;
as the universe expanded it cooled/wavelength increased;
so the radiation present today is in the microwave region / has temperature of 2.7 K;
or
the early universe contained high energy neutrons/protons;
as the universe expanded and cooled (to 109 K) nucleosynthesis could start, producing helium;
as the temperature dropped further, nucleosynthesis stopped leaving an excess of protons/hydrogen;
the current abundance of hydrogen and helium is consistent with the predictions of the Big Bang/expansion;
or
Olbers’ paradox asks “why is the night sky dark?”;
this cannot be explained if universe is infinite and static / OWTTE;
in an expanding universe some light is red-shifted out of visible range;
in a Big Bang universe some light from distant galaxies has not reached us yet;
Examiners report
In (a) far too many candidates just repeated the question rather than stating that expansion refers to galaxies moving further apart.
CMB radiation was usually mentioned in (b)(i). The fact that CMB was a specific prediction of the Big Bang model, long before its discovery, was sometimes mentioned in (b)(ii). Most were able to refer to cooling and wavelength increase of CMB as being consistent with the Big Bang model.
