Light from distant galaxies, as seen by an observer on Earth, shows a red-shift. Outline why this observation suggests that the universe is expanding.

The future development of the universe is determined by the relationship between the apparent density of the universe and the critical density.

(i) Define the term critical density.

(ii) Discuss how the density of the universe determines its future development. Your discussion should include one problem associated with determining the density of the universe.

because of the Doppler effect;
light from sources moving away from an observer is observed to have a lower frequency than from the sources when stationary / redshift indicates motion away from observer/Earth;

(i) this is the value of density for which the universe will begin to contract after an infinite amount of time;
Do not accept “density at which universe is flat”.

(ii) if the density of the universe is less than the critical density it will continue expanding forever;
if the density is greater than the critical density then it will after a certain amount of time begin to contract;
the behaviour of galaxies suggests that there is more matter in the universe than is actually observed; { (allow other relevant comment about dark matter)
without knowing the mass of this matter the density cannot be determined;