Theoretical studies indicate that the universe may be open, closed or flat.

(i) State, by reference to critical density, the condition that must be satisfied for the universe to be flat.

(ii) In a flat universe, the rate of expansion would be slowing down. Suggest a reason for this.

(iii) Outline why it has been difficult to determine whether the universe is open, closed or flat.

Outline one piece of experimental evidence that supports the fact that the universe is expanding.

(i) a universe whose density is equal to the critical density;

(ii) the mutual gravitational attraction would slow the expansion down;

(iii) the density of the universe needs to be determined;
this involves many uncertainties related to measurement of distances/volume;
this involves many uncertainties related to presence of dark matter;

light from galaxies is observed to be red-shifted/to have a longer wavelength than that emitted;
indicating that the distance between galaxies is getting bigger/galaxies move away from each other/from us;
Award [1 max] if galaxies are not mentioned.

or

the presence of the cosmic microwave background radiation;
is evidence of cooling of the universe/increase in wavelength/red-shift due to expansion;