Explain, with reference to the possible fate of the universe, the significance of the critical density of matter in the universe.

Suggest one reason why it is difficult to estimate the density of matter in the universe.

if less than critical density, universe expands without limit;
if equal to critical density universe stops expanding after an infinite amount of time;
if greater than critical density, universe expands first then contracts;
Award [1 max] if terms open, flat and closed are used and not defined.

there is matter that cannot be detected;
which is likely to consist of dark matter/neutrinos;

or

difficulty of measuring volume accurately;
because of difficulty of measuring distances accurately;

or

matter is not evenly distributed;
so density may vary from place to place;