Listed following are three possible models for the long-term expansion (and possible contraction) of the universe in the absence of dark energy:

a) recollapsing universe
b) coasting universe
c) critical universe
rank each model from left to right based on the ratio of its actual mass density to the critical density, from smallest ratio (mass density much smaller than critical density) to largest ratio (mass density much greater than critical density).

1. Recollapsing universe

2. Critical universe

3. Coasting universe

Explanation:

Recollapsing universe has dark matter density greater than critical density. While critical universe has its matter density equal to the critical sensity. Coasting universe on the other hand has much smaller matter density compared to critical density.

Note that the critical density is approximately 10^-20 grams/cm3

car a and b are moving in opposite directions

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vapour or gas assuming atmospheriv pressure