Final answer:
If Ωmatter = 0.5 today with no dark energy, the universe would be a closed universe, eventually contracting in a 'big crunch'.
Step-by-step explanation:
If Ωmatter = 0.5 today and there were no dark energy, the universe would follow a trajectory where the density of the universe is higher than the critical density. This scenario leads to a model of the universe known as a closed universe. In this model, the universe will eventually stop expanding and start contracting, culminating in a scenario termed the "big crunch". John Wheeler characterized the big crunch as the inverse of the Big Bang: an implosion where matter, energy, space, and time are all compressed to an infinitesimal point.