Alevel Physics/Cosmology/How the universe may evolve
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The question of whether the Universe is infinite or not depends crucially on the value of the quantity know as Ω. Such a value, although not determined yet, depends basically on the rate of expansion, or the Hubble Constant, of the Universe.
In simple terms, the quantity Ω is the ratio of the density of the universe (mass per unit volume) to the critical value which determines expansion or collapse. There are three possible outcomes from this value 
 Steady State: the Universe will continue to expand up to a point, whereupon the Universe will stay in a constant state. This would occur if Ω = 1.
 Continuous Expansion: The Universe will continue to expand. This will occur if Ω<1. The result of continuous expansion would be, according to the 1st Law of Thermodynamics, a Universe which would gradually cool down until the temperature became 0 Kelvin.
 Regression: The Universe will grow, and then collapse on itself. This will occur if Ω>1. It has been postulated that if the Universe were to carry on this course, all matter would recondense into a singularity, and recreate another big bang.
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