A-level Physics/Cosmology/How the universe may evolve

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.