The Cosmic Microwave Background by Rhodri Evans

Author:Rhodri Evans
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

As we have already discussed, Einstein’s theory of General Relativity relates gravity to a distortion of space. Put simply, if the Universe has a lot of mass in it, it will have a positive curvature like a ball. If it has very little mass in it, it will have negative curvature like the surface of a saddle. And, if the amount of mass density is just right then it will have a flat geometry like a flat surface (see Fig. 4.6). The density required for a flat Universe is called the ‘critical density’. The ratio of the actual density to the critical density is called Ω. We often write this as Ω 0, the total Ω due to adding the contributions from different components such as normal matter, dark matter and anything else. The three possible scenarios are shown in Fig. 4.6.

Fig. 4.6The three possible geometries of the Universe, which depend in the ratio of its density to the so-called ‘critical density’ Ω 0. If Ω 0 = 1 (bottom diagram) the Universe is flat, if Ω 0 < 1 (middle diagram) it is open, and if Ω 0 > 1 (top diagram) it is closed

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