The Universe

Dark Matter and Dark Energy

How do astronomers describe the concentration of matter in the universe?

Astronomers use the Greek capital letter omega (Ω) to represent the concentration, or density, of matter in the universe. Sometimes, a subscript M is added (ΩM) to make clear that this is the concentration of matter; at other times, two subscripts are used to distinguish the concentration of dark matter (ΩDM) and that of baryonic or non-dark matter (ΩB).

If dark energy does not exist, then the matter density in the universe alone determines the geometry and final fate of the cosmos. In that case, there are three possibilities. If Ω is larger than one, then the universe would have a closed geometry and ultimately collapse in a Big Crunch. If it is equal to one, then the universe would have a flat geometry and would expand forever. If it is less than one, then the universe would have an open geometry and would also expand forever.


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