If the density of the universe great than a critical value, then it might continue expanding...

If the density of the universe great than a critical value, then

it might continue expanding forever.
		it could expand to a fixed size and remain.
		it might eventually stop expanding and start collapsing.
		there's probably less dark matter than luminous.

Expert Solution

it might continue expanding forever.

We know from general relativity that expansion of the universe is slowed down by the mutual gravity of all the matter inside it. Whether or not the expansion will continue forever depends on whether or not there is enough matter in the universe to reverse it. If the density of matter in the universe is less than a certain critical value, then the universe will never stop expanding. If, on the other hand, the density of matter is greater than the critical value, then the pull of gravity will eventually be strong enough to stop the expansion and the universe will begin contracting. In Section III we saw that whether or not the universe is finite or infinite depends on whether the density of matter is above or below a critical value. That value turns out to be exactly the same as the critical value that determines whether or not the expansion will reverse. In other words, general relativity says that an open (infinite) universe will expand forever and a closed (finite

genius_generous answered 1 hour ago

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