## No matter how fast the universe is expanding, shouldn't gravity eventually make it recollapse? (Intermediate)

Even if the universe is expanding or even accelerating, wouldn't gravity eventually slow the expansion? No matter how big the force of velocity, wouldn't gravity, however small, eventually whittle down the expansion?

No, it actually wouldn't. This isn't the greatest analogy in the world, but it's sort of like cutting a string in half, and then cutting one of the halves of the string in half, and then cutting that in half, etc. You can keep cutting forever, and you'll never be completely out of string. You'll never be out of string, because you keep cutting off less and less each time.

Likewise, gravity can slow down the expansion of the universe, but it keeps doing so by less and less as time goes on (because it's not as strong over larger distances). So it's not hard to imagine a universe in which the expansion never gets below a certain rate, because the effects of gravity can never get it quite that low.

#### Christopher Springob

Chris studies the large scale structure of the universe using the peculiar velocities of galaxies.  He got his PhD from Cornell in 2005, and is now a Research Assistant Professor at the University of Western Australia.

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