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The Question

(Submitted January 09, 1998)

I was confronted today at school with a wild possibility: is it true that some astrophysicists have concluded that there is not enough mass in space for a Big Crunch? I know it is possible for this to happen, I'm just curious as to how space can always be expanding, but can never collapse.

The Answer

It may seem like a wild possibility, but current observations say that there is NOT enough matter in the universe to reverse the expansion and head us towards a "Big Crunch". There certainly isn't enough "visible" matter (stars and gas and stuff), but there is evidence that there is matter that we can't see, so called "Dark Matter". See http://imagine.gsfc.nasa.gov/docs/introduction/dark_matter.html for more information on this. Current measurements give a value for "Omega" (the density of the universe divided by the density required to halt the expansion) of 10 - 40% (including dark matter). Many theorists still prefer an Omega of one, but if the matter is out there, we haven't seen it yet.

Thanks for your question.

Eric Christian
for Ask an Astrophysicist

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