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[–] black_listed 0 points 4 points (+4|-0) ago 

What do you mean "as densely as usual"? The universe is currently filled with iron at its usual density.

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[–] killer7 [S] 1 point 0 points (+1|-1) ago 

The density about 7.6 tons per cubic meter.

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[–] VIP740 0 points 2 points (+2|-0) ago 

If the iron was evenly spread the gravitational pull of all the different iron particles would cancel each other out. You probably wouldn't get any black holes that way.

If you had black holes and neutron stars to suck in some of the metal you may end up with some black holes cutting tunnels through the metal, but I don't think you'd get everything to collapse all at once.

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[–] privacy_first 0 points 2 points (+2|-0) ago 

The most logical meaning of the question is to replace every atom of any substance with iron

To calculate the new mass you need to know what is the percentage of every atom in current universe

I have no idea of what that is .... :) sorry

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[–] curomo 0 points 1 point (+1|-0) ago 

I would assume that you'd get multiple collapses, but sounds like a question for Randall Monroe over at http://whatif.xkcd.com

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[–] thelma ago  (edited ago)

All the iron would magnetize, forming 2 black holes of opposite polarity causing my bowels to become obstructed.

As usual.