That's not necessarily true. Look at crystal growth from solution. The crystal gets bigger and bigger, yet its density remains the same. IIRC, Guth's...
It's a convenient axiom. Trying to decide whether we are fish, dreaming we are men does not get the garden tilled, or contribute in any useful way to...
That "less than a mile" estimate is dubious because we don't even know how much material exploded at this point. For a real answer, you'll need that,...