Hi everyone,

I was reading through lecture slides, and i came across one in the black hole lecture that was particularly confusing (its 36 of 44). It is part of the argument that a black hole is like the big bang in reverse.

The calculation shown results in ρbh (universe), which is the critical density for a star with a gravitational radius equal to the radius of the observable universe. It goes on to say that this density is equal to the density required for the universe to be flat, which in an earlier lecture was said to be the actual density of the universe. Um, yes, but only approximately. We don't know the actual density of the universe. Jason

my conclusion from this was that the universe is at its critical black hole density, the density at which it will begin to collapse into a black hole. This doesn't seem right to me, but i cant see whats wrong with my conclusion. On the other hand, in the lecture the conclusion reached from this is that we may be inside the event horizon of our universe. Does this mean that if we are the baby universe, then the part of the parent universe that reached that critical density collapsed into a black hole, and our universe emerged with the same density??

I hope someone replies soon, Im going to have nightmares abt our universe collapsing into a black hole now :S

~Sharmila (u 4848798)

[[red I did see the :S, but anyway … there needn't be anything scary about being inside a black hole. When viewed from inside, matter inside a black hole behaves normally (except at the very end of its collapse, which current physics seems to be unable to describe. OK, but we're not at the end of a collapse, so that's fine). The scary effects that you get inside some black holes, e.g. huge tidal forces, are ordinary gravitational effects which depend on the (local) density. Now SMALL black holes have to have a very high density. But the minimum density of a black hole depends on its size. The minimum average density of a universe-sized black hole is actually extremely low. So we could be in a black hole and no worries. So I agree with your :S and I'll add a :-).

Jason ]]