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Author Topic:   Black Hole Universe Model Questions
Son Goku
Posts: 1169
From: Ireland
Joined: 07-16-2005
Member Rating: 10.0

Message 11 of 69 (663394)
05-24-2012 6:12 AM
Reply to: Message 8 by New Cat's Eye
05-21-2012 11:43 PM

Isn't our universe less dense than a black hole?

Black Holes can be less dense than water, it's a pretty cool fact which can be seen with an easy calculation.

Density is mass divided by volume, so (p is density, M is mass, V is volume)

p = M/V

A black hole is a sphere which has volume V = (4/3)*(pi)*(R^3), with R the radius. Substituting for V in the first equation we have:

p = 3M/(4pi*R^3)

The radius of a black hole is its Schwarschild radius, which is:

R = (2GM)/(c^2)

G is Newton's constant and c is the speed of light. So replacing R in our formula for p with this, we get:

p = (3M*c^6)/(32pi*G^3*M^3)

Canceling the M factors and separating out the M factors:

p = ((3c^6)/(32pi*G^3))*(1/M^2)

The numbers in front are just some number which doesn't depend on the properties of the black hole (it's just multiples of the speed of light and G and pi), which I'll call C (= 7.6 x 10^79), so

p = C/M^2

So the density of a black hole is proportional to the inverse square of its mass. So the bigger the mass, the less dense the black hole. Make the mass big enough and the black hole can be less dense than water.

This message is a reply to:
 Message 8 by New Cat's Eye, posted 05-21-2012 11:43 PM New Cat's Eye has acknowledged this reply

Replies to this message:
 Message 12 by 1.61803, posted 05-24-2012 10:34 AM Son Goku has not yet responded

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