B Are black holes infinitely dense?

[Aliam1]

If they are infinitely dense than, they would have infinite gravity, so if they have infinite gravity then the universe would have been sucked into a black hole. I was told that black hole has Infinite density so I want to clarify with you guys because it's logically didn't make sense to me.

 

[jedishrfu]

https://www.insidescience.org/news/black-hole-cores-may-not-be-infinitely-dense

 

[russ_watters]

You are correct; black holes are not infinitely dense. In fact, very large black holes have very low density; like below 1 s.g.

What *might* be infinitely dense is the "singularity" at the center of the black hole, if it exists. What would make it infinitely dense isn't infinite mass, but zero volume, which resolves the contradiction you noted.

 

[PeroK]

If they are infinitely dense than, they would have infinite gravity, so if they have infinite gravity then the universe would have been sucked into a black hole. I was told that black hole has Infinite density so I want to clarify with you guys because it's logically didn't make sense to me.

At a sufficient distance from a black hole, its gravitational attraction is no different from a star of the same mass.

The reason you cannot get "infinite" gravity with a star is that you cannot get close enough without reaching the surface of the star.

Even in good old Newton's gravity:

$g = \frac{GM}{r^2}$

You have "infinite" gravity close to a point mass.

 

[russ_watters]

At a sufficient distance from a black hole, its gravitational attraction is no different from a star of the same mass.

The reason you cannot get "infinite" gravity with a star is that you cannot get close enough without reaching the surface of the star.

Even in good old Newton's gravity:

$g = \frac{GM}{r^2}$

You have "infinite" gravity close to a point mass.

The way I read the OP, though he didn't come out and say it, was that a "black hole" has finite volume, so infinite density would indeed result in infinite mass and therefore infinite "gravity" everywhere in the universe

 

[Nugatory]

I was told that black hole has Infinite density

Told by whom? If you don't provide a reference there's no way for us to know whether the source is wrong or you misunderstood it.

But with that said...
- It's somewhat tricky defining the "density" of a black hole, but one intuitive definition would be the volume of a hypothetical sphere with area equal to $4\pi{r_S}^2$ where $r_S$ is the Schwarzschild radius divided by the mass. By this definition, not only the is the density not infinite but the larger the black hole the less dense it is.
- Although the density as defined above is not infinite, even an infinitely powerful outwards force will not stop something from falling through the horizon. Thus, we could reasonably say that the gravitational force is infinite there.

 

[Aliam1]

The way I read the OP, though he didn't come out and say it, was that a "black hole" has finite volume, so infinite density would indeed result in infinite mass and therefore infinite "gravity" everywhere in the universe

Another evidence to what you said russ is the expanding universe which says that gravity is not infinite. What do you think russ