Black holes and our galaxy

[Ian]

Did anyone ever wonder whether or not our galaxy is a black hole? Has anyone ever looked at it's shape and thought of that? Our galaxy spins around and the stars near it's center get sucked into the black hole "assuming that there is a black hole near the center of our galaxy" all of the stars that are in our galaxy are making either gravitational wells or gravitational feilds. If you think about everything in our galaxy that has huge amounts of gravity, then, all the objects' gravity together can form a black hole. That's my theory. Does anyone agree?

 

[Bikewer]

No.

The laws of physics are functioning normally in the galaxy, and we can readily observe these this, and the objects in the galaxy as well.
In a black hole (according to the current model, at any rate) the laws of physics break down on the other side of the Event Horizon, and light cannot excape.

Objects such as stars that are drawn near a black hole are gradually torn apart, with material from the star gradually spiraling into the event horizon, with a large acompanying release of energy.
The star does not survive the transit intact, as the gravitational effects are too localized to "pull in" the entire star in one chunk, as it were.

 

[Correa Neto]

Bad Astronomer can confirm or not the following- The avaliable data suggests that at the centre of our galaxy there is a massive black hole. But that's a far cry from saying that out galaxy IS a black hole. Got the difference?

BTW, I must also not agree with the following-

"That's my theory. "

Because it seems to me your post does not fullfils the requirements to be labelled a theory.

 

[Crossbow]

No for me as well!

While Black Holes may be far more numerous than previously thought, there is also data that shows that Black Holes do evaporate over time. Therefore, they may actually be transitional objects as opposed to something that lasts virtually forever.

 

[Ian]

Black Hole

How strong is the black hole at the center of our galaxy?

 

[SGT]

No.

The laws of physics are functioning normally in the galaxy, and we can readily observe these this, and the objects in the galaxy as well.
In a black hole (according to the current model, at any rate) the laws of physics break down on the other side of the Event Horizon, and light cannot excape.

Objects such as stars that are drawn near a black hole are gradually torn apart, with material from the star gradually spiraling into the event horizon, with a large acompanying release of energy.
The star does not survive the transit intact, as the gravitational effects are too localized to "pull in" the entire star in one chunk, as it were.

The laws of physics break down at the event horizon. We have no means to know what lies beyond the horizon.
The mass and dimensions of our galaxy are not compatible with what is required for a black hole. The escape velocity at the limits of the galaxy is well beyond the speed of light, so our galaxy cannot be a black hole.
I am not sure if the same is true for the entire Universe. We have a quite good idea about its dimensions, but not of the mass. Perhaps our Universe is a black hole. One thing we are sure of: light does not leave the Universe.

 

[Ziggurat]

The laws of physics break down at the event horizon.

Depends what you mean. General relativity certainly doesn't break down at the event horizon, it keeps working quite well all the way to the singularity.

I am not sure if the same is true for the entire Universe. We have a quite good idea about its dimensions, but not of the mass.

No, we've got little idea about it's dimensions. And if it's an open universe, it's infinite.

Perhaps our Universe is a black hole. One thing we are sure of: light does not leave the Universe.

That's a meaningless turn of phrase to use for the entire universe. Yeah, light can't escape, but nothing enters the universe from outside it either. The idea of a black hole is only useful if there's both an inside and an outside, otherwise you're just playing word games.

 

[SGT]

The idea of a black hole is only useful if there's both an inside and an outside, otherwise you're just playing word games.

Why do you think there must be an outside for a black hole to exist? The only requisite is that the escape velocity must be greater than the speed of light. We don't know if it is true for our Universe, but it is possible, even if the probability is low.

 

[Crossbow]

Re: Black Hole

How strong is the black hole at the center of our galaxy?

Well, since it is the biggest black hole in all of the galaxy,
and since all of the other objects in our galaxy are in orbit about it,
then one can infer that it is very, very, very strong indeed.

Or are you looking for something more quanitative (such as an estimate of its mass)?

 

[Ian]

Gravitational field of the black hole. "if it exists"

Do gravitational fields exist or is it a gravitational well? If a gravitational field exists for the black hole at the center of our galaxy, how far out does it extend to? Does it extend to the edge of our galaxy?

 

[SGT]

Re: Gravitational field of the black hole. "if it exists"

Do gravitational fields exist or is it a gravitational well? If a gravitational field exists for the black hole at the center of our galaxy, how far out does it extend to? Does it extend to the edge of our galaxy?

A black hole is a mass. So it has all the properties of a mass: it has inertia, it generates a gravitational field and responds to gravitational fields generated by other masses. As any other field, the gravitational field generated by the black hole extends to the infinity, but its intensity decays with the square of the distance. So, yes, the field generated by the black hole at the center of the galaxy extends to the edge, but it should be very weak there.

 

[Ziggurat]

Why do you think there must be an outside for a black hole to exist? The only requisite is that the escape velocity must be greater than the speed of light. We don't know if it is true for our Universe, but it is possible, even if the probability is low.

I don't think you quite understood my point. For one thing, even if you go faster than c, you still can't "escape" the universe. Even more importantly, though, it's just useless to talk about the entire universe being a black hole. It doesn't actually help you to understand anything, it just becomes a label.

 

[SGT]

I don't think you quite understood my point. For one thing, even if you go faster than c, you still can't "escape" the universe. Even more importantly, though, it's just useless to talk about the entire universe being a black hole. It doesn't actually help you to understand anything, it just becomes a label.

I don't agree with you. We really don't know what lies inside a black hole. A really massive black hole has its events horizon so great that its density can be very low, so there could be stars, planets and galaxies inside this black hole. If people lived in one of the planets, they would consider their black hole a universe.
I don't say that this is true for our Universe and I don't even know if the picture of a universe inside a black hole is real, but it is not against the laws of physics AFAIK.

 

[zer0vector]

Re: Re: Black Hole

Well, since it is the biggest black hole in all of the galaxy,
and since all of the other objects in our galaxy are in orbit about it,
then one can infer that it is very, very, very strong indeed.

Or are you looking for something more quanitative (such as an estimate of its mass)?

Actually our galaxy isn't in orbit due solely to the mass of the black hole (thought to be about 2x10^6 Solar Masses). The black hole gravity does have a small effect, but the main potential which shapes the galaxy is the distribution of the stars themselves.

It's pretty easy to think about if you consider that there are 10^10-10^11 stars in the galaxy. Assuming an average mass of 1 solar mass, that's orders of magnitude greater than the mass of the black hole.

 

[espritch]

It isn't the amount of mass that makes a black hole, it's the density. For instance, our sun is not a black hole, but if you squeezed all of it's mass into an area the size of the moon, it would become one. Matter tends to resist such compression due to electromagnetic repulsion. This force is much stronger than gravity. It takes a tremendous amount of energy (like that released in a super nova) to overcome this resistance. However, once matter has been sufficiently compressed, the concentration of gravity will overcome electromagnetic resistance (and anything else) and the matter will continue to collapse in on itself to a singularity.

Consider two stars of equal mass, one having collapsed into a black hole. At any distance beyond the circumference of the star, the gravitational attraction of the star and the black hole are identical. When you begin to move past the surface of the star toward it's center, the gravitational attraction actually decreases because the matter behind you is pulling in the opposite direction. When you reach the center of the star, the gravitational attraction from all directions is equal, so there is effectively no gravitational attraction at all. But there is no surface for the black hole. So as you get closer to the singularity, the gravitational attraction just keeps increasing. Eventually, it is so strong, light cannot escape (meaning you've reached the event horizon). The event horizon is not a surface in any physical sense, just the distance from the singularity at which neither light nor anything else can escape the pull of gravity.

 

[SGT]

It isn't the amount of mass that makes a black hole, it's the density.

Not exactly. What makes a black hole is a combination of high mass and relatively small dimensions. If a star becomes a black hole, since the mass is not so great, the radius of the event horizon will be small and the density would be really huge. But with a really massive black hole, like the one supposed to be in the center of our galaxy, the radius of the event horizon would be large enough that the density would be very low.

 

[espritch]

Not exactly. What makes a black hole is a combination of high mass and relatively small dimensions.

After some thought, I agree. Volume increases as the cube of distance while gravitational attraction falls as the square of distance, so a very large aggregation of mass does not need to be as dense to form a black hole as a very small one.

However, regarding the original question (is the universe is a black hole), I think the answer is no. Whether a black hole has the mass of a single sun or the mass of 3.5 billion suns (the estimated mass of our galactic black hole), the matter in the event horizon will still collapse to a singularity with infinite density. Since matter in our universe has not collapsed to a singularity (quite the opposite, it is expanding at an apparently increasing rate), this would argue that the universe is not a black hole.

 

[SGT]

However, regarding the original question (is the universe is a black hole), I think the answer is no. Whether a black hole has the mass of a single sun or the mass of 3.5 billion suns (the estimated mass of our galactic black hole), the matter in the event horizon will still collapse to a singularity with infinite density. Since matter in our universe has not collapsed to a singularity (quite the opposite, it is expanding at an apparently increasing rate), this would argue that the universe is not a black hole.

There is no need of collapse to a singularity with infinite density. All that is demanded for a black hole is that nothing can cross the event horizon from the inside out. I don't have the data with me and googling turned too many results, but it seems that if the mass of the dark matter of the Universe were about ten times the mass of the light matter, the event horizon would be of 15 billion light years, the estimated size of the Universe.
Nobody knows how much dark matter there is, but it is not probable that it is enough to make the Universe a black hole. But the idea is not so crazy.

 

[SquishyDave]

There is no need of collapse to a singularity with infinite density. All that is demanded for a black hole is that nothing can cross the event horizon from the inside out. I don't have the data with me and googling turned too many results, but it seems that if the mass of the dark matter of the Universe were about ten times the mass of the light matter, the event horizon would be of 15 billion light years, the estimated size of the Universe.
Nobody knows how much dark matter there is, but it is not probable that it is enough to make the Universe a black hole. But the idea is not so crazy.

I'm no scientitian, but the latest scientific thinking is that the size of the universe is 156 billion light years, not 15 billion. 13.7 billion years is the age of the universe.

Now as for the universe being a black hole, if your definition for a black hole is only light cannot escape from it then yes the universe could be considered one. But why not just call it the universe, because that's what it is?

If your definition for a black hole is matter that has collapsed in on itself such that the gravity is so big light cannot escape then the universe probably isn't one. And I think most sciencey type guys use the collapsed matter definition, also light cannot escape because the universe is expanding not because gravity is too high meaning according to the proper use of the word, the universe is not a black hole.

To summarise my points for ease of shooting down.

Light cannot escape a black hole because the gravity is too high.

Light cannot escape the universe becuase the universe is expanding too fast

Seems fundamentality different. And no black hole is expanding at or faster than the speed of light, luckily for us I would say.

 

[espritch]

There is no need of collapse to a singularity with infinite density. All that is demanded for a black hole is that nothing can cross the event horizon from the inside out.

That used to be my assumption. However, as someone on this board explained to me a while back, the situation is more complicated. I'll try to explain this the way it was explained to me (take with a grain of salt - I'm no expert on relativity).

In the relativistic universe, distances in space-time are measured as separations given by the equation:

ds^2 = -C^2dt^2 + dx^2 + dy^2 + dz^2

This includes the three dimensions of space plus a time component with a negative coefficient. The event horizon for a black hole is defined by the Schwarzschild radius given as R = 2GM/C^2. The Scharwzschild metric describes the behavior of space-time for a non-rotating black hole. Using polar coordinates (r, T, O) (I'm using T for theta and O for omega because I don't know how to specify these symbols*)

ds^2 = -(1-R/r)C^2dt^2 + (1/(1-R/r))dr^2 + r^2(dT^2 + sin^2TdO^2)

As long as the radius 'r' is larger than the Schwarzschild radius 'R', we are in normal space and the time component has a negative coefficient. But when 'r' becomes smaller than 'R', the time coefficient becomes positive while the coefficient for the distance becomes negative. This, in effect, means that time and spatial distance have switched places. Progress in time (which in uni-directional) becomes progress toward the singularity. This implies that collapse toward the singularity becomes inevitable for anything within the event horizon.

*For a clearer version, try this link:
http://www.astro.ku.dk/~cramer/RelViz/text/exhib1/exhib1.html