Thread: I have a Cosmology question

This one’s really off topic. I have a question concerning Cosmology and Physics.
I’ve heard this (I'm paraphrasing)- “expansion of the universe is accelerating, scientists don’t know why.”
And this-“there is a super massive black hole at the center of the milkyway galaxy”.
My question is could the gravitational field generated by the super massive black hole at the center of our galaxy be warping our local time/space, literally causing time to slow down, and that is why the universe beyond our local time/space seems to be expanding at an increasing rate? If I remember right, Einstein’s theory states that if we came under the influence of a black holes gravitational field, the slower time would flow as we approach it. We wouldn’t notice because time is relative to the observer, and time would appear to be flowing at the same rate to us. We would only notice when looking beyond our local time/space because everything outside would appear to be accelerating. And as we continue to approach the event horizon, to an outside observer, our time/space would appear to slow until it came to a complete stop at the edge of the event horizon.
Why can’t we see the super massive black hole at the center of the galaxy? Again, if I remember right, there is a large cloud of dust blocking our view. Or is that all the galactic debris heading for the event horizon. I took an astronomy class about thirty years ago, so my info may be a little outdated. If my info is incorrect please say so, we must learn from our mistakes or they have no value.

The answer in both cases is that its too far away. For noticeable time distortion you need to have very extreme speed or very extreme gravity. As far seeing it with the naked eye we're on one of the spiral arms and there is an enormous distance to the galactic center. I think that you may be correct in that there is alot of interfere between us and the hole that stopped other observation (telescope, radio telescope), I think that they actually figured out we have a supermassive hole in or galactic center by observing supermassive holes in other galaxies then looking for evidence of one in our galaxy. All this is from my watching the science channel and not any formal education, so take it with a grain of salt.

Well, it's been about 5 years since my last astrophysics class, and physics was only my minor, but I'll take a stab at this one.

1. The universe is expanding at an accelerating rate: This is actually still being debated. There are several theories on the expansion of the universe, and this is only one of them. Because we are not on an inertial frame, newtonian physics break down looking at the rest of the galaxy. They also break down because we are talking about really big forces and really high velocities. Quantum mechanics is more precise, but is still not advanced enough to use for most of astrophysics.

2. There is a large black hole in the center of the milky way. Well, I've never actually heard that before, but it's certainly possible.

3. A large black hole in the center of out galaxy could cause time warping here: The answer is no. First of all, the idea that black holes can warp time is still a theory, but it's based on the fact that they are so massive that their gravitational pull could accelerate matter to near light speed. However, this is only true when you get close to them. From a distance, a black hole has no more gravitational pull than the sun which collapsed to form it.

The idea with black holes, is that a massive object, typically a sun, collapses in on itself. It retains all of it's mass, but get's much smaller. MUCH smaller. We're still not sure how small, but think marble vs earth.

The gravitational pull of a body is determined by two things, as long as you stay outside it's surface. How massive it is, and how close it is to it's center.

(As an interesting aside, assuming a spherical body of uniform density, the gravity towards the center does not change as you bore into the object. If you dug a shaft half way to the core of the earth, and stood in the bottom of it, you'd feel the same gravity as on the surface. The reason is that you are getting closer to the center increasing gravity but also increasing the gravitational forces of the earth above you trying to lift you back out.)

***NOTE: The previous section in parenthesis is completely wrong, as pointed out by gugi, but I left it because I'd already been quoted. For the correct explanation of gravity inside a solid sphere, see gugi's explanation in the next post.

When a star collapses to form a black hole, because it retains the exact same mass in a drastically smaller size, you can get MUCH closer to the center of it, without going inside it. As long as you are outside what would have been the surface before it collapsed, it has no more effect on you than it would have as a sun. Once you cross that threshold, fun things start happening.

The reason they are called black holes, is that just like the sun they once were, anything that get's close to them will get sucked in, and either sling-shotted across the galaxy at incredible speeds (near light speed if it goes close to the actaul body of the black hole) or it will get sucked into the black hole, and become part of it. It would transition to the same state of collapsed matter, and increase the mass of the black hole. Unlike a sun, instead of emmiting an incredible amount of light and electromagnetic radiation, they are so dense, and therefore have such incredible gravitational pull at close range, that light cannot excape them. The lack of nearby debree, combined with the fact that they cannot emit or reflect any light, causes a figurative "black hole" in space. Hense the name.

Why can't we see a black hole: If you've read everything I've had to say so far, you already know the anser to this question. We can't see any black holes at all. We know they are there because we can measure the effect of their gravity, and in very rare circumstances, we can see them form countless lightyears away. Once they have formed, all we see if a spot of nothingness surrounded by bodies that behave as though they are still orbiting a solid object.

Originally Posted by VeeDubb65

(As an interesting aside, assuming a spherical body of uniform density, the gravity towards the center does not change as you bore into the object. If you dug a shaft half way to the core of the earth, and stood in the bottom of it, you'd feel the same gravity as on the surface. The reason is that you are getting closer to the center increasing gravity but also increasing the gravitational forces of the earth above you trying to lift you back out.)

I have to correct this. The gravity increases linearly from zero at the very center to the maximum value at the surface. The gravitational force of a sphere is exactly zero inside of it, so the only thing that contributes is the sphere with a radius at the point of interest. The volume and the mass of that sphere is proportional to the cube of the radius, while the gravitational force decreases with the square of the radius, this the net effect is that the gravity is proportional to the radius.
Works the same way with electricity.

Dang gugi, nice razors and good physics.


Well, I did say that it had been a long time, and it was only my minor.

I'll have to pull out my old text books this week, but suddenly I'm thinking that I was mixing up parts of the rules for a solid sphere and hollow sphere and ending up with something that was wrong for both. I did remember that there was no net gravity at the center, but didn't bother to mention since I was already pretty far afield from the point.

Correct me if I'm wrong again, but isn't it right that there is no net gravity at any point inside a hollow sphere with a massive & uniform shell, but standing on the outer surface, the gravity would be the same as if the sphere was solid with the same mass and outer volume?

The theoretical super massive black hole in the centre of the galaxy seems quite plausible, but I must agree that the kinds of distortions scientists are seeing which lead them to suggest that the expansion of the universe is accelerating could hardly be a relativity puzzle for us. The gravity from that body or whatever the material makeup of the centre of our galaxy is, has less pull on our little piece of space than the sun does, which in turn has less than our moon does. I should like to point out that it doesn't matter if the mass is condensed into a black hole or spread out over stellar clusters, it's the same mass and gravity in the end, outside the immediate vicinity of an event horizon that is.

X

PS I'm a hack. Cosmology is a hobby subject for me.

I think I remember doing a science project on black holes in the 5th grade. Other than that, I haven't done much research about them.

But I have heard, not sure where, that some of the answers about how weird behavior is being perceived in our universe might come from the presence of dark matter?

Originally Posted by crazycliff200843

I think I remember doing a science project on black holes in the 5th grade. Other than that, I haven't done much research about them.

But I have heard, not sure where, that some of the answers about how weird behavior is being perceived in our universe might come from the presence of dark matter?

Quite right, but dark matter is a separate topic entirely. Dark matter is simply physicists' tool for explaining the difference between observed behavior of gravity in the universe and apparent total mass. Nobody agrees on what dark matter is, or if there even is such a thing.

Some scientists insist that "dark matter" is a real phenomenon that may make up a significant portion of our universe. Matter which has mass but no volume or tangible form. If you listen to Coast to Coast AM, you'll get guys on there that think this is the be-all, end-all of astronomy.

Another group thinks that dark matter isn't dark at all. It's just plain old matter that we haven't spotted yet.

There are others who say that the whole idea is bunk, and what we are observing is the result of the fact that we are not observing the universe from an inertial frame, and newtonian physics are therefore useless to do anything more than approximate what we are seeing.

I wouldn't even begin to venture a guess at which camp is right, except to say that in any case, it's a perfect example of how little we really understand about the universe, despite all how far we have come.

Originally Posted by VeeDubb65

Correct me if I'm wrong again, but isn't it right that there is no net gravity at any point inside a hollow sphere with a massive & uniform shell, but standing on the outer surface, the gravity would be the same as if the sphere was solid with the same mass and outer volume?

That is correct, and also applies to both gravity and E&M. The statement comes directly from Gauss' law, which is one of Maxwell's equations.

Edit: Well, it does not "come from" Gauss' law, but that is a very clear example, and often the first that physics students see.

Originally Posted by VeeDubb65

Quite right, but dark matter is a separate topic entirely. Dark matter is simply physicists' tool for explaining the difference between observed behavior of gravity in the universe and apparent total mass. Nobody agrees on what dark matter is, or if there even is such a thing.

Some scientists insist that "dark matter" is a real phenomenon that may make up a significant portion of our universe. Matter which has mass but no volume or tangible form. If you listen to Coast to Coast AM, you'll get guys on there that think this is the be-all, end-all of astronomy.

Another group thinks that dark matter isn't dark at all. It's just plain old matter that we haven't spotted yet.

There are others who say that the whole idea is bunk, and what we are observing is the result of the fact that we are not observing the universe from an inertial frame, and newtonian physics are therefore useless to do anything more than approximate what we are seeing.

I wouldn't even begin to venture a guess at which camp is right, except to say that in any case, it's a perfect example of how little we really understand about the universe, despite all how far we have come.

An interesting tidbit about dark matter - a "true" definition of dark matter is any matter that cannot be seen (or easily seen) by observing electromagnetic radiation from it. So it is undeniable that there is dark matter (such as brown dwarfs, asteroids, and other similar bodies). But when most scientists talk about "dark matter," they are referring to abundant bodies of dark matter; objects such as those in the () above are nowhere near the order of magnitude of mass of the "dark matter" that scientists are hunting for.