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Quick question that I was just hoping to get some clarification on. Current theories state that the universe is expanding; with those planets farther away moving away from us at a faster rate.

Is it truly that the universe is expanding or could it be a result of us being "pulled" towards the black hole at the center of our universe while being whipped around in a vortex?

Due to angular momentum its not a direct free fall towards the black hole but orbiting around it. It would account for the reason why the only planets not seemingly moving away from us are in our direct vicinity. I'm sure there is some fault in this logic, and hope those more knowledgeable than I can clarify.

Qmechanic
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Bret
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  • Related: http://physics.stackexchange.com/q/25591/2451 , http://physics.stackexchange.com/q/136860/2451 and links therein. – Qmechanic May 08 '16 at 10:05

2 Answers2

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First of all, the Universe isn't expanding according to "current theories". It is an observational fact.

Second, there is no center of the Universe. Space was created, and started expanding. This expansion pulls everything away from each other. Galaxies lie approximately still in space, but space is expanding. This means that no matter where you are located in space, you see all the other stuff recede from you.

An often-used analogy is a balloon with ants on the surface. The ants' world is the surface of the balloon. In this 2D analogy of a 3D Universe, there is no up or down, only left, right, forward, and backward. When you inflate the balloon, every ant will see all the other ants move away, even though none are actually moving across the surface.

Similarly, we see all galaxies move away from us, no matter if we look north or south. If your scenario were true, we would move in a certain direction. If this motion were on small scales (the black hole were nearby), then if galaxies in, say, the northern hemisphere were receding, galaxies in the southern hemisphere would be approaching. If it happened on much larger scales and all galaxies were moving in the same direction, we would see no apparent motion of the other galaxies.

Thus, your scenario — while in principle not physically impossible — is inconsistent with observations.

pela
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  • While I agree with this, wouldn't galaxies from any location in the sky appear to be receding from us if we weren't just spinning around our center but also towards it? The only bodies of matter that wouldn't appear to be retreating from us would be those within our own galaxy as they would be caught in the same system. Im not trying to be difficult, more playing devils advocate as to why this idea isn't possible. When I specify "Center" im referring to the black hole in the center of each galaxy, not referring that our universe has a "center". – Bret May 08 '16 at 10:48
  • @Mmaibl: I'm not sure I understand. Do you mean if we are pulled toward MW's BH, but the galaxies themselves are still moving away from each other? So that we move radially through the MW? If we did, we would be able to observe a blueshift of the matter in the center (in fact we do move inwards at ~10 km/s, but this is just because our orbit is not completely circular). – pela May 08 '16 at 12:30
  • Galaxy A & B are completely identical. To the observer in Galaxy A (whom is being drawn around & in towards the center of the galaxy); Galaxy B would be redshifted as its ALSO being drawn around & in towards its center. To the observer in Galaxy A, it appears as though its moving away from the observer. However its point in space hasnt really changed, just the galaxy itself has contracted (over a huge period of time). This contraction gives the impression it is moving away from us. – Bret May 08 '16 at 13:18
  • @Mmaibl: Okay, I see. If I understand correctly, you mean that the galaxies themselves are contracting because their stars are being "sucked into" the BH. But if this were true, then 1) another galaxy's stars in "the front" as seen from us would be redshifted while stars "on the other side of its BH" would be blueshifted, and 2) all galaxies would exhibit more or less the same red/blueshift, whereas in reality, the redshift of a galaxy is larger, the farther away it is from us. – pela May 08 '16 at 19:40
  • Also, forget about planets. Planets orbit stars nicely, and stars orbit their galaxy's center nicely. Only galaxies recede from one another. A star in a given galaxy "feel" the gravity of everything that's inside its orbit; both the other stars, the central BH (which is negligible for stars in the outskirts), the gas, and, in particular, the dark matter. – pela May 08 '16 at 19:44
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First of all, there is NO centre in the universe.

I know it's not a good analogy, but think of the universe as the surface of a balloon. Forget the interior, we're only looking at 2 dimensions, whereas the real universe has 3 of them. Put some ink dots on the balloon, which represent galaxies (note: NOT planets).

Now inflate that balloon. You'll see that every dot moves away from every other dot, yet there is no centre to the expansion. The further 2 dots are apart, the faster they're moving away from each other. The same kind of thing happens in the universe - but in 3 dimensions, not 4. No galaxy can claim to be the centre, every one moves away from every other. The separation speeds is determined by the distance between them, as stated in Hubble's law:$$v = H_0D$$ where $H_0$ is the Hubble constant. Recent measurements set it at about $68 km/s$ per megaparsec, or about $224 km/s$ per million light-years distance. Hence it really only becomes important when we are talking about distances of tens of millions of light-years, or more.

You will not see the expansion within a galaxy, or even within a cluster of galaxies, as the internal gravity is stronger than the expansion force. And you will certainly not see it within a planetary system.

Of course, if black energy keeps hold, in the really distant future (trillions of years) even planets may be ripped apart. But we need not worry about just yet!

hdhondt
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  • Thank you so much for your response. That is a great analogy representing the current theory regarding the expansion of space. My question is though is it the only solution that fits? I have no knowledge of how fast we are careening around the black hole at our center, but just around the sun we are going around 66,500 mph (I think). As galaxies further away take longer for light to reach us they would be more red shifted (wouldn't they?) If all planets are "falling inward" towards center wouldn't other galaxies appear to be moving away from us? – Bret May 08 '16 at 09:17
  • There is no black hole at the centre - and there is NO centre. Redshift is caused by the galaxies' speed away from us. It's called the Doppler effect, and you can notice it also in the sound a car makes as it passes you: the sound is higher on approach than when it is leaving. – hdhondt May 08 '16 at 09:43
  • I was saying the black hole at the center of each galaxy. Are you saying the the central point of a galaxy is not run by a black hole? – Bret May 08 '16 at 10:32
  • Yes, most galaxies have a black hole at the centre. But, we are not "falling into it", nor are the planets "falling inwards". All the stars in the galaxy are circling around the central black hole, just as the planets are going around the sun. Note that there are many other (much smaller) black holes within our galaxy, which are also circling around the massive black hole at the centre. It's mass is about 4 million suns, which is tiny compared to the total mass of the galaxy (more than 100 billion suns). – hdhondt May 08 '16 at 10:54
  • $86,\mathrm{km},\mathrm{s}^{-1}$ $\rightarrow$ $68,\mathrm{km},\mathrm{s}^{-1}$ – pela May 08 '16 at 12:31
  • I guess its me just being thick, and for that I apologize. Ill try to explain how I see it and perhaps you can tell me where im going wrong. Lets assume a tub of water with a drain in the center. I spin the water with the drain as the center point, and the release the drain. The vortex will be compounded and amplified by the void in the middle. The water will be drawn to the center but due to its momentum it will go around and around the vortex. As it gets closer to the drain the speed in which it circles the drain (void) would increase. Isn't this what happens on a planetary scale? – Bret May 08 '16 at 13:23
  • If there is no force pulling planets inwards to the center of its galaxy then why don't galaxies on the outskirts of the vortex simply get ejected into space? What is driving the rotation? If there isn't a "pulling" force then wouldn't galaxies that we can see that are millions and millions of years old simply have ejected all their planets eons ago and left only a black hole in the center? Black holes have noticeable effects on matter around them that we can track; they pull in matter. It seems logical to assume that's happening in our galaxy but is offset w/ angular momentum. – Bret May 08 '16 at 13:30
  • Sorry for the rapid comments but I only have a set amount of characters to work with per response. A spinning vortex with a continuous rotation source at center, especially a rapidly rotating one is going to try and eject outwards. I think (at least logically) that the black hole in the center is offsetting this by pulling them back in towards the center. So the black hole is both the "pushing" & "pulling" force at once. "Pushing" due to its rotational energy & "pulling" due to the properties of a black hole. – Bret May 08 '16 at 13:36
  • You should forget about comparisons with vortices and water going down a hole. Contrary to a tub of water, in space there is no friction. Planets and stars want to go in a straight line, but the gravitational force of the central object pulls them away from that straight line, so they move in an elliptical orbit around the centre. The earth does not spin into the sun, and neither does the sun spin into the black hole at the centre. – hdhondt May 08 '16 at 23:39
  • Im not suggesting that they do. The angular momentum would keep the planet or star from "falling into" the black hole. However if a rogue planet without that established rotation was to enter the area of a black hole it wouldnt be gravitationally attracted to it? Without the offset momentum wouldnt it go right into it? – Bret May 13 '16 at 11:50
  • I realize space has no friction but, like water, it is drawn to the void in the middle. Whether it be a drain or a massive body compacted to such a point that it gravitationally affects everything around it. – Bret May 13 '16 at 11:52
  • The gravitational "area" of a black hole - and that of every other massive body in the universe - extends to infinity. We are always in the "area" of our galaxy's black hole, as well as in that of every other back hole in the universe. If there was no rotation then indeed, planets would fall into the sun, and the entire galaxy would collapse into the black hole. However, there IS rotation. If a star did not have enough angular momentum to stay in its orbit, then it would move closer to the centre, speed up and get more momentum that way, until it achieved a stable orbit again. – hdhondt May 14 '16 at 05:02