# Universe Expanding

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.

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.

• 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". – Mmaibl May 8 '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 8 '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. – Mmaibl May 8 '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 8 '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 8 '16 at 19:44

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!

• 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? – Mmaibl May 8 '16 at 9:17
• 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 8 '16 at 9:43
• 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? – Mmaibl May 8 '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 8 '16 at 10:54
• $86\,\mathrm{km}\,\mathrm{s}^{-1}$ $\rightarrow$ $68\,\mathrm{km}\,\mathrm{s}^{-1}$ – pela May 8 '16 at 12:31