Why do galaxies such as milky way appear flat

My simple interpretation of this is that it’s a bit like holding a number of objects on ropes and swinging them around your body they will tend to become coplanar in orbit around ones person.

The thing is, if everything expanded unabated from a singularity into open space, why does the orbit have to be essentially 2 dimensional at all? An unabated explosion of dynamite for example doesn’t follow a coplanar rule, neither does adding food colouring to a bucket of water or releasing rotten egg gas in a school classroom experiment. Also, when we look at atomic particles orbiting a nucleus, are not these orbits random and 3 dimensional?

So why do galaxies such as the milky way not follow the same habits when everything came from a singularity…how do we go from a 3 dimensional spacial expansion to what appear to be 2 dimensional or at least largely coplanar galaxies?

It’s flat mathematically in a four-dimensional space and difficult (impossible?) to visualize. The most helpful analogy that I’ve run across is that of a deflated balloon, ground zero, so to speak of big bang cosmology. There is no space at the beginning – space did not even exist(!) – but as the balloon is inflated, three-dimensional space is the ‘flat’ surface of the balloon, and everything, generally speaking, gets further and further apart. (The ‘inside’ of the ballon does not exist, only the cosmos on the three-dimensional flat surface.) Physics scholars, please adjust, correct or add as might be helpful. ; - ) That is also the source of the term spacetime fabric.

Because they are. Due to rotation of the core black hole.

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I understand this analogy, however, wouldnt that mean that only objects that are touching the external surface of the balloon are flat? everything “inside” that void would not be flat, they would be 3 dimensional and therefore not restricted to a coplanar dimension.

This again leads to the same gas problem i illustrated earlier. Lets say the ballon is inflated due to an explosion/expansion/addition of just normal air (transpose - the big bang)…if one was to then release rotten egg gas (transpose - a supernova that may form a galaxy), if the rotten egg gas is released from within a single point inside that inflating ballon would it not be 3 dimensional in its diffusion through the space inside it?

So how then is it possible for something like a supernova to be the beginnings of what is essentially a flat disc galaxy such as the Milky Way?

I have another question to add to this but wonder if i would be better off asking it separately…my next question is, what starts the angular momentum for an orbiting object around a sun contained “inside” that balloon?

If i were to illustrate the orbiting question:

I have 5 elastic strings attached to a single anchor point and each string connected to a single rock. If i set off an explosion and all of those rocks are propelled outwards away from the anchor point, will they not all get pulled back to that point of origin ny the elastic?

I think you are still thinking in only three dimensions. The balloon analogy is an imperfect one, as are all analogies. (“An analogy is a 3-legged horse.”) The balloon’s surface only represents three dimensional space. You cannot really use two dimensions to visualize three, and you cannot really use three dimensions to visualize four.

Have you ever heard of a Klein bottle? It represents the problem because can be illustrated, but only inaccurately, since in the concept, a Klein bottle does not actually intersect its own surface.

this is just playing games and it is an explanation that starts to sound utterly stupid.

I have also read the following answer on quora.com about how angular momentum came about…its equally as stupid btw…

The question proposes a hypothetical (an “if”) in which the the Big Bang involved an expansion from a central point. Then it goes on to ask how the angular momentum came about.

The answer is: “the same way as when there is no center to expand from”.

There is no special difficulty in getting angular momentum in expansion from a central point compared with expansion without a central point. (Current big-bang models do not include a center, so there is no need for the hypothetical: it makes no difference.)

The total angular momentum of the Universe came about by the same process that the Universe itself came about. Measurements suggest that the total angular momentum is zero. This suggests that the Universe came into being from a zero angular momentum state. But even if it were non-zero, that does not pose any special problem.

Rotations in the Universe are local effects that cancel out on the large scale.

the issue i have with the above answer is that it is almost exactly the same fundamental idea that a mathematician attempted to prove in an equation by claiming that the origin of the energy and matter that started the big bang occurred for such an infinity small period of time and scale that it essentially remained nothing…it never existed, however, all life emerged from this apparent nothing which is also claimed to be something therefore not entirely in breach of Einstein theory of relativity.

Honestly, that is even more stupid than the Christian view of God in that at least the historical authenticity of the Bible is well proven (and not theoretical)!

also, another quote from Quora that has me wondering…

The same analogy applies to black holes which are thought to be at the center of all galaxies, e.g. stars get caught in the gravitational pull of the black holes and spin along with the holes themselves.

when we consider the “water down the drain” anaology, if our solar system came from a black hole for example (I am not sure it did, just guessing here), when we consider a rock on the end of an elastic band analogy, why should a black hole spin when it implodes…does the center point suddenly make a sideways step as an object comes hurtling back towards it?

our earth is claimed to be 4.5 billion years old in a 13.54 billion year old universe. Clearly, there are other solar systems and galaxies beyond ours…so how did ours become coplanar if we were never likely on the outer surface of the balloon illustration?

It’s a great question. I think Dale’s on the right track, but I’m way over my head here. My doctorate is in medicine, not physics. I found this brief video interesting, but I’d have to read more.

Why Do Galaxies Rotate? - Bing video

My family watched a video by Neil DeGrasse Tyson last weekend in Chicago’s Museum of Science and Industry–and it sounded like it would also have relevance. Have you read his book, “Astrophysics for People In a Hurry”? I think that he comments on it there–but I’m also working tonight, and can’t comment adequately. I look forward to this discussion.


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You have posed a question that is much easier in the asking than the telling, and has some pretty deep implications. Another form of this inquiry is, does the universe have a net angular momentum? The answer seems to be that if you add up all the rotational vectors of galaxies and other objects in the universe, they all cancel out to zero. That is consistent with a big bang “explosion”.

How individual structures in the cosmos flatten out is pretty well understood and predicted under gravity and conservation laws; there is no magic to it if your math and physics is up to task. Mine is not, but be assured should you wish to verify this for yourself, I’m afraid you would have a lot of work cut out for yourself.

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Are you trying to figure out how galaxies become flat over time? It’s a really interesting question and there are lots of all kinds of facets to answering this question. Here is a popular level piece from a few years ago discussing some advancements in our understanding of the formation of our galaxy:

But there is always more to unravel. Our galaxy today is pretty good about hiding evidence of any non-flatness, usually ripping apart things thanks to tidal forces. Here is a slightly older article talking about galactic streams and what they can teach us about our galaxy:

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This is an incoherent

creationist question in sheep’s clothing that, as always…, can’t cope with the greatest single fact: eternity. Existence does not need intent. Why would it? All that does is make the infinitely complex problem infinitely worse.

Why large dynamic systems (Earth - moon, Saturn & its rings, the asteroid belt, the Solar system, the sun itself, the Earth itself; both are oblate spheroids) spin flat is high school physics.

The shape of galaxies has nothing to do with the expansion of the univers.
It down to orbital mechanics, centrifugal force and conservation of angular momentum.
First the probability that a cloud of gas has 0 spin is extremely low, so the cloud of gas that made the galaxy will almost certainly spin and be flattened as a consequence of the centrifugal force. This spin is overall conserved as stars and planets form leading to an overall spin.
Next, once these planets and stars for they have their own gravitational field and they will start to pull on each other. If two object meet perpendicular to each other they will each modify the others orbit changing in particular of use each other inclination leading them to slowly become coplanar along the average orbital plane. This will essentially mean that the orbit will trend towards the average orbital plan leading to galaxies being flat. The same applies to solar systems and planets which is why Saturn has rings rather than a cloud of dust.


My immediate reaction is… they’re not. The only thing which is flat are simplistic representations of them. The Milky Way isn’t even close to being flat. I suppose the visually spiral portion of it could be called somewhat flat. If you are asking why some galaxies have such spiral portions to them, the obvious answer would be angular momentum. Interestingly enough this is same answer to why black holes do not eat up everything. In which case, one answer could also be that a huge black hole in the center would tend to swallow up thing which didn’t have that angular momentum – but that sounds a bit dubious to me.

It made me wonder… how many times would the Milky Way have rotated since the beginning of the universe and since the rotation takes 250 million years then that would be around 50 times.

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It might be worth mentioning disk shaped whirling masses that form in our own atmosphere:

When you get groups of masses moving in different directions and running into one another, you often get spin. From that spin, you get disks.

We also see the same shapes in proto-planetary disks.


I have had to jump over some of the points in posts above…I hope that is ok. I have done so because they do not address some of the very problems with the theory…
One post rubbished my statement about the mathematician…sorry but thats not my algorithm. It’s a secular mathematician who came up with it and is a widely accepted explanation of how the origin of the big bag could ignore Einstein’s theory of relativity!

Angular momentum argument falls flat…that is a 2 dimensional claim…it does not explain 3 dimensional angular momentum…there is no reason for galaxies to have such momentum only in a single plane [edit] and others not. If so, why does the nucleus of an atom not follow this principle?
I have issue with the net zero claim…a fly travelling inside a moving car is in its own space moving as it flies…it has its own airspeed in the car…and you still feel it when it bumps into your skin. But it does not hit you at a different momentum one way to the other relative to the direction of travel of the car! Also a circling glider does not experience any difference in a moving airmass downwind to upwind…it is only the appearance of drift the pilot visually sees…but he doesn’t feel anything different. Nevertheless, he is still drifting as he circles.
Another analogy was the comparison with earth atmospheric weather systems…ah sorry but these are constrained to the surface of the earth and are also influenced additionally in this manner by the coriolos effect.

I have another observation on the topic of expansion…there are I believe 2 planets in our solar system that do not rotate counter clockwise…Venus and Uranus. This suggests the water down the drain idea doesn’t apply across the board in space.

This might be a bit of a side question, I wonder, would our planet be livable if it rotated clockwise?

Sure there is.

Take a look at Saturn:

Those rings are aligned perpendicular to Saturn’s axis of rotation. Galaxies do the very same thing with respect to the rotation of the black hole at their centers.

All of the planets are in the same plane.


yes i understand this point about angular momentum…the point is, there are some galaxies where this is not the case, they are not flat. My understanding is that they are known as irregular galaxies such as the [edit] Small Magellanic Cloud (i notice the Large has been reclassified)
here is an image of an irregular galaxy File:Irregular galaxy NGC 1427A (captured by the Hubble Space Telescope).jpg - Wikipedia

Wikipedia goes on to define irregular galaxies in the following manner:

An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy.[1] Irregular galaxies do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure.[2]

Collectively they are thought to make up about a quarter of all galaxies. Some irregular galaxies were once spiral or elliptical galaxies but were deformed by an uneven external gravitational force. Irregular galaxies may contain abundant amounts of gas and dust.

My query about the wikipedia explanation…how does a galaxy that was once uniform in its spiral, end up irregular from another gravity source. Im interested in knowing how it is that objects propelled outwards from a singularity might do this to one another billions of years down the track?

I would like to think of it in a similar manner to a swimming pool whirlpool, however, i dont think this can work because the pool has a surface tension above, a floor below…can this model work in space?

I agree angular momentum is not a complete answer. I think @gavin_kemp 's explanation fills in the gap, however. But I wonder if everyone understood it. So I will reiterate this explanation in my own words. The point is that when you have two bodies, their orbit is going to be two dimensional. But then the orbit of the most massive body will pull others into that same orbital plane. The only difference with the galaxy is the number of bodies involved.

As I explained, the Milky way is not flat at all, because the spiral is only part of it. So you might say that the question really comes down to why don’t we have galaxies with multiple spirals in different directions? And that is where gavin_kemp’s expanation comes into play. They would gravitationally interfere with each other, the same way that unaligned planetary orbits would interfere with each other. Each pulling the other out of shape with the more massive one dominating.

Irregular galaxies are most likely the result of a collision and/or merging of two or more galaxies. And we can see examples of such collisions in all the different stages that we would expect.

In fact, I am beginning to wonder if spiral galaxies are all the ultimate result of galactic collisions. Comparing the stars of in the spiral of the Milky Way to those in the spherical portion (halo and bulge), they are both much younger and more metal rich. The metal richness means they are not first generation stars but stars which formed from the gases of stars that have burnt out and died. Those in the spherical portion (halo and bulge) are old stars and metal poor. This suggest to me the explanation that the spiral is the result of a galactic collision which would both accelerate the life cycle of stars and give birth to more (and larger younger) stars.

Though an alternate explanation is simply that spirals are simply more dynamic and that by itself would cause more star creation as well as larger younger stars. After all, spiral galaxies are the majority 60-77%. Though since spiral galaxies are brighter, it could be we just SEE more of them. It has been more recent evidence that there are far more galaxies than we thought and perhaps this might significantly shift the proportion which are spiral galaxies.


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