# B Why don't galaxies obey gravity?

1. Jul 28, 2017

### Wallsy

Hi all,my second question is this.Why don't galaxies obey gravity? In our solar system the closer to our sun the planet is the faster it rotates.So the outer planet takes much longer for a solar year.
When we look at galaxies this gravitational law doesn't apply.The outside stars spin at the same rate as the inside stars.
My question is why?

2. Jul 28, 2017

### davenn

where did you get that idea from ?, please show a link

also you cannot compare the simple planetary orbital velocities around their parent star, with the more complex orbital systems of star around their parent galaxy
they are very different

there is plenty of info on the net that disproves it

here's just one ....

https://en.wikipedia.org/wiki/Galaxy_rotation_curve

3. Jul 28, 2017

### phinds

Dark Matter, and of course they DO obey gravity, it's just that there is a different distribution of matter in the galaxy than in the solar system.

4. Jul 28, 2017

### rootone

Because it only makes sense if some kind of invisible matter exists. which has mass like normal matter, but does not interact with it other than through gravity.
There are numerous experiments ongoing to detect what it might be, best guess at present is some kind of supersymmetric particle.
You may well say as others have, well maybe our understanding of gravity is incorrect.
So far though speculation of modified gravity has been fruitless, nobody has produced a theory which explains GR, yet also includes a 'dark' component.

5. Jul 29, 2017

### Wallsy

My mind says our understanding of gravity is spot on.Our understanding of galaxies isn't.Thinking outside the box-Maybe black holes aren't sucking in planets and stars but spitting them out?Creating stars?

6. Jul 29, 2017

### rootone

You will not get far with alternative cosmnology here unless you have data

7. Jul 29, 2017

### phinds

This is nonsense. Thinking outside the box only works well when you first understand what's IN the box.

8. Jul 30, 2017

### Janus

Staff Emeritus
1. The dark matter hypothesis is based on the idea that our understanding of gravity is correct and thus, if the matter we can see doesn't have enough mass to produce the gravity needed to hold galaxies from flying apart, then there is matter that we can't see that is providing that extra mass and gravity.

2. Black holes spitting out stars would be contrary to our understanding of gravity and thus this suggestion conflicts with your first sentence.

3. How would black holes producing stars explain the observations we have made about galaxies?

9. Jul 30, 2017

### Mike Holland

I have just been re-reading "Stars in the Making" by Cecelia Payne-Gaposchkin. I first read it in my teens, about 1960. The book is way out of date, and her claims may no longer be correct, but she says that the visible matter in the spiral arms and core of the galaxy accounts for only 1-10% of the mass of the galaxy, and that the balance is in the globular star clusters and high-speed stars that form a (mostly invisible to us) sphere. I.e. our galaxy is spherical with just a tiny sliver across the middle where the gas, new-born bright stars and spiral arms are.

All the write-ups on stellar motion in the galaxy refer to the distribution of the visible matter. They seem to ignore most of the matter of the galaxy. If we take into account that the stars are in a great sphere of matter rather than a disc, then we would get a much flatter velocity curve. If the sphere was of uniform density (which of course it isn't), then the gravity would fall off as the inverse of the radius instead of its square! Do we really need dark matter to explain the observed curves?

10. Jul 30, 2017

### phinds

Mike, do you seriously think that something this obvious has been overlooked by thousands (probably 10's of thousands) of scientists?

11. Jul 30, 2017

### stoomart

Is there a macroscopic concept of "binding energy" for galaxies, along the same lines as atoms having more mass than the sum of their constituents, or mass defect?

12. Jul 30, 2017

### Janus

Staff Emeritus
The binding energy is due to the forces holding the atom together. The force holding the galaxies together is gravity (we have no evidence of any other significant binding forces.) Gravity is the weakest of the known forces. So if you are asking whether or not the binding energy of the gravity itself is enough to generate a mass defect large enough to make up for the "missing mass", then no, not by a long shot.

If you are thinking of a different "binding energy", what force would provide it? Why have we never noticed it? And any force capable of producing a large enough mass defect to make up for the missing mass would have to be so strong that in of itself it would capable of holding the galaxy together many times over, rather than having to rely on the mass defect it produced. (in fact it would be so strong that we would be wondering why galaxies don't collapse in on themselves rather than why they don't fly apart.)

13. Jul 30, 2017

### stoomart

My understanding is gravity is a non-factor at the quantum scale because it is simply too weak, hence why we have nuclear forces; I suspect a similar case at galactic scales, but I don't want to start coming up with personal theories. This excerpt from the wiki article in my last post is what got me thinking about this:

The nuclear force must be stronger than the electric repulsion at short distances, but weaker far away, or else different nuclei might tend to clump together. Therefore, it has short-range characteristics. An analogy to the nuclear force is the force between two small magnets: magnets are very difficult to separate when stuck together, but once pulled a short distance apart, the force between them drops almost to zero.​

14. Jul 30, 2017

### Mike Holland

Phinds, here is a quote from Wikipedia -
"When mass profiles of galaxies are calculated from the distribution of stars in spirals and mass-to-light ratios in the stellar disks, they do not match with the masses derived from the observed rotation curves and the law of gravity."
Whoever wrote that had no concept of the sphere of matter surrounding our galaxy (and presumably other galaxies). I have not found a single write-up on the rotation curves and dark matter that refers to the 90% of the mass outside the plane of the spirals. Or was Cecelia's estimate totally wrong?

15. Jul 30, 2017

### Janus

Staff Emeritus
But again, we have no reason to suspect any type of short range force on the galactic scale to provide this binding energy. But let's use the nuclear binding energy model. For a uranium atom, the total mass defect amounts to less than 1% of the total mass, of the atom. But with galaxies, the missing mass isn't equal to just a tiny fraction of the mass of the visible matter, but more than it. So what makes more sense? To hypothesize some unknown short range force that is capable of making up that much mass in the form of binding energy ( which, by the way, would not solve the rotation curve problem. It isn't just how fast the stars orbit, it is also how those orbital speeds change as you move outward from the center. Assuming a short range force at the galactic center would mean that there is where the mass defect mass would be, and the orbital speeds would behave like those of our solar system.). or that there is some form of matter that is not detectable by electromagnetic radiation that is providing that extra mass (considering that we already know of 1 particle that fits this bill, the neutrino)?

16. Jul 30, 2017

### Janus

Staff Emeritus
The point isn't that the mass providing the observed rotation curves isn't spread out in a sphere surrounding the center of the Galaxy,( if fact , DM is based on this assumption, it is that this mass could be made of regular baryonic matter, and remain undetectable at with any wavelength of the electromagnetic spectrum. If that mass where made up of ordinary matter, we would see it.

17. Jul 31, 2017

### Mike Holland

I don't know how one would determine the spread of dark matter vertical to the plane of the galaxy by observing the movement of stars in the plane of the galaxy. But there is an awful lot of matter that is spread out in a sphere, about 90% according to Cecelia, and this matter would certainly flatten the velocity/distance curves to some degree.

Is Cecelia;'s estimate still valid, or have more recent X-ray surveys etc given a different view of the matter distribution of the spiral galaxy and surrounding stars and clusters? I have read dozens of books on Astronomy, and subscribe to New Scientist, but have never seen any other mention that the spiral arms and visible stars are just a small part of our galaxy. The halo of globular clusters is commonly mentioned, but never its contribution to the total mass of the galaxy.

18. Jul 31, 2017

### UsableThought

Although I'm a big fan of books as opposed to online reading, I often Google for specific questions, given that web searches are both far broader & far more specific than is possible with hard-copy books and magazines. I think that would be helpful in your case also.

Indeed, a search for +'Globular cluster' +'dark energy' +'halo' pulls up what seem to be quite relevant hits; for example here is a 2015 study titled "Dark Matter Halos in Galaxies and Globular Cluster Populations. II: Metallicity and Morphology":

https://arxiv.org/abs/1504.03199

The study starts off like this:

A remarkably simple empirical correlation has emerged between two global properties of galaxies that belong to early phases of galaxy formation: these two quantities are $M_h$ (the total halo mass of a galaxy including all visible and dark matter in its potential well); and $M_GCS$ (the total mass in its globular cluster system).​

Of course a study like this is narrowly technical; but even at a glance it seems to speak to the point of whether scientists interested in dark matter had somehow forgotten about either globular clusters or the galactic halo.

Last edited: Jul 31, 2017
19. Jul 31, 2017

### Mike Holland

Thanks, UseableThought. I will study those links.

20. Jul 31, 2017

### Janus

Staff Emeritus
In this link, they estimate the mass of the stellar halo as being between 1e8 and 1e9 solar masses: