[Lvl999Noob]

I thought newtonian gravity was already proven to be inaccurate with Einstein's Special Relativity (or General Relativity?) giving better results on cosmic scales (basically analogous to an approximation vs an exact formula)?

[magicalhippo]

General Relativity reduces to Newtonian gravity as the curvature goes to zero, that is when you're very far away from objects relative to their masses, for slow non-relativistic objects like stars and galaxies.

Galaxies are typically so far away from another they're almost like point sources to each other, hence Newtonian gravity explains their motion very well.

However, inside galaxies things do not behave as expected, as stars in almost all the galaxies we've measured does not move like Newtonian (nor GR) behaves based on the matter in the galaxy we see. One alternative to the mainstream theories of dark matter is to modify Newtonian gravity, called MOND.

This work tested if MOND fit the motion of galaxies in galaxy clusters. They found it did not.

MOND already does not explain other phenomena that dark matter can so it's not terribly surprising. Here[1] is a nice accessible talk going through all the evidence for dark matter.

But it is technically a possibility that there's two things are going on, something MOND-like as well as dark matter, so worth checking.

[1]: https://pirsa.org/26030070

[NewEntryHN]

Why is the article titled "Newton's law of gravity passes its biggest test" if it doesn't explain the movement more than MOND?

[magicalhippo]

Other way around. Newtonian dynamics explains the data very well, MOND did not.

In particular, Newtons law of gravity says the effect of gravity falls off as 1/r^2 where r is the distance from the mass. MOND modifies the standard equations so that gravity starts like 1/r^2 when r is small, and acceleration is large, but for greater distances, when the acceleration is low, instead falls off like 1/r.

MOND explains the movement of the stars in (most) galaxies very well. However this result showed that MOND was not consistent with the motion of the galaxies in the cluster. On the other hand the motion was consistent with plain Newtonian dynamics. Hence Newtons law of gravity (and by extension GR) passed the test.

[rhdunn]

Special Relativity is an extension of Galilean/Newtonian mechanics (motion of projectiles and other objects) to the case where the object is travelling at speeds that are a fraction of the speed of light. It deals with non-accelerating frames of reference. Satelites need to use this to correct for time dilation effects, but tracking the trajectory of an arrow/etc. or a car/etc. travelling from one location to another then classical mechanics is sufficient.

General Relativity is an extension of Newtonian gravity. It is also an extension of Special Relativity to cover accelerating frames of reference. Satelites need to use this, as does tracking the orbit of Mercury. However, for the orbits of other planets and the moon, using Newtonian gravity is sufficient for a reasonable degree of accuracy, and is used for tracking things like equinoxes/solstices, full moons, etc..

[GuB-42]

At these scales (entire galaxies, very weak forces), it doesn't make a significant difference.

There are ways of adapting MOND to match general relativity, should it turn to be correct at explaining what it is supposed to explain (like the movement of galaxies).

[NewEntryHN]

I think OP's question is more how could Newton's law "pass" a test any more than General Relativity would, considering that it's merely an edge case of GR?

[GuB-42]

Oh yes, I think the title is a bit misleading.

It is an argument against MOND, a theory that says that gravity has to be modified to account for some observations. But for these particular observations, general relatively and Newton's laws gives the same results in practice, the difference is negligible, so showing that these observations can be explained by Newton's laws implicitly mean that they can also be explained by general relativity. No need for modifications.

[DonaldFisk]

General Relativity. It explained the anomaly in the precession of Mercury's perihelion, and the bending of starlight by the Sun (double the value predicted by Newton's law).

The test here is for the inverse square law of gravity. The rival theory in this case isn't GR, but MOND: https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics