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Comment by twilightfog | Hacker News Reader

Another promising theory is that dark matter isn't even necessary, we only assume its present due to its gravitation effect, and that can be explained by other phenomenon.

Gravity decreases in proportion to distance squared between objects, but its not observed beyond a certain distance. To compensate, that space was assumed to be filled with dark matter. This theory suggests that gravity switches from a inverse-square law to a different set of rules after a certain distance.

https://www.quantamagazine.org/20161129-verlinde-gravity-dar...

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> Gravity decreases in proportion to distance squared between objects, but its not observed beyond a certain distance.

This isn't true. F = GMmr^-2 has infinite range. This is true from a theoretical standpoint, and it's also borne out in observations. The Andromeda Galaxy, presently 2.5 million light-years away, is being pulled toward our galaxy and will eventually collide with it. Observations on an even larger scale support the idea that the equation has infinite range.

The above theoretical construct could in principle be contradicted by observations, and if it were, someone would win a Nobel Prize, so there's every incentive to find persuasive evidence that contradicts it.

The Andromeda Galaxy, presently 2.5 million light-years away, is being pulled toward our galaxy and will eventually collide with it.

To support an inverse square law, we'd need to know the mass of our galaxy, and the mass of the Andromeda galaxy. We have the speed, computed from various blueshift methods. We'd also need to know the rate at which the speed changes.

Do we know any of those three? The mass of the Andromeda galaxy is very different if you assume its filled with dark matter, vs. only has visible matter. Which mass is used to confirm the inverse square law? If it includes dark matter, you have circular reasoning: dark matter is hypothesized to make the inverse square law work within the Andromeda galaxy. If it doesn't include dark matter, then why does the dark matter in the Andromeda galaxy contribute to its rotational motion, but not the attraction to the Milky Way?

My reply was only in response to this comment --

> Gravity decreases in proportion to distance squared between objects, but its not observed beyond a certain distance.

-- not to the question of what detailed properties gravitation possesses.

Ah. I took the comment to say the inverse square isn't observed beyond a certain distance, not that gravity wasn't observed at all beyond a certain distance.

Verlinde's work is exciting, but it's just a hypothesis for now.

Just be careful to not confuse it with the old MOND theories, which don't work at all.

Verlinde's work is exciting, but it's just a hypothesis for now.

So is dark matter. :) I wonder which has more evidence supporting it, and which has more evidence against it?

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