# Dark matter gravitational force

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## Main Question or Discussion Point

Is there any astronomical indication that gravitational force between dark matter might be weaker than between visible matter?

## Answers and Replies

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jbriggs444
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Is there any astronomical indication that gravitational force between dark matter might be weaker than between visible matter?
Since dark matter is characterized entirely by its gravitational effects, those effects are, by definition, the same as for regular matter.

Unless the question was between dark matter and itself and between regular matter and itself. Am I correct in stating that regular matter will attrack itself stronger than Dark Matter will attract itself or is it the same do we believe?

Orodruin
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Unless the question was between dark matter and itself and between regular matter and itself. Am I correct in stating that regular matter will attrack itself stronger than Dark Matter will attract itself or is it the same do we believe?
The question does not make sense within GR. It follows from the equivalence principle that gravity must work the same way regardless of the source.

Staff Emeritus
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Is there any astronomical indication that gravitational force between dark matter might be weaker than between visible matter?
How could you even tell? All we know gravitationally is the product GM. If G is weaker (and I don't even know what that would mean) the inferred M is bigger. Or vice versa.

If bigger / heavier particles attrack each other stronger than smaller / lighter particles, and DM is a heavy particle then it should collapse / attrack itself faster than baryonic matter. That's where I was going with the question, can we tell the likely size of DM particles by their rate of collapse / attraction? Although thinking about it, we don't really have a good enough picture of them to be able to answer the question I don't think.

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Again, how could you even tell?

Write down an equation, any equation, that has G-for-dark-matter on the left side, and anything you like on the right.

anorlunda
Mentor
If bigger / heavier particles attrack each other stronger than smaller / lighter particles
Carefully re-read and digest the import of the following answer you were given earlier. That answer says it all.

How could you even tell? All we know gravitationally is the product GM. If G is weaker ... the inferred M is bigger. Or vice versa.

jbriggs444
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Take the formula for Newtonian gravity: $F=\frac{(Gm_a)m_p}{r^2}$ where $m_a$ is the active gravitational mass and $m_p$ is the passive gravitational mass. The best sense I can make of the proposal is that it is equivalent to making $m_a$ different from $m_p$.

In Newtonian physics, that is a prescription for a reactionless drive and perpetual motion. In General Relativity, it is a violation of the equivalence principle, as has been pointed out already.

Janus
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Just to expound a bit on what jbriggs444 said. The entire dark matter model depends on both the gravitational interaction between dark matter and visible matter and the gravitational interaction dark matter has with itself.

Now a dark matter halo contains more mass than
If bigger / heavier particles attrack each other stronger than smaller / lighter particles, and DM is a heavy particle then it should collapse / attrack itself faster than baryonic matter. That's where I was going with the question, can we tell the likely size of DM particles by their rate of collapse / attraction? Although thinking about it, we don't really have a good enough picture of them to be able to answer the question I don't think.
It's not the size of the individual particles that make up the mass that counts, it is the total mass. If you start with two static clouds of equal mass and size, one made up of smaller particles and the other, larger ones, they would collapse at the same rate. The cloud made of larger particles would have fewer particles spaced further apart than the cloud with the smaller particles in order for the clouds to have the same total mass and distance between particles also plays a role in the strength of gravitational attraction.

The cloud made of larger particles would collapse faster if the particles were spaced as far apart as those in the small particle cloud, but this would mean the cloud would either have to be smaller to have the same mass, or more massive to have the same size. It would have to be denser overall.
With the DM halo, we have an idea of what size and mass it needs to be to exhibit the total gravitational effect it has, and thus its overall density. And it is this density that would determine how it behaves overall, and not how "granular" it is. The only way that you would note a difference is if the "grains" are of a significant size compared to the total size of the cloud. With dark matter halos measured in 100's of thousands of light years in extent, it would take some pretty large "particles" to be noticeable.

Orodruin
Staff Emeritus