Dark matter gravitational force

In summary, there is no indication that gravitational force between dark matter and visible matter is weaker than between dark matter and each other.
  • #1
Ranku
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Is there any astronomical indication that gravitational force between dark matter might be weaker than between visible matter?
 
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  • #2
Ranku said:
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.
 
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  • #3
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?
 
  • #4
MikeeMiracle said:
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.
 
  • #5
Ranku said:
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.
 
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  • #6
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.
 
  • #7
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.
 
  • #8
MikeeMiracle said:
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.

Vanadium 50 said:
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.
 
  • #9
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.
 
  • #10
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
MikeeMiracle said:
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.
 
  • #11
Janus said:
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.
This assumes collisionless particles. In reality, normal matter is not collisionless but heats up and radiates away energy. This is why normal matter coalesces into stars, planets, etc, while dark matter forms more loosely bound halos.
 
  • #12
Thanks for clarifying :)
 

1. What is dark matter gravitational force?

Dark matter gravitational force is a hypothetical force that is believed to be responsible for the gravitational pull of dark matter, a type of matter that does not emit or absorb light and is therefore invisible to traditional telescopes. It is thought to make up about 85% of the total matter in the universe.

2. How is dark matter gravitational force different from regular gravity?

Dark matter gravitational force is different from regular gravity in that it is believed to be much stronger and able to act over much larger distances. It is also thought to be the dominant force in the universe, while regular gravity is just one of the four fundamental forces.

3. What evidence do we have for the existence of dark matter gravitational force?

The main evidence for dark matter gravitational force comes from observations of the rotation of galaxies. The observed rotational speeds of stars in galaxies are much higher than what would be expected based on the visible matter in the galaxy. This suggests the presence of an invisible force, like dark matter gravitational force, that is holding the galaxy together.

4. How does dark matter gravitational force affect the expansion of the universe?

Dark matter gravitational force is thought to play a major role in the expansion of the universe. It is believed to counteract the effects of dark energy, another mysterious force that is causing the universe to expand at an accelerating rate. Without dark matter gravitational force, the universe would not be able to hold itself together and would expand at a much faster rate.

5. Can we harness dark matter gravitational force for practical use?

At this time, there is no known way to harness dark matter gravitational force for practical use. It is still a largely unknown and mysterious force, and scientists are still working to understand its properties and behavior. However, further research and understanding of dark matter gravitational force could potentially lead to new advancements in our understanding of the universe and perhaps even new technologies in the future.

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