[talanum1 (OP)]

Do we need to model Dark Matter as gravitationally repulsive on small distances (O(1 light year))?

Or does it occur in the solar system?

[Kryptid]

I don't see why we would.

[Bored chemist]

Do we need to model Dark Matter as gravitationally repulsive on small distances (O(1 light year))?

Or does it occur in the solar system?

That's not an "either / or" thing.

[Origin]

Do we need to model Dark Matter as gravitationally repulsive on small distances (O(1 light year))?

No.

Or does it occur in the solar system?

Dark matter is theorized to be throughout the galaxy so there would of course be dark matter in the solar system.

[talanum1 (OP)]

If there is dark matter on Earth, we won't see it, but would we feel it?

[Kryptid]

If there is dark matter on Earth, we won't see it, but would we feel it?

Nope, for the same reason we don't feel neutrinos.

[evan_au]

If there is dark matter on Earth...

I wouldn't say "on Earth", because that implies that Dark Matter is stopped by the rocks of the Earth's surface.

The hypothetical Dark Matter particles pass straight through the Earth, just like the very real Neutrinos pass straight through the Earth (and straight out of the Sun, and straight out of a supernova).

If there is dark matter on Earth, we won't see it, but would we feel it?

The hypothetical Dark Matter particles are "felt" via their gravity.

However, there is none on the Earth [and if there were, they would just add to the Earth's gravity].
Those that are in the vicinity of Earth pass through and keep on going, so their gravitational field is very small.
- Their density is also small in the whole Solar System
- Their density is even small between the Sun and Proxima Centauri (the nearest star).

It is only when you look on the scale of a whole galaxy (or a cluster of galaxies) that you see the gravitational effects of Dark Matter.

So we humans will not personally "feel" Dark Matter - but our instruments can detect its effects by looking at galaxy rotation curves, for example.
https://en.wikipedia.org/wiki/Galaxy_rotation_curve

[talanum1 (OP)]

Dark Matter must be "cold". Zipping through the Earth suggests the particles are moving fast therefore "hot".

The only reason for us not feeling it is if it is like air. Then there needs to be an explanation for why it does not clump like rocks.

[Bored chemist]

Dark Matter must be "cold".

Why?

[evan_au]

Dark Matter must be "cold". Zipping through the Earth suggests the particles are moving fast therefore "hot".

It is true that the Dark Matter model that best fits the observed universe is the "Cold Dark Matter" hypothesis (at least, in 2022).
https://en.wikipedia.org/wiki/Cold_dark_matter

Cold refers to the fact that the dark matter moves slowly compared to the speed of light

One of the Dark Matter candidates was neutrinos, which are emitted at close to the speed of light.
- This means that they exceed the escape velocity of galaxies
- So they are not gravitationally bound to galaxies
- So they won't form a Dark Matter "Halo" around galaxies
- And won't affect the rotation curves of galaxies in the observed manner
- And won't distort the visible shape of distant galaxies as they pass nearer galaxies
- There are other reasons that relic neutrinos from the Big Bang were excluded...

So we can refine "Cold" to mean that most Dark Matter particles have a velocity less than the escape velocity of their host galaxy.
- The escape velocity of Earth (from the surface) is 11 km/second = "Not Hot"
- The escape velocity of the Sun (from the Earth's Orbit) is 42 km/second = "Not Hot"
- The escape velocity of the Galaxy (from the Sun's orbit) is 530 km/sec = "Not Hot" = Most likely order of magnitude
- The speed of light is 300,000 km/sec = "Fast" = "Hot"

[Origin]

The only reason for us not feeling it is if it is like air.

We can feel air.  Air can knock you off of your feet.

[Kryptid]

Then there needs to be an explanation for why it does not clump like rocks.

It's down to a lack of ways to lose energy. Since it doesn't interact with electromagnetism, it can't produce electromagnetic radiation to cool off the way that hot, normal matter does.

[talanum1 (OP)]

Can't it cool down by emitting gravitational waves? It is bound by gravity.

[talanum1 (OP)]

Dark Matter must be "cold".
Why?

I think it applies better to our observations. They gravitationally lenses background objects.

[Kryptid]

Can't it cool down by emitting gravitational waves? It is bound by gravity.

It's a much slower process because of how weak gravity is compared to the other forces.

[evan_au]

Quote from:

Then there needs to be an explanation for why it does not clump like rocks.

It's down to a lack of ways to lose energy. Since it doesn't interact with electromagnetism, it can't produce electromagnetic radiation to cool off the way that hot, normal matter does.

Since the hypothetical Dark Matter Particle is beyond the Standard Model of particle physics, some cosmologists have wondered if there might be another Force, beyond the Standard Model?
- This new force would allow Dark Matter Particles to interact with each other
- So Dark Matter Particles might be able to clump into equivalents of "Dark Atoms", "Dark Molecules", "Dark Dust", Dark Sand, Dark Rocks, Dark Planets or even Dark Stars!
- But this force would not interact with "normal" matter, just like Dark Matter does not interact with normal matter. A Dark Rock would still pass right through the Earth, just like a Neutrino.
- Cosmologists have taken this serious enough to modify their computer simulations to allow for clumping of Dark Matter. As I recall, the clumpiness must be kept at very small scales or it impacts the structure of the universe in visible ways.
https://en.wikipedia.org/wiki/Dark_matter#Composition

PS: Some have called this hypothetical force "Dark Light"; "the Dark Force" sounds too much like Star Wars...

[Origin]

So Dark Matter Particles might be able to clump into equivalents of "Dark Atoms", "Dark Molecules", "Dark Dust", Dark Sand, Dark Rocks, Dark Planets or even Dark Stars!

I wouldn't think that is viable since dark matter that clumps would develop a disk just like normal matter and that is not is what is observed.
Edit for clarity.

[talanum1 (OP)]

A Dark Rock would still pass right through the Earth, just like a Neutrino.

If it clumps we would feel it due to it's influence on the mass of our atoms.

[Origin]

If it clumps we would feel it due to it's influence of the mass of our atoms

No.  Normal matter interacts with us electrically and dark matter does not interact electrically.

[Bored chemist]

If you had a really big lump of dark mater moving implausibly slowly then you could hypothetically measure the gravitational effect from it.
If it had passed through this experiment it could, hypothetically have been observed.
https://web.archive.org/web/20110605043202/http://ethesis.helsinki.fi/julkaisut/mat/fysik/vk/virtanen/studieso.pdf

But, as we understand it, big clumps are unlikely and so is slow movement.