What does it mean to model the distribution of galaxies as cosmic dust?

[Jerome Wang]

I read a paper published in General Relativity and Gravitation:

On the local geometry of rotating matter

Some of the content in Section 5 raised my doubts, and the content is as follows:

In cosmology it is customary to model the distribution of galaxies as a dust where each galaxy is a small object, relative to the scales of interest in cosmology. If neighboring galaxies and gas clouds have orbital angular momentum which are correlated with each other, then the resulting cosmic dust will appear to have intrinsic angular momentum, when modeled on a sufficiently large scale.

and

The intrinsic angular momentum density and torsion of the macroscopic model are average moments of finer pseudo-Riemannian structures (like rotating galaxies) which have no intrinsic angular momentum and no torsion.

There are two aspects to my doubts, one is about the structure and the other is about the rotation curve:

On galaxy structure

In astronomy, C.C. Lin and Frank Shu proposed the density wave theory to explain the spiral arm structure of spiral galaxies.

If according to the paper:

The intrinsic angular momentum density and torsion of the macroscopic model are average moments of finer pseudo-Riemannian structures (like rotating galaxies) which have no intrinsic angular momentum and no torsion.

, then modeling the distribution of galaxies as cosmic dust seems to be combining the concepts of mean-field and quasiparticle with Lin–Shu density wave theory and effectively reformulate it in terms of Einstein–Cartan theory.

About galaxy rotation curve

It is well known that the galaxy rotation problem is an unsolved problem in current astrophysics, while the proton spin crisis is an unsolved problem in current particle physics.

According to the paper:

If neighboring galaxies and gas clouds have orbital angular momentum which are correlated with each other, then the resulting cosmic dust will appear to have intrinsic angular momentum, when modeled on a sufficiently large scale.

, then modeling the distribution of galaxies as cosmic dust also seems to transform the rotation problem into a spin crisis.

Including the above doubts, I would like to ask:

What does it mean to model the distribution of galaxies as cosmic dust?

 

[Quantum Call]

1. Cosmic Dust in Cosmology:​

• In cosmology, "cosmic dust" is not literal dust but rather a conceptual tool used to describe the matter content in the universe on large scales. It refers to the continuous, diffuse matter that fills space, similar to how dust would spread throughout an area.
• This cosmic dust is modeled as a continuous, fluid-like substance that represents the average density of matter (galaxies, gas, dark matter, etc.) spread across vast scales.
• Galaxies and other structures (such as gas clouds) are treated as small perturbations or localized regions within this "dust." These perturbations, when looked at on a large enough scale, give rise to the collective behavior of matter in the universe.

2. Intrinsic Angular Momentum in Galaxies:​

• Galaxies themselves rotate and have their own angular momentum due to the movement of stars, gas, and other matter. However, when we look at the large-scale structure of the universe (the distribution of galaxies on cosmological scales), the focus shifts to the average properties rather than the detailed specifics of individual galaxies.
• The intrinsic angular momentum of cosmic dust comes from the collective motion of galaxies. The idea is that neighboring galaxies and gas clouds have orbital angular momentum (spin) that is correlated on large scales. This correlation of motion can result in an apparent, macroscopic angular momentum of the "cosmic dust."
• The torsion mentioned refers to a geometric property in general relativity related to the curvature of spacetime in the presence of spinning matter. In this cosmic dust framework, the torsion and angular momentum are seen as emergent properties from averaging over smaller, finer-scale structures (like galaxies).

3. Galactic Rotation Curves and Spin Crisis:​

• The rotation curve problem refers to the observation that galaxies do not rotate as predicted by the visible mass alone. According to Newtonian mechanics, or even more refined models using general relativity, stars in the outer parts of galaxies should be moving slower than what is observed. This discrepancy is often attributed to dark matter or modifications to gravity (such as MOND, Modified Newtonian Dynamics).
• By modeling the universe as cosmic dust, the rotation curve problem can be interpreted in a different way. The large-scale average motion of galaxies might be influenced by the collective angular momentum of the dust, leading to this apparent rotation.
• The spin crisis in particle physics refers to the unresolved issue of understanding how proton spins emerge. Similarly, the apparent large-scale rotational effects of galaxies could point to underlying collective effects, such as spin correlations in cosmic dust.

4. Link to Einstein–Cartan Theory:​

• The reference to Einstein–Cartan theory suggests that this approach goes beyond traditional general relativity by incorporating torsion into spacetime, which is linked to spinning objects.
• In this framework, galaxies and other structures contribute to the pseudo-Riemannian geometry of spacetime through their spin, and the average moments of these finer structures (galaxies) shape the macroscopic properties of the universe.
• This theory may help explain phenomena like galaxy rotation curves without needing dark matter, by considering the emergent effects of spinning matter on spacetime geometry.

Conclusion:​

To model the distribution of galaxies as cosmic dust means to treat galaxies as localized, small perturbations within a continuous medium, much like how dust would spread throughout space. On large scales, the matter appears as a fluid-like "dust" with properties such as angular momentum and torsion that emerge from the collective behavior of galaxies. This approach aligns with ideas from Einstein–Cartan theory, where the macroscopic geometry of the universe reflects the average properties of finer structures.

 

[Jerome Wang]

1. Cosmic Dust in Cosmology:​

• In cosmology, "cosmic dust" is not literal dust but rather a conceptual tool used to describe the matter content in the universe on large scales. It refers to the continuous, diffuse matter that fills space, similar to how dust would spread throughout an area.
• This cosmic dust is modeled as a continuous, fluid-like substance that represents the average density of matter (galaxies, gas, dark matter, etc.) spread across vast scales.
• Galaxies and other structures (such as gas clouds) are treated as small perturbations or localized regions within this "dust." These perturbations, when looked at on a large enough scale, give rise to the collective behavior of matter in the universe.

2. Intrinsic Angular Momentum in Galaxies:​

• Galaxies themselves rotate and have their own angular momentum due to the movement of stars, gas, and other matter. However, when we look at the large-scale structure of the universe (the distribution of galaxies on cosmological scales), the focus shifts to the average properties rather than the detailed specifics of individual galaxies.
• The intrinsic angular momentum of cosmic dust comes from the collective motion of galaxies. The idea is that neighboring galaxies and gas clouds have orbital angular momentum (spin) that is correlated on large scales. This correlation of motion can result in an apparent, macroscopic angular momentum of the "cosmic dust."
• The torsion mentioned refers to a geometric property in general relativity related to the curvature of spacetime in the presence of spinning matter. In this cosmic dust framework, the torsion and angular momentum are seen as emergent properties from averaging over smaller, finer-scale structures (like galaxies).

3. Galactic Rotation Curves and Spin Crisis:​

• The rotation curve problem refers to the observation that galaxies do not rotate as predicted by the visible mass alone. According to Newtonian mechanics, or even more refined models using general relativity, stars in the outer parts of galaxies should be moving slower than what is observed. This discrepancy is often attributed to dark matter or modifications to gravity (such as MOND, Modified Newtonian Dynamics).
• By modeling the universe as cosmic dust, the rotation curve problem can be interpreted in a different way. The large-scale average motion of galaxies might be influenced by the collective angular momentum of the dust, leading to this apparent rotation.
• The spin crisis in particle physics refers to the unresolved issue of understanding how proton spins emerge. Similarly, the apparent large-scale rotational effects of galaxies could point to underlying collective effects, such as spin correlations in cosmic dust.

4. Link to Einstein–Cartan Theory:​

• The reference to Einstein–Cartan theory suggests that this approach goes beyond traditional general relativity by incorporating torsion into spacetime, which is linked to spinning objects.
• In this framework, galaxies and other structures contribute to the pseudo-Riemannian geometry of spacetime through their spin, and the average moments of these finer structures (galaxies) shape the macroscopic properties of the universe.
• This theory may help explain phenomena like galaxy rotation curves without needing dark matter, by considering the emergent effects of spinning matter on spacetime geometry.

Conclusion:​

To model the distribution of galaxies as cosmic dust means to treat galaxies as localized, small perturbations within a continuous medium, much like how dust would spread throughout space. On large scales, the matter appears as a fluid-like "dust" with properties such as angular momentum and torsion that emerge from the collective behavior of galaxies. This approach aligns with ideas from Einstein–Cartan theory, where the macroscopic geometry of the universe reflects the average properties of finer structures.

You do know that fluid-like "dust" is made up of dust particles that are highly idealized models of galaxies, right?
But your response mention nothing about what happens to the geometry inside of the dust particles, that is the difference between Einstein–Cartan theory and general relativity.
There is also no mention of how the galaxies rotation curve problem transforms when galaxies are modeled as dust particles.