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So-called cold dark matter [1] is the currently favored hypothesis. The Dark Star hypothesis additionally assumes that the CDM particles interact with each othe

So-called cold dark matter [1] is the currently favored hypothesis. The Dark Star hypothesis additionally assumes that the CDM particles interact with each other even though they do not interact with baryonic ("normal") matter (other than via gravity). If they do, they'd be their own antiparticles and would annihilate on collisions. This process would produce the energy, in the form of ordinary photons, that powers the Dark Stars. Like all proper scientific hypotheses, the DS hypothesis makes testable predictions, and now it appears that some relevant JWST observations support, or at least do not conflict with, the DS hypothesis. At the same time, the more mainstream model of protogalaxies and Population III stars [2] has some difficulties explaining the same observations. Of course, this is only very slight evidence in favor of the DS model, but that's science for you. Small steps. [1] https://en.wikipedia.org/wiki/Cold_dark_matter https://en.wikipedia.org/wiki/Cold_dark_matter [2] https://en.wikipedia.org/wiki/Stellar_population#Population_III_stars https://en.wikipedia.org/wiki/Stellar_population#Population_...

Pxtl 3y ago
So wait, the hypothesis is that the reason Dark Stars are so diffuse that they look like galaxies is that maybe dark matter self-annihilates, so it can't coalesce into true stars? But what keeps it from just all annihilating quickly? Can't ever reach the critical mass of gravity needed for rapid annihilation because it's destroyed as fast as it accumulates? The annihilation provides repulsive force keeping them from collapsing too fast? Or is there no repulsive force?
- gjhan 3y ago
  In the context of this paper, the DSs are significantly smaller than galaxies, but JWST doesn't have enough resolution to distinguish a galaxy from the much smaller DS. In their DS model, what seems to limit the rate at which the DM annihilates is that any interaction between DS particles has a low probability of happening (a cross section). We can imagine this as particles just whizzing around each other, gravitationally bound (i.e. confined to a nearby volume) but so small that they have a low probability of actually interacting.
- Sharlin 3y ago
  Not that diffuse; the objects in question are so far away that they're not resolved by JWST. They look like point sources, no matter whether galaxy-sized or vastly smaller (the hypothetical DS objects would "only" be solar-system sized). DM self-annihilation would be rare enough (both due to low DM density and tiny reaction cross-section) that it wouldn't burn out very quickly. The heat (and thus pressure) generated by the annihilation would indeed keep the DSs from collapsing into "real" stars.
  - djkorchi 3y ago
    Is there actually a balance needed between pressure and collapse? Radiation pressure presumably doesn’t do anything to the constituent dm particles. Similarly, wouldn’t the particles in the star be on various elliptical trajectories and not collapse?
    - CydeWeys 3y ago
      You're forgetting about friction. A diffuse cloud is eventually going to become tighter and tighter as gravity draws it together and the individual orbiting particles within that cloud lose momentum from hitting each other.
      - Pxtl 3y ago
        Well, they say the DS would also have a decent amount of baryonic matter in it - some diffuse hydrogen and helium. If the pressure from the annihilations pushes the hydrogen and helium outwards, that could create some outward gravity to pull on the dark matter, right? Wait, except the outward gravitational pull inside of a hollow shell is zero because it all cancels. Would there even be friction though? It sounds like the only interactions these hypothetical particles have is gravity and annihilation.
        Sharlin 3y ago
        Yes, no friction for the DM particles. WIMP DM cannot collapse by shedding kinetic energy as heat like visible matter can. Thus the dark matter halos around galaxies. But gravity would still cause there to be a higher average density of DM inside and near the dark star (or indeed a modern-day galaxy) than far away from any massive objects.

Pxtl 3y ago

So wait, the hypothesis is that the reason Dark Stars are so diffuse that they look like galaxies is that maybe dark matter self-annihilates, so it can't coalesce into true stars? But what keeps it from just all annihilating quickly? Can't ever reach the critical mass of gravity needed for rapid annihilation because it's destroyed as fast as it accumulates? The annihilation provides repulsive force keeping them from collapsing too fast? Or is there no repulsive force?

gjhan 3y ago
In the context of this paper, the DSs are significantly smaller than galaxies, but JWST doesn't have enough resolution to distinguish a galaxy from the much smaller DS. In their DS model, what seems to limit the rate at which the DM annihilates is that any interaction between DS particles has a low probability of happening (a cross section). We can imagine this as particles just whizzing around each other, gravitationally bound (i.e. confined to a nearby volume) but so small that they have a low probability of actually interacting.
Sharlin 3y ago
Not that diffuse; the objects in question are so far away that they're not resolved by JWST. They look like point sources, no matter whether galaxy-sized or vastly smaller (the hypothetical DS objects would "only" be solar-system sized). DM self-annihilation would be rare enough (both due to low DM density and tiny reaction cross-section) that it wouldn't burn out very quickly. The heat (and thus pressure) generated by the annihilation would indeed keep the DSs from collapsing into "real" stars.
- djkorchi 3y ago
  Is there actually a balance needed between pressure and collapse? Radiation pressure presumably doesn’t do anything to the constituent dm particles. Similarly, wouldn’t the particles in the star be on various elliptical trajectories and not collapse?
  - CydeWeys 3y ago
    You're forgetting about friction. A diffuse cloud is eventually going to become tighter and tighter as gravity draws it together and the individual orbiting particles within that cloud lose momentum from hitting each other.
    - Pxtl 3y ago
      Well, they say the DS would also have a decent amount of baryonic matter in it - some diffuse hydrogen and helium. If the pressure from the annihilations pushes the hydrogen and helium outwards, that could create some outward gravity to pull on the dark matter, right? Wait, except the outward gravitational pull inside of a hollow shell is zero because it all cancels. Would there even be friction though? It sounds like the only interactions these hypothetical particles have is gravity and annihilation.
      - Sharlin 3y ago
        Yes, no friction for the DM particles. WIMP DM cannot collapse by shedding kinetic energy as heat like visible matter can. Thus the dark matter halos around galaxies. But gravity would still cause there to be a higher average density of DM inside and near the dark star (or indeed a modern-day galaxy) than far away from any massive objects.