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Comment by Pxtl | Hacker News Reader

I'm surprised to read an article with so much assumed knowledge about dark matter - about how it heats, etc. Is there a good place to read about the current best-guesses of the properties of dark matter?

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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 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

[2] https://en.wikipedia.org/wiki/Stellar_population#Population_...

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?

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.

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.

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?

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.

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.

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.

>Yes, no friction for the DM particles

Chandrasekhar dynamical friction?

https://en.wikipedia.org/wiki/Dynamical_friction

> 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?

But it's not on net being pushed outwards. It's in equilibrium. The outward push is on average exactly canceled out by the baryonic matter falling inwards due to friction. If it weren't, then everything would get either denser/sparser until equilibrium were regained. The point is that the forces are working in such a way as to maintain a stable equilibrium, like in normal stars.

The DM particles would indeed be on random/chaotic orbits, unlike visible matter which can shed kinetic energy via EM interaction (collisions). But DM density near a mass concentration would still be higher than far away from anything massive.

Normal stars are in hydrostatic equilibrium, a density where the inward force exerted by gravity and the outward force exerted by the pressure of the hot plasma are balanced. In a dark star the situation would be similar, except the heat would be generated by DM annihilation rather than fusion (the heat from annihilation would keep the star too "puffy" to reach the core pressure and temperature required for fusion.

> Similarly, wouldn’t the particles in the star be on various elliptical trajectories and not collapse?

That's a common misunderstanding. Orbits around many bodies do not work that way, and the particles exchanging momentum so they collide or escape the cloud is normal.

This is the Proceedings of the National Academies of Science. It's getting shared here, but the target audience is other physicists, who would have this background knowledge.

Basically, there are 2 hypothesis: WIMPs and MACHOs.

WIMPs stands for Weakly Interacting Massive Particles, and proposes that DM consist of yet unknown massive particles that don’t interact much with regular matter. The problem with that hypothesis is that no such particles are predicted by Standard Model and decades of searching for any traces of those particles didn’t yield anything.

MACHOs stands for Massive Compact Halo Objects and assumes that DM in galactic halos consists of some known dark objects, most likely Black Holes. The question is: where those black holes come from. There is a model of cyclical universe by Gorkavyi, Mathers et al, where those are primordial black holes left from the previous cycles of the universe. It also explains galaxy formation in early universe (observed by JWST) and predicted gravitational wave background recently discovered by NANOGrav.

Isn't the concept of dark matter just an assumption in itself? Seems to follow that any knowledge about it would be an assumption. In fact, that seems like good science, on instead of the word assumption, one might use theory. Once the theory is built up to test, it might actually one day become fact (or proven wrong).

The word they should be using is hypothesis, not theory. A theory stands up to scrutiny.

there you go, an even better word. good thing i don't call myself a scientist

That's not how the term "theory" is used in science. Only hobbyists care about the distinction, actual scientists infer the implied certainty from context. There is Born's rule, Noether's theorem, Newton's law (which we know is wrong), Einstein's theory of GR, the standard model (which is the best thing we have), ... sometimes we use the word "a quantum theory" to mean a certain Lagrangian, even one which we know does not describe reality at all.

> Noether's theorem

is not like the other entries on your list. It's a full-blown mathematically rigorous theorem (that incidentally also happens to be of central importance in physics), not a physical model.

This paper predicts that dark stars would have a distinct spectrum that would distinguish them from ordinary galaxies.

> Isn't the concept of dark matter just an assumption in itself?

Dark matter is not an assumption. Dark matter is not a hypothesis. Dark matter is not a theory.

Dark matter is a series of observations of the universe. Galaxies spin are observed to spin differently than our models and estimates of their mass say they should. Velocities of galaxies in galaxy clusters are much faster than the sum total of the gravitational effects of the cluster can account for. The bullet cluster lenses gravity in a distribution that is not in accordance with the matter that we see. The CMB (cosmic microwave background) is lumpy, too lumpy for our models. This is dark matter; dark matter is a series of observations where the stuff we see does not line up with what our models predict. Dark matter is not an assumption. Dark matter is opening our eyes and looking at the sky.

Now, we can have different theories of dark matter. Hypotheses or theories that attempt to explain the observations. Currently the leading theory is WIMPS, but MACHO and MOND were in the running for a while.

By way of analogy, we have known that light was a thing for thousands of years. Light is not an assumption. Light is not a hypothesis. Light is not a theory. Light is the observation that we are able to see. Light is the observation that we see better when the Sun is up than when a full Moon is up, and sometimes barely at all if there's a new moon or if we're in a cave. Light is the observation that the Sun is brighter than the Moon. Light is the observation that we can make a fire, perhaps a campfire, or a candle, or a torch, that can enable us to see in the dark. Light is the observation that if we put the fire out, we can't see anymore. There were several theories that tried to explain what light is; the Greek theory about our eyes sending out feelers, or waves in the luminiferous aether, or a stream of billiard ball-like particles, or waves in the electromagnetic field. We can have a meaningful discussion about which of these theories is the best one, but we can't have a meaningful discussion about whether the phenomena known as "light" is an assumption: We can see. Therefore light, whatever it happens to be, does exist.

Dark matter is no different.

To translate more what he means. I think the person you replied to is asking:

"Is what we see the result of a form of invisible matter?"

Invisible matter aka dark matter is the "theory" he is referring to.

>> I'm surprised to read an article with so much assumed knowledge about dark matter - about how it heats, etc.

Exactly! I'm astounded at the amount of - quite literally - made up unsubstantiated assumptions about DM. My favorite is to "explain" a galaxy rotation curve by assuming a spherical distribution of DM around the galaxy, but never explaining why or how it would take on such a distribution. Just don't ask questions...

The paper is trying to explain some unexpected observations by the JWST. It starts with the idea that maybe these observations are dark stars, works backwards to show that there are a couple of models of dark matter where this would work, and then predicts that, if these really are dark stars, they'll have a measurably different spectrum than regular stars. Measuring the spectrum would then substantiate (or not) a model of dark matter. This sort of thing is how they might nail down a more likely model of dark matter.

This is not unlike trying to explain those observations with ordinary matter, working backwards to try and work out what distribution of ordinary matter could produce the observations and what sort of physics would create such a distribution.

> Just don't ask questions...

The insinuation packed in this statement is beyond ridiculous. As if there was a giant conspiracy by Big Cosmology. What makes you think asking questions isn't welcome? Go visit your local college, they probably have a weekly seminar open for everybody. Observe how they interact. Scientists and students ask each other questions (and I mean hard questions) all the time. Pointing out failures in each other theories is the scientists' favorite past time. It's literally how science works.

The structure formation of dark matter is extensively studied and simulations are in good agreement with observations (not perfect though, look up the dwarf galaxy problem).

> I'm astounded at the amount of - quite literally - made up unsubstantiated assumptions about DM.

This is done all the time in cosmology: let's assume X is true just for the heck of it, what does that mean for Y? Could we observe it? Would it perhaps explain several observations at once?

Even toy worlds or toy models are explored all the time, by which I mean worlds or models that we know do not describe our world. Valuable insights can still be gained.

And why wouldn't they make some assumptions? Would you prefer it if certain ideas are forbidden to be explored?

>> As if there was a giant conspiracy by Big Cosmology. What makes you think asking questions isn't welcome?

Because they're not? Proposing a specific distribution of fairy dust immediately begs two questions. "What is it?" - ok I'll let that slide, but "why that distribution?" is critical. It is claimed to influence regular mater via gravity, so why should it take on a different distribution? It "solves" one problem but creates many more. Hey if there's a math model to explain one phenomenon, why does it differ from the existing stuff under the same influence? If peer review doesn't force them to address such questions, there is no hope for me to do so.

There is no shortage of explantions aimed at a smart highschool or undergraduate student which will answer most of those questions. Obviously not "what is it?", that is one of the biggest open questions in cosmology.

The answer is you have to understand the math to understand the theory. Popular accounts are always an approximation, but if the theory were easily refuted by lay people, scientists would not be basing entire research programs on it.

The inevitable process of science involves raising questions that, at first, do not have answers. Publications are not exam papers where peer reviewers have the answer key. The process of science is also a communal activity, where one scientist raises a question in one forum, and the answer comes from a different scientist years later. Peer review should not and does not (per your own admission) suppress the raising of unanswered questions. And this contradicts your earlier claim that asking questions isn't welcome.

You're are implicitly assuming something that cosmologists do not assume: that DM even exists. It is not assumed to exist. It is proposed to exist as an explanation for observations we have made. As such, the proposing scientists are free to set the terms of their own proposal.

The whole reason DM was proposed is because, if true, it explains gravitational phenomena of galaxies that we can observe but can't otherwise explain under the current best theory of gravity. (Some alternative proposals include very different theories of gravity, rather than DM. [0])

The weakly-interacting property of the proposed DM (again, this property is not an assumption; it's part of the proposal) is what leads to the spherical distribution. It is a (very well explained) mathematical consequence of the lack of interaction that the DM retains a spherical distribution while the ordinary matter collapses to a plane.

[0] https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics

But if the dark matter gravitationally affects the visible matter in the galaxy, then the visible matter in the galaxy should gravitationally affect the dark matter. So even if there was reason for the dark matter to initially assume that shape, what's keeping it in that shape against the pull of the visible matter?

> what's keeping it in that shape against the pull of the visible matter?

Dark matter halos do flatten out, just much more slowly than baryonic matter, since gravitational interactions radiate off energy much more slowly than electromagnetic ones.

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