So, IIUC, this is an alternative to the paper that was discussed a couple of days ago saying the Universe might be twice as old as previously thought. The "problem observations" both of them try to address are what appear to be huge galaxies at an early time (according to redshift) when there shouldn't have been such large galaxies yet. The aforementioned paper says maybe the Universe is actually much older. This paper says maybe those huge early galaxies aren't actually galaxies but "Dark Stars" instead.
> The first phase of stellar evolution in the history of the universe may be Dark Stars (DS), powered by dark matter (DM) heating rather than by nuclear fusion.
Is "heating" the right way to think about the power from dark matter? Isn't "heating" a function of regular energy and matter?
Heat is an abstract thermodynamic concept that does not depend on the particle species. For instance, the leading class of theory of dark matter is called "cold dark matter", and this literally refers to the temperature of the dark matter.
>The different theories on dark matter (cold, warm, hot) refer not to the temperatures of the matter itself, but the size of the particles themselves with respect to the size of a protogalaxy
That article is wrong. (More generally, phys.org cannot be trusted.) The cold/warm/hot absolutely refers to temperature, specifically whether the typical thermal speed is much less than (cold) or comparable to (hot) the speed of light.
Thermal speed depends on the mass of the particle at a given temperature, correct? Like a volume of hydrogen has a much higher thermal speed than the same volume of radon at the same temperature.
The article from Symmetry Magazine says the same thing as the Phys.org article:
>Light, fast particles are known as hot dark matter; heavy, slow ones are cold dark matter; and warm dark matter falls in between.
The dark matter in the early universe would have been at the same temperature whether of the "warm" or "cold" type, it is just that the speed of the "warm"/non-heavy types would have been too fast to have caused the clumping we see. Or at least that's how I understood it. But that Symmetry article is a good one as well.
If a heavy species and a light species are in thermal equilibrium in the early universe and decouple from everything else (because the interaction rate falls due to expansion) at the same time (i.e., same temperature), the heavy species cools faster with the expansion of the universe than the light one does. This is because the temperature of a relativistic gas cools like ~1/a while the temperature of a non-relativistic gas cools like ~1/a^2, where a is the expansion factor. Thus, in that simplified case, the heavier one really is colder, not just moving slower.
Now, it turns out there's an opposing effect where, depending on how the DM couples to normal matter, the heavier particle decouples earlier (and thus at a hotter temperature) because creation/annihilation have to stop once the temperature drops below the energy scale associated with the rest mass, and this effect can often dominate the previous one.
The thing that actually matters, functionally, (and thus the thing that is used to classify DM types) is the thermal velocity relative to the speed of the Hubble flow during structure formation. It is of course true that, as you say, at fixed temperature a species of low mass will have higher velocity than a species of high mass, but CDM and HDM are not at the same temperature due to the opposing expansion and decoupling effects mentioned above (among other things).
My original point, in response to andsoitis, was that heat is an abstract thermodynamic concept that does not depend on the particle species. The names "hot" and "cold" were not metaphorical, even if the boundaries between CDM and HDM are not literally drawn using a threshold temperature. For instance, per the symmetry article:
> “Even though the universe was very hot at the time, axions would have been very cold at birth and would stay cold forever, which means that they are absolutely cold dark matter.” Even though axions are very light, Graham says, “because they exist at close to absolute zero, the temperature where all motion stops, they are essentially not moving. They’re kind of this ghostly fluid, and everything else moves through it.”
This is why it's wrong for phys.org to say that "cold", "warm", and "hot" do not refer not to the temperatures of the matter itself. Likewise, even if "hard" and "soft" cheeses are formally defined by whether they are dried and aged (such that there could be a few "hard" cheeses that are softer than the hardest "soft" cheeses), it would be wrong to say "'hard' and 'soft' do not refer to the firmness of the cheese itself".
The authors are supposing that dark matter at this stage of the universe has its own particles and antiparticles and they are annihilating each other, the energy released by that annihilation is generating the heat.
As far as models model, most of the regular matter did annihilate. If I'm remembering correctly, the fact that anything we can see and feel still exists is an asymmetry between matter and antimatter somewhere in the arena of 30,001 particles for every 30,000 antiparticles. Obviously, very nearly all of this annihilated, and all of the matter that still exists is the residue of this early asymmetry. I don't exactly keep up to date on this stuff, but this either was or still is one of the bigger open questions regarding the Big Bang. What caused this asymmetry? It was one of the classic examples of apparent fine tuning.
As for why dark antimatter/matter pairs would persist longer rather than run out (I guess that's the opposite of what you're saying, but it's important to remember what we're seeing even looking back this far is way after the normal matter/antimatter annihilated immediately after inflation), it's effectively the same reason dark matter doesn't clump and retains it's roughly spherical form at larger than entire visible galaxy sizes. Regular matter interacts via the electromagnetic force, which has an infinite interaction radius. Charged particles repel and attract each other from large distances. Thus, fundamental particles don't need to get that close to each other to form atoms and molecules. Dark matter only interacts via the weak force, which has a tiny interaction radius. The fundamental particles need to more or less make a direct beeline to the same point in spacetime to ever touch each other, which has an extremely low probability of ever happening. It's the same reason Earth can be bombarded nonstop with unimaginably large numbers of neutrinos every second from the sun, yet virtually all of them go straight through everything. All of space is mostly empty space, even things that look solid to us because the wavelengths we can discriminate are much larger than the spaces between atoms and molecules. It wouldn't look that way to dark matter. It would look actually empty.
> Dark matter only interacts via the weak force, which has a tiny interaction radius.
And gravity, or so I'm told.
But both gravity and the weak force are fields, and so just like EM, they pervade space. Isn't that right? The weak force weakend dramatically with distance, but it doesn't disappear - I thought one property of a field is that it pervades spacetime.
Not that that makes any difference to your argument.
They are both fields, but gravity is different in that spacetime itself is the field.
As for the weak force, I think that's not necessarily a known property of DM, but rather a property of hypothetical candidates for being the dark matter known as WIMPs(weakly interacting massive particles).
Anti-matter is very rare with normal matter, so we don’t see it running out. But if dark matter and dark anti-matter are both just as common then you could see it play out as they suggest.
Including the one used in the article: "If the DM particles are their own antiparticles, then their annihilation provides a heat source that stops the collapse of the clouds and in fact produces a different type of star, a Dark Star, in thermal and hydrostatic equilibrium."
If you prefer, then, neutrinos are neutral but have an anti-particle (although it's still possible that neutrinos are Majorana particles, in which case they're their own anti-particle).
One process could be that the radiation is absorbed within the ball of gas, leaving us to see only what's being radiated by the outer surface of the ball. Likewise the light that we get from the sun is produced by a thin shell near its surface.
That depends on the number density and annihilation cross section. There has been a gamma ray excess from around the galactic core that's been puzzling for a number of years; one explanation was annihilation of dark matter, although other more mundane explanations (like emissions from a population of neutron stars) I think are preferred now.
Baryonic matter is not symmetrical with its antiparticles. I forget the percentage given that it is not my subfield, but it was very high, meaning the existing amount of matter is just a sliver of what was initially "created"/coagulated
But, it's the DM that would provide the heat/glow for the star:
> If the DM particles are their own antiparticles, then their annihilation provides a heat source that stops the collapse of the clouds and in fact produces a different type of star, a Dark Star, in thermal and hydrostatic equilibrium.
> Three key ingredients are required for the formation of DSs:
> 1) sufficient DM density
> 2) DM annihilation products become trapped inside the star
> 3) the DM heating rate beats the cooling rate of the collapsing cloud.
> If the DM particles are their own antiparticles, then their annihilation provides a heat source
How much if this is speculation? Also, do other particles behave like this?
I didn't realise particles could be their own antiparticles, but it transpires that e.g. photons are, because all photons are neutral, not charged somehow.
However, even though a proton is its own antiparticles, two photons do not annihilate, right?
Yes, as you note, photons are their own antiparticles.
The maths doesn't have a preferred time direction, so two photons can annihilate into an electron-positron pair.
I'm not sure if this has actually been observed given how hard it is. That said, my favourite type of supernova is caused by pair creation, though I don't know the proportion of that which comes from 2-photon interactions: https://en.wikipedia.org/wiki/Pair-instability_supernova
There's also Majorna particles, but as I understand it the only known particles that are definitely Majornas are also quasiparticles:
Notably it does bound the energy of gamma rays over long distances (as the higher the energy the more likely it will annihilate with other photons along the way.)
The wikipedia article on the Breit-Wheeler process has some history of the work on experimental observations, although I don't know how accurate or up to date it is https://en.wikipedia.org/wiki/Breit–Wheeler_process
> The maths doesn't have a preferred time direction, so two photons can annihilate into an electron-positron pair.
But only if their energy is high enough. So by that reckoning, photons below 511 keV don't have antiparticles, and those above it do. That's pretty weird. So maybe it's better to say that photons aren't really their own antiparticle, but they might theoretically destroy each other in some rare circumstances.
Nobody is sure, but some people think that neutrinos are they own antiparticle https://en.wikipedia.org/wiki/Neutrino#Majorana_mass I never liked that theory, but some people that know more than me about particle physics liked it.
There were some experiment using atoms that decay ejecting two neutrinos, and hopping that in some case the two neutrinos will annihilate each other. https://en.wikipedia.org/wiki/Neutrinoless_double_beta_decay . IIRC, none of the experiments found the strange annihilation, so perhaps neutrinos are not their own antiparticle :) .
> Isn't "heating" a function of regular energy and matter?
I'm a bit confused at what made you think this. If there is any way to transfer energy from it to something else (which there must be, else it would be impossible to ever interact with it), then it can heat in exactly the same way as anything else.
> If it could heat up, Dark Matter wouldn't be dark.
I don't think this is correct at all. A system doesn't have to interact with EM to be thermodynamic. If you can define a temperature for it and there is the possibility for energy transfer, then it can heat up
I think this is where the debate is. I’m not a physicist but my understanding of the current dark matter models is that it doesn’t interact with itself in a way that could be thought of as “energy transfer” (ie. like particles that collide), but only gravitationally. This would mean there’s no real way for a dark matter “particle” to transfer momentum to another particle, and thus no real way for “heat” to exist as such.
Conceivably there could be interactions which are sufficiently constrained to prevent the equipartition theorem from coming into play in practice. For example, an interaction with a massive carrier may not be able to support thermal transfer except at very high energies.
Gravitational heat transfer would, I assume, work for everything, but it would also be very very slow.
We're a little bit spoiled by EM - it makes thermal interactions happen quickly and at all energy scales.
I think the confusing thing for me is that dark matter doesn’t interact with the electromagnetic field so it doesn’t reflect, absorb, or emit electromagnetic radiation.
That seems wrong. "Dark matter" usually means matter that doesn't interact with photons, not just "matter that is dark because no light is shining on it". But if it emits photons, even in a matter-antimatter reaction, then it couples with photons, and so it can't be dark matter in that sense.
Apparently (eg. [1]) there would be a couple of different pathways available for WIMP annihilation, to a W⁺W⁻ boson pair, or to μ⁺μ⁻ muon pair, or a e⁺e⁻ electron-positron pair, so the immediate annihilation products would be charged non-dark matter particles which would either quickly decay or annihilate into photons or simply shed their energy via normal EM interactions.
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?
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.
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?
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.
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.
> 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.
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).
> 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.
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.
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.
>> 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 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.
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.
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.
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.
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.
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.
Not attempting to claim malfeasance or anything but PNAS has a weird "contributed article" track (this article is one of them) in which the authors _can pick their own reviewers_ [1].
"Only 18% of direct submissions were published in 2013, whereas more than 98% of contributed papers were published, according to figures on the journal's website" [2].
Seems like a reasonably professional paper by real authors, but the abstract seems suspiciously handwavy while the content is a bit thick for me. Can we get a BS check from the physicists here? Is this a good area for inquiry or just mining a giant data set for confirmation of a wonky idea?
Also: what's the "heating" mechanism proposed? Dark matter infall would pick up energy I guess, but if it's expected to interact only with gravity that doesn't seem likely to do anything meaningful vs. the same cloud contracting on its own? What's being heated?
