"It means it doesn’t emit any electromagnetic radiation for all we can tell."
Its lack of interaction with electromagnetic radiation means that it is transparent. Of course, something can be dark and transparent, though transparent is probably not what first comes to mind when most people think of dark matter.
It's also probably not completely clumpily distributed, at least not in the way that galaxies are. Back in the 1970s, it was thought that when calculating the brightness of distant objects from the absolute brightness, the redshift, and the assumed cosmological model, one should take into account that since we can see distant objects only if we look between closer objects, one should consider the effect of the light propagating in an "empty beam", rather than through an average-density universe. ("Dyer-Roeder distance" is the buzz phrase.)
Of course, the comparison of calculated and observed brightness is one of the classic cosmological tests, and the 2011 Nobel Prize in physics was awarded for applying this test.
It does make a difference, as I pointed out in arXiv:1505.02917. There is also some evidence for lack of clumpiness, at least if this is understood appropriately (a rather thorny subject), as I pointed out in arXiv:1508.05544. (For what it's worth, the arXiv:1503.08506 by Kaiser and Peacock for the gory details. The published in MNRAS (today) but I haven't had time to update the arXiv reference.
Dark matter (DM) as gravitation-only spherical atmosphere inflated by primordial temperature runs the Tully-Fisher effect. Spiral galaxies are stable across redshifts. DM is not progressively scavenged by stellar black holes and galaxy center giant black holes. Bad. Tully-Fisher can be MoND's Milgrom acceleration. Source on a bench top by measuring vacuum trace chiral anisotropy toward hadrons (fermion quarks) only, then Noetherian angular momentum leakage. No DM.
DM as thin megaparsec filaments, Tully-Fisher rules, is bad. Minimize energy by coalescing into blobs. Filaments resemble stretched elastic sheets puckering. Pucker spacetime by universal expansion. Filament intensity declines with decreasing redshift and slowing expansion. No DM
"We know that dark matter can’t be constituted by dim brown dwarfs or black holes. The main reason is that we know the total mass dark matter brings into our galaxy, and it’s a lot, about 10 times as much as the visible matter. If that amount of mass was made up from black holes, we should constantly see gravitational lensing events – but we don’t."
Brown dwarfs would be baryonic, and we know from big-bang nucleosynthesis that all dark matter can't be baryonic. Primordial black holes, formed before nucleosynthesis, wouldn't be subject to this restriction, but the lack of enough lensing events rules them out. Another argument, made by some Swedish colleagues and myself, is that if most dark matter were in primordial black holes, it would cause variability in QSOs due to microlensing, but the variability as actually observed looks quantitatively different. (This was a good hypothesis, in that it made testable predictions, and was an interesting idea, but has been ruled out by having one of its predictions falsified.)
CIP: I think this site explains it well. The axion might seem somewhat confusing because it is not a "massive" particle as the M in WIMPs demands. Instead, it is a very light particle. How can that be? This works because it isn't the particle itself that makes up the dark matter, but a condensate of the field that is generated by symmetry breaking in the early universe. This much like the Higgs field isn't the same as the Higgs particle (though the mass parameter comes about differently). Best,
Thanks for adding these details :) I only want to object on that we know from nucleosynthesis that dark matter can't be entirely baryonic. Unless you mean something different, what we know is that it needs to have certain cross-section or interaction probability respectively. Best,
The axion is a peculiar beast which forms large homogeneous patches, known as "domains". Inside these domains the field is constant, but it makes a jump at the border of the patches (the "domain walls"). While one cannot measure the constant field itself, it might actually be possible to measure this passage though the domain walls - I wrote about this here. But what most experiments look for is the particle itself, not the condensate. Best,
I disagree with your first point. E.g. gamma-rays from the decay of WIMPs might well be found in the future. In this case DM would emit EM radiation and we would "see" it. "Dark" does just mean that we do not see it (yet).
What you mention isn't a direct signal, it's a secondary signal that comes from the decay of intermediary particles. You are right if you go by word - but technically what "dark" means here is that there is no direct coupling. Best,
I see that two of my hand-written links don't work, but rather give a "javascript void()". Not sure why. Another try:
The details of the effect of local inhomogeneities in observational cosmology is still an area of active research; see arXiv:1503.08506 by Kaiser and Peacock for the gory details. The latter paper has been published in MNRAS (today) but I haven't had time to update the arXiv reference.
I only want to object on that we know from nucleosynthesis that dark matter can't be entirely baryonic. Unless you mean something different, what we know is that it needs to have certain cross-section or interaction probability respectively.
We know from big-bang nucleosynthesis how many baryons are in the universe (as a fraction of the total density of matter, which we also know from various cosmological tests). From this, we can conclude that most of the dark matter cannot be in the form of brown dwarfs, bricks, planets, back issues of the ApJ, cold gas, and so on, quite apart from whether or not we (could in principle) detect it. Even though black holes have no hair, black holes formed from baryonic objects, such as those which form from collapsed stars, would also be "baryonic dark matter" in this context.
The only way around this is a) to form these objects before nucleosynthesis and b) have them be immune to nucleosynthesis. For primordial black holes, no problem: their baryonic nature is unknown to the baryons involved in nucleosynthesis. Otherwise, one would have to come up with forming an object out of baryons before the universe was cool enough to form helium, and also keep them from reacting with the other baryons. See slide 28 of Starkman's talk which you recently posted; I'm referring to what is crossed out. Of course, Starkman's macro dark matter falls into the but-dark-matter-could-be-baryonic category, but I think it is fair to say that it is still relatively speculative. If it can come about, OK, but it is far from clear that there is any sort of prediction that such objects must exist.
Also, such macro dark matter forms objects much larger than an atomic nucleus, but we know (from mircolensing surveys) that even objects the size of planets are ruled out. Other lensing observations rule out galaxy-size objects other than galaxies, whatever they are made of.
I'm not sure what you mean when you say it's probably not smoothly distributed. I understand that the dark matter should be more dense toward the center of the galaxy but as far as I can tell the density gradient should be smooth. If there were clumps of denser dark matter I would expect them to be dispersed by even small tidal forces.
What do you make of the cusp problem, where dark matter refuses to agglomerate in galactic cores?
Ref: See wikipedia but basically there should be a strong peak of dark matter in the core of each galaxy, but they are not there. Instead dark matter seems not want to get too clumpy.
MRS Hawkins has published papers (also available for free on axriv.org) that argue the case for primordial black holes.
He has argued that at least some of the variability of QSOs and AGNs could be due to microlensing by primordial black holes.
In his most recent paper he demonstrates how the previously assumed galactic model might have underestimated the true abundance of MACHOs, which are most easily explained as primordial black holes. There are MWG models that are consistent with 100% of the dark matter in the Galaxy being constituted by PBHs.
MACHOs have been detected, unlike hypothetical particles which have failed every test so far. The discovery that at least 100 billion MACHOs populate our Galaxy has stood the test of time and repeated attacks by those who want a different answer.
Quite frankly, I do not think having fixed ideas on this crucial issue for all of physics is a good idea. MACHOs exist; the question is whether they contribute 10% or 100% of the DM, or something in between.
"the question is whether they contribute 10% or 100% of the DM, or something in between."
Or less than 10 per cent. A "significant fraction" has been ruled out. Practically no-one other than Hawkins believes his claim. Yes, he published some good papers, in part good because they made unadjustable, testable, prior predictions. But the predictions were falsified. That's science. In his latest paper on the subject, he moves the goal posts by arguing that if one pushes all uncertainties in the right direction, then his theory can just survive. However, in other cases which supported his theory in the past where there are now more and better data, he doesn't take this into account. Also, as noted above, his theory makes predictions which he himself didn't highlight. These have also been ruled out.
And his original claim was (and still is) that his primordial black holes make up most of the dark matter.
Tully-Fisher relation, filament lensing, microwave background thermal inhomogeneities' multipole power spectrum, demand lots of gravitating something absent all other interactions. That all three share one origin is arbitrary.
Dark matter models, quantum gravitation, and SUSY are hierarchies of excuses. Only general relativity works - as pure geometry. Fixed geometry of mass (atoms) is crystallography (0.1 nm^3 scale chemistry) - the differential empirical test of spacetime geometry trace chiral symmetry breaking, baryogenesis onward. Physics specifically excludes it. Look.
>What you mention isn't a direct signal, it's a secondary signal that comes from the decay of the intermediary particles.
So what? Why does a secondary signal not qualify as "emission of EM radiation of DM" in this context, as long as the radiation comes from the direction of the DM particles (as it well may to very good precision, due to the expected very small lifetime of the secondary particles)?
> but technically what "dark" means here, is that there is no direct coupling
Even if I would accept your restriction to primary signals: what excludes that there is a weak primary EM signal from DM, say if it would be milli-charged? Even with your restriction "dark" still just means that we do not see primary EM radiation from DM (yet).
I was just trying to explain how the word is commonly used. Complaining about this is entirely pointless. The problem with a 'weak primary signal' is that it wouldn't be weak, unless you make some very strange assumptions like, as you seem to hint at, fractional charges. We know what the coupling constant of QED is. It's not weak. How do you get the primary signal to be weak then? Best,
Wouldn't the dark matter content of a galaxy evaporate due to gravitational interactions? Dark matter particles would be constantly slingshoted out of the galaxy.
And I have always wondered what the population of dark matter orbits would look like. Would they be random? They are interacting with stars and their orbit isn't randomly distributed.
I wonder if I could model this on universal sandbox.
Yes but a gravitational slingshot can give a particle escape velocity. For example voyager 1 reached escape velocity by a close approach to both Jupiter and Saturn. About twenty stars have been found to have galactic escape velocity. This may be from interactions with black holes or globular clusters or whatever. But if it happened to stars then it can happen to dark matter particles.
ppnl: It happens equally often as our galaxy attracts dark matter that was not previously bound. It's an equilibrium situation. (At least to good approximation, neglecting galaxy mergers and so on.)
Still it seems like the process would transfer momentum from the heavy solar masses to the dark matter halo. This would heat the halo over time. I don't know how much.
> I was just trying to explain how the word is commonly used. Complaining about this is entirely pointless.
Its not commonly used like this. An example from a paper by UCSC authors: "A generic feature of WIMP DM is the emission of photons ... resulting from ... DM pair annililation." (first sentence of "Searching for DM annihilation in x-rays and gamma-rays" by T. Jeltema, S.Profumo, in: Proc. of Dark 2009 conf., Heidelberg).
>The problem with a 'weak primary signal' is that it wouldn't be weak, unless you make some very strange assumptions like, as you seem to hint at, fractional charges.
No, another example: brown dwarf DM. They emit primary EM radiation too weak to be detected.
Look, everybody makes mistakes (especially somebody churning out blog contributions at your pace), so don't be immune to advice ;-).
I don't know these particular proceedings you are referring to, and I'm not sure what this sentence is supposed to demonstrate. Nobody doubts that dark matter can annihilate into photons. The point is that it doesn't do so *directly*. The sentence you quote explicitly refers to WIMPS. The "W" in WIMP stands for "weakly" interacting. WIMPS, by definition, do not couple to photons. It's an indirect process, as I said above. The WIMPS couple to some carriers of the weak force and these subsequently decay into other stuff, in the end much of it is photons.
It is correct that dark matter can in principle directly couple to photons if you come up with another explanation for why that happens very rarely. Macro dark matter is an example for this. In this case the reason the interaction is so small is that the stuff is highly clumped.
So I agree to the point that the statement that it doesn't couple is highly simplified. At second order everything that gravitates couples anyway. But this is a blogpost and not a research paper.
"It is correct that dark matter can in principle directly couple to photons if you come up with another explanation for why that happens very rarely. Macro dark matter is an example for this. In this case the reason the interaction is so small is that the stuff is highly clumped."
I would like to understand this little better. Do you think dark matter particles could be closer to each other than atomic, nuclear or quark *radii*?
What I meant wasn't so much a referral to the actual distance, but to the clumping. See, what matters for the interaction probability (or rate) isn't only the strength of the interaction, but also how often the particles meet each other. The more thinly they are dispersed, the more likely they are to meet. If they are very clumped together on the other hand, they meet very rarely, and the probability of interaction can be small even if the interaction strength isn't small.
Consider you have a bucket of water in a large empty room. You will only interact with it if you happen to run into the bucket. But now take the same water and finely sprinkle it in the air. You will find no way to avoid it. It's similar with dark matter. The normal assumption is that it's 'finely sprinkled' (particle dark matter) and in this case the only way you can get a small interaction probability is if the interaction itself is weak. If it is very clumped however (macro dark matter), then the interaction probability might be small even though the interaction, if it happens, is not weak. Best,
"Still it seems like the process would transfer momentum from the heavy solar masses to the dark matter halo. This would heat the halo over time. I don't know how much."
One of the ways that you test a dark matter model is to run a stimulation in which it exists and has certain properties and see what kind of universe that produces in many runs of the simulation with some randomness inserted. If the universe produced often looks like ours, you are on the right track and if it doesn't, you're doing something wrong.
One of the really key factors in devising those simulations is trying to figure out in a simplified model how ordinary matter and dark matter interact, because those interactions via gravity have a huge impact on the overall structure produced. Ordinary matter in isolation tends to produce one kind of structure. Dark matter alone tends to produce another kind of structure. But them together in the same model and you get a kind of structure that is intermediate between the two pulled one way by the dark matter component and another by the ordinary matter component. Earlier simulations modeled pure dark matter thinking that it would be approximately right because most of the stuff in the universe is dark. But, like seasonings in a recipe, the presence of feedback with even modest amounts of ordinary matter in dark matter dominated galaxies dramatically influences the overall structure produced.
2MASS Redshift Survey: "stringy" galaxy distribution in the nearby universe, colors based on redshifts. Filamentary matter and voids distribution may arise from an anisotropic shape of space as the universe expands. Consider Gore-Tex packing and Zetix with negative Poisson's ratios (auxetic materials). They expand in orthogonal diameter when pulled, opening voids amidst strings.
http://physics.aps.org/assets/88e08c5f-b99d-492d-81ad-64ede2698b3b/e90_1.png Astrophys. J. Suppl. Ser.199 26 (2012) arXiv:1108.0669v2 10.1088/0067-0049/199/2/26
We show that particles orbiting a central body (e.g., Saturn's rings) can be assembled into one or more dense (e.g., opaque) independent rings without interparticle collisions taking place despite the inevitable particle oscillations about the ring plane. The resultant apparent bulk motion is a slow “rolling” motion of the ring, as it orbits, the individual rings describing a “helical” motion. Such rings would only evolve due to external perturbations or (slow) internal gravitational perturbations, since the particles need never collide. This picture opens up the possibility of having hollow rings, for example. Moreover, it is possible that an initially uniform disk of randomly moving particles may spontaneously separate into a series of such rings. The consequence would be a striated disk having virtually zero internal viscosity.
End quote.
I suppose there is a field of study of the stable structures that can be formed gravitationally from collisionless particles. Surely that would help in trying to figure out where to find dark matter.
Is it possible that dark matter isn't matter at all, but pure gravity in itself, or what might be called "naked gravity"?
Probably not, unless (though I don't think so) you mean a Brill-Hartle geon, in which case Sabine might be able to say whether such objects can be ruled out as dark-matter candidates.
I'm not really sure what you mean there. Do you mean gravitons or solitons of some kind? It has been argued that gravitons might be able to condense and appear like dark matter. The problem with this idea is that it only works if gravitons are massive. And that is very difficult to combine with General Relativity. This isn't to say it's impossible. Then there is of course 'modified gravity' which, in some sense, is 'pure gravity' just not the normal gravity. In case you mean it's that we don't really understand general relativity, this is exceedingly unlikely to be an explanation for dark matter and I actually don't know of any such argument. It has been argued though that dark energy might actually be an artifact of misunderstanding spatial averages in non-linear theories. Best,
" It has been argued though that dark energy might actually be an artifact of misunderstanding spatial averages in non-linear theories."
I guess you mean not dark energy itself but rather observations interpreted to indicate dark energy might be due to other effects related to the inhomogeneity of the universe (though on large scales it is still homogeneous enough): the propagation of light is affected so that distances are miscalculated and/or the inhomogeneity influences the local expansion rate (which in turn influences distance calculation).
What about geons? Objects consisting of gravitational waves with an energy density high enough that they are bound objects? The idea goes back to Wheeler. Bizarre, but ruled out as dark-matter candidates?
I'm no expert on this, but I believe that technically geons are one type of solitons, but there are other types of solitons as well.
I don't know of any mechanism which could produce them in a way to account for dark matter, but I don't know if they are definitively ruled out observationally.
8:53 AM, September 23, 2015
[Image]
Dark matter filaments. Image Credits: John Dubinski (U of Toronto).
1. “Dark” doesn’t just mean we don’t see it.
It means it doesn’t emit any electromagnetic radiation for all we can tell. Astronomers haven’t been able to find neither light visible to the eye, nor radiation in the radio range or x-ray regime, and not at even higher energies either.
2. “Matter” doesn’t just mean it’s stuff.
What physicists classify as matter must behave like the matter we are made of, at least for what its motion in space and time is concerned. This means in particular dark matter dilutes when it spreads into a larger volume, and causes the same gravitational attraction as ordinary, visible, matter. It is easy to think up “stuff” that does not do this. Dark energy for example does not behave this way.
3. It’s not going away.
You will not wake up one day and hear physicists declare it’s not there at all. The evidence is overwhelming: Weak gravitational lensing demonstrates that galaxies have a larger gravitational pull than visible matter can produce. Additional matter in galaxies is also necessary to explain why stars in the outer arms of galaxies orbit so quickly around the center. The observed temperature fluctuations in the cosmic microwave background can’t be explained without dark matter, and the structures formed by galaxies wouldn’t come out right without dark matter either.
Even if all of this was explained by a modification of gravity rather than an unknown type of matter, it would still have to be possible to formulate this modification of gravity in a way that makes it look pretty much like a new type of matter. And we’d still call it dark matter.
4. Rubin wasn’t the first to find evidence for dark matter.
Though she was the first to recognize its relevance. A few decades before Vera Rubin noticed that stars rotate inexplicably fast around the centers of galaxies, Fritz Zwicky pointed out that a swarm of about a thousand galaxies which are bound together by gravity to the “Coma Cluster” also move too quickly. The velocity of the galaxies in a gravitational potential depends on the total mass in this potential, and the too large velocities indicated already that there was more mass than could be seen. However, it wasn’t until Rubin collected her data that it became clear this isn’t a peculiarity of the Coma Cluster, but that dark matter must be present in almost all galaxies and galaxy clusters.
5. Dark matter doesn’t interact much with itself or anything else.
If it did, it would slow down and clump too much and that wouldn’t be in agreement with the data. A particularly vivid example comes from the Bullet Cluster, which actually consists of two clusters of galaxies that have passed through each other. In the Bullet Cluster, one can detect both the distribution of ordinary matter, mostly be emission of x-rays, and the distribution of dark matter, by gravitational lensing. The data demonstrates that the dark matter is dislocated from the visible matter: The dark matter parts of the clusters seem to have passed through each other almost undisturbed, whereas the visible matter was slowed down and its shape was noticeably distorted.
The same weak interaction is necessary to explain the observations on the cosmic microwave background and galactic structure formation.
6. It’s responsible for the structures in the universe.
Since dark matter doesn’t interact much with itself and other stuff, it’s the first type of matter to settle down when the universe expands and the first to form structures under its own gravitational pull. It is dark matter that seeds the filaments along which galaxies later form when visible matter falls into the gravitational potential created by the dark matter. If you look at some computer simulation of structure formation, what is shown is almost always the distribution of dark matter, not of visible matter. Visible matter is assumed to follow the same distribution later.
7. It’s probably not smoothly distributed.
Dark matter doesn’t only form filaments on supergalactic scales, it also isn’t entirely smoothly distributed within galaxies – at least that’s what the best understood models say. Dark matter doesn’t interact enough to form objects as dense as planets, but it does have ‘halos’ of varying density that move around in galaxies. The dark matter density is generally larger towards the centers of galaxies.
8. Physicists have lots of ideas what dark matter could be.
The presently most popular explanation for the puzzling observations is some kind of weakly interacting particle that doesn’t interact with light. These particles have to be quite massive to form the observed structures, about as heavy as the heaviest particles we know already. If dark matter particles weren’t heavy enough they wouldn’t clump sufficiently, which is why they are called WIMPs for “Weakly Interacting Massive Particles.” Another candidate is a particle called the axion, which is very light but leaves behind some kind of condensate that fills the universe.
There are other types of candidate particles that have more complex interactions or are heavier, such Wimpzillas and other exotic stuff. Macro dark matter is a type of dark matter that could be accommodated in the standard model; it consists of macroscopically heavy chunks of unknown types of nuclear matter.
Then there are several proposals for how to modify gravity to accommodate the observations, such as MOG, entropic gravity, or bimetric theories. Though very different by motivation, the more observations have to be explained the more similar the explanations through additional particles have become to the explanations through modifying gravity.
9. And they know some things dark matter can’t be.
We know that dark matter can’t be constituted by dim brown dwarfs or black holes. The main reason is that we know the total mass dark matter brings into our galaxy, and it’s a lot, about 10 times as much as the visible matter. If that amount of mass was made up from black holes, we should constantly see gravitational lensing events – but we don’t. It also doesn’t quite work with structure formation. And we know that neutrinos, even though weakly interacting, can’t make up dark matter either because they are too light and they wouldn’t clump strongly enough.
10. But we have no direct experimental evidence.
Despite decades of search, nobody has ever directly detected a dark matter particle and the only evidence we have is still indirectly inferred from gravitational pull. Physicists have been looking for the rare interactions of proposed dark matter candidates in many Earth-based experiments starting already in the 1980s. They also look for astrophysical evidence of dark matter, such as signals from the mutual annihilation of dark matter particles. There have been some intriguing findings, such as the PAMELA positron excess, the DAMA annual modulation, or the Fermi gamma-ray excess, but physicists haven’t been able to link any of these convincingly to dark matter.
"10 Things you should know about Dark Matter"
44 Comments -
That is interesting.
8:58 AM, September 15, 2015
"It means it doesn’t emit any electromagnetic radiation for all we can tell."
Its lack of interaction with electromagnetic radiation means that it is transparent. Of course, something can be dark and transparent, though transparent is probably not what first comes to mind when most people think of dark matter.
10:31 AM, September 15, 2015
"It’s probably not smoothly distributed."
It's also probably not completely clumpily distributed, at least not in the way that galaxies are. Back in the 1970s, it was thought that when calculating the brightness of distant objects from the absolute brightness, the redshift, and the assumed cosmological model, one should take into account that since we can see distant objects only if we look between closer objects, one should consider the effect of the light propagating in an "empty beam", rather than through an average-density universe. ("Dyer-Roeder distance" is the buzz phrase.)
Of course, the comparison of calculated and observed brightness is one of the classic cosmological tests, and the 2011 Nobel Prize in physics was awarded for applying this test.
It does make a difference, as I pointed out in arXiv:1505.02917. There is also some evidence for lack of clumpiness, at least if this is understood appropriately (a rather thorny subject), as I pointed out in arXiv:1508.05544. (For what it's worth, the arXiv:1503.08506 by Kaiser and Peacock for the gory details. The published in MNRAS (today) but I haven't had time to update the arXiv reference.
10:47 AM, September 15, 2015
Thanks Bee, for another lovely summary. However, I didn't understand the bit about the axion and its "left behind" condensate. Care to clarify?
10:49 AM, September 15, 2015
Dark matter (DM) as gravitation-only spherical atmosphere inflated by primordial temperature runs the Tully-Fisher effect. Spiral galaxies are stable across redshifts. DM is not progressively scavenged by stellar black holes and galaxy center giant black holes. Bad. Tully-Fisher can be MoND's Milgrom acceleration. Source on a bench top by measuring vacuum trace chiral anisotropy toward hadrons (fermion quarks) only, then Noetherian angular momentum leakage. No DM.
DM as thin megaparsec filaments, Tully-Fisher rules, is bad. Minimize energy by coalescing into blobs. Filaments resemble stretched elastic sheets puckering. Pucker spacetime by universal expansion. Filament intensity declines with decreasing redshift and slowing expansion. No DM
10:50 AM, September 15, 2015
"We know that dark matter can’t be constituted by dim brown dwarfs or black holes. The main reason is that we know the total mass dark matter brings into our galaxy, and it’s a lot, about 10 times as much as the visible matter. If that amount of mass was made up from black holes, we should constantly see gravitational lensing events – but we don’t."
Brown dwarfs would be baryonic, and we know from big-bang nucleosynthesis that all dark matter can't be baryonic. Primordial black holes, formed before nucleosynthesis, wouldn't be subject to this restriction, but the lack of enough lensing events rules them out. Another argument, made by some Swedish colleagues and myself, is that if most dark matter were in primordial black holes, it would cause variability in QSOs due to microlensing, but the variability as actually observed looks quantitatively different. (This was a good hypothesis, in that it made testable predictions, and was an interesting idea, but has been ruled out by having one of its predictions falsified.)
10:55 AM, September 15, 2015
Does axion condensate behave like chunks of matter as far as gravity is concerned. How?
10:58 AM, September 15, 2015
What
about dark Matter trapped in Black Holes?
Shouldn't the Black Hole(s) in the center of Galaxies
gobble up some/a lot of it?
Georg
11:08 AM, September 15, 2015
CIP: I think this site explains it well. The axion might seem somewhat confusing because it is not a "massive" particle as the M in WIMPs demands. Instead, it is a very light particle. How can that be?
This works because it isn't the particle itself that makes up the dark matter, but a condensate of the field that is generated by symmetry breaking in the early universe. This much like the Higgs field isn't the same as the Higgs particle (though the mass parameter comes about differently). Best,
B.
11:18 AM, September 15, 2015
Phillip,
Thanks for adding these details :) I only want to object on that we know from nucleosynthesis that dark matter can't be entirely baryonic. Unless you mean something different, what we know is that it needs to have certain cross-section or interaction probability respectively. Best,
B.
11:22 AM, September 15, 2015
Kashyap,
The axion is a peculiar beast which forms large homogeneous patches, known as "domains". Inside these domains the field is constant, but it makes a jump at the border of the patches (the "domain walls"). While one cannot measure the constant field itself, it might actually be possible to measure this passage though the domain walls - I wrote about this here. But what most experiments look for is the particle itself, not the condensate. Best,
B.
11:25 AM, September 15, 2015
George,
Yes, they do. But since the dark matter is very thinly dispersed (compared to normal matter which is strongly clumped) it doesn't amount to much.
11:42 AM, September 15, 2015
I disagree with your first point.
E.g. gamma-rays from the decay of WIMPs
might well be found in the future. In this
case DM would emit EM radiation and we would
"see" it. "Dark" does just mean that we do not
see it (yet).
12:36 PM, September 15, 2015
Maurice,
What you mention isn't a direct signal, it's a secondary signal that comes from the decay of intermediary particles. You are right if you go by word - but technically what "dark" means here is that there is no direct coupling. Best,
B.
12:54 PM, September 15, 2015
I see that two of my hand-written links don't work, but rather give a "javascript void()". Not sure why. Another try:
The details of the effect of local inhomogeneities in observational cosmology is still an area of active research; see arXiv:1503.08506 by Kaiser and Peacock for the gory details. The latter paper has been published in MNRAS (today) but I haven't had time to update the arXiv reference.
12:57 PM, September 15, 2015
Hello Sabine,
Couldn't the dark matter adopt a very dense state within the black hole, as opposed to a thinly dispersed state outside ?
Greetings, Koen
1:07 PM, September 15, 2015
I only want to object on that we know from nucleosynthesis that dark matter can't be entirely baryonic. Unless you mean something different, what we know is that it needs to have certain cross-section or interaction probability respectively.
We know from big-bang nucleosynthesis how many baryons are in the universe (as a fraction of the total density of matter, which we also know from various cosmological tests). From this, we can conclude that most of the dark matter cannot be in the form of brown dwarfs, bricks, planets, back issues of the ApJ, cold gas, and so on, quite apart from whether or not we (could in principle) detect it. Even though black holes have no hair, black holes formed from baryonic objects, such as those which form from collapsed stars, would also be "baryonic dark matter" in this context.
The only way around this is a) to form these objects before nucleosynthesis and b) have them be immune to nucleosynthesis. For primordial black holes, no problem: their baryonic nature is unknown to the baryons involved in nucleosynthesis. Otherwise, one would have to come up with forming an object out of baryons before the universe was cool enough to form helium, and also keep them from reacting with the other baryons. See slide 28 of Starkman's talk which you recently posted; I'm referring to what is crossed out. Of course, Starkman's macro dark matter falls into the but-dark-matter-could-be-baryonic category, but I think it is fair to say that it is still relatively speculative. If it can come about, OK, but it is far from clear that there is any sort of prediction that such objects must exist.
Also, such macro dark matter forms objects much larger than an atomic nucleus, but we know (from mircolensing surveys) that even objects the size of planets are ruled out. Other lensing observations rule out galaxy-size objects other than galaxies, whatever they are made of.
1:32 PM, September 15, 2015
I'm not sure what you mean when you say it's probably not smoothly distributed. I understand that the dark matter should be more dense toward the center of the galaxy but as far as I can tell the density gradient should be smooth. If there were clumps of denser dark matter I would expect them to be dispersed by even small tidal forces.
5:32 PM, September 15, 2015
Sabine,
What do you make of the cusp problem, where dark matter refuses to agglomerate in galactic cores?
Ref: See wikipedia but basically there should be a strong peak of dark matter in the core of each galaxy, but they are not there. Instead dark matter seems not want to get too clumpy.
10:13 PM, September 15, 2015
MRS Hawkins has published papers (also available for free on axriv.org) that argue the case for primordial black holes.
He has argued that at least some of the variability of QSOs and AGNs could be due to microlensing by primordial black holes.
In his most recent paper he demonstrates how the previously assumed galactic model might have underestimated the true abundance of MACHOs, which are most easily explained as primordial black holes. There are MWG models that are consistent with 100% of the dark matter in the Galaxy being constituted by PBHs.
MACHOs have been detected, unlike hypothetical particles which have failed every test so far. The discovery that at least 100 billion MACHOs populate our Galaxy has stood the test of time and repeated attacks by those who want a different answer.
Quite frankly, I do not think having fixed ideas on this crucial issue for all of physics is a good idea. MACHOs exist; the question is whether they contribute 10% or 100% of the DM, or something in between.
10:26 PM, September 15, 2015
"MACHOs exist;"
No-one doubts this.
"the question is whether they contribute 10% or 100% of the DM, or something in between."
Or less than 10 per cent. A "significant fraction" has been ruled out. Practically no-one other than Hawkins believes his claim. Yes, he published some good papers, in part good because they made unadjustable, testable, prior predictions. But the predictions were falsified. That's science. In his latest paper on the subject, he moves the goal posts by arguing that if one pushes all uncertainties in the right direction, then his theory can just survive. However, in other cases which supported his theory in the past where there are now more and better data, he doesn't take this into account. Also, as noted above, his theory makes predictions which he himself didn't highlight. These have also been ruled out.
And his original claim was (and still is) that his primordial black holes make up most of the dark matter.
7:26 AM, September 16, 2015
Tully-Fisher relation, filament lensing, microwave background thermal inhomogeneities' multipole power spectrum, demand lots of gravitating something absent all other interactions. That all three share one origin is arbitrary.
Dark matter models, quantum gravitation, and SUSY are hierarchies of excuses. Only general relativity works - as pure geometry. Fixed geometry of mass (atoms) is crystallography (0.1 nm^3 scale chemistry) - the differential empirical test of spacetime geometry trace chiral symmetry breaking, baryogenesis onward. Physics specifically excludes it. Look.
http://thewinnower.s3.amazonaws.com/papers/95/v1/sources/image004.png
Nucl. Phys. B 185 (1) 20 (1981), doi:10.1016/0550-3213(81)90361-8; Erratum, ibid. 195(3) 541 (1982), doi:10.1016/0550-3213(82)90011-6
Discrete Math. 313(12) 1289 (2013), doi:10.1016/j.disc.2013.02.010, arXiv:1109.1963
Acta Cryst. A 59(3) 210 (2003) Section 3ff. doi:10.1107/S0108767303004161
Publ. Inst. Math., Nouv. Sér. 49(63) 51 (1991), Section 2
10:50 AM, September 16, 2015
>What you mention isn't a direct signal, it's a secondary signal that comes from the decay of the intermediary particles.
So what? Why does a secondary signal not qualify as "emission of EM radiation of DM" in this context, as long as the radiation comes from the direction of the DM particles (as it well may to very good precision, due to the expected very small lifetime of the secondary particles)?
> but technically what "dark" means here, is that there is no direct coupling
Even if I would accept your restriction to primary signals: what excludes that there is a weak primary EM signal from DM, say if it would be milli-charged? Even with your restriction
"dark" still just means that we do not see primary EM radiation from DM (yet).
11:02 AM, September 16, 2015
Maurice,
I was just trying to explain how the word is commonly used. Complaining about this is entirely pointless. The problem with a 'weak primary signal' is that it wouldn't be weak, unless you make some very strange assumptions like, as you seem to hint at, fractional charges. We know what the coupling constant of QED is. It's not weak. How do you get the primary signal to be weak then? Best,
B.
1:22 PM, September 16, 2015
Wouldn't the dark matter content of a galaxy evaporate due to gravitational interactions? Dark matter particles would be constantly slingshoted out of the galaxy.
And I have always wondered what the population of dark matter orbits would look like. Would they be random? They are interacting with stars and their orbit isn't randomly distributed.
I wonder if I could model this on universal sandbox.
12:10 AM, September 17, 2015
ppnl: They are bound by their own gravitational potential.
1:07 AM, September 17, 2015
Yes but a gravitational slingshot can give a particle escape velocity. For example voyager 1 reached escape velocity by a close approach to both Jupiter and Saturn. About twenty stars have been found to have galactic escape velocity. This may be from interactions with black holes or globular clusters or whatever. But if it happened to stars then it can happen to dark matter particles.
But how often will it happen?
3:18 AM, September 17, 2015
ppnl: It happens equally often as our galaxy attracts dark matter that was not previously bound. It's an equilibrium situation. (At least to good approximation, neglecting galaxy mergers and so on.)
3:58 AM, September 17, 2015
Hmm, yes I see.
Still it seems like the process would transfer momentum from the heavy solar masses to the dark matter halo. This would heat the halo over time. I don't know how much.
4:31 AM, September 17, 2015
> I was just trying to explain how the word is commonly used. Complaining about this is entirely pointless.
Its not commonly used like this. An example from a paper by UCSC authors: "A generic feature of WIMP DM is the emission of photons ... resulting from ... DM pair annililation." (first sentence of "Searching for DM annihilation in x-rays and gamma-rays" by T. Jeltema, S.Profumo, in: Proc. of Dark 2009 conf., Heidelberg).
>The problem with a 'weak primary signal' is that it wouldn't be weak, unless you make some very strange assumptions like, as you seem to hint at, fractional charges.
No, another example: brown dwarf DM. They emit primary EM radiation too weak to be detected.
Look, everybody makes mistakes (especially somebody churning out blog contributions at your pace), so don't be immune to advice ;-).
4:54 AM, September 17, 2015
ppnl,
What you say is correct, in principle. In the end everything strives towards equilibrium. But it happens very, very slowly.
5:45 AM, September 17, 2015
Maurice,
I don't know these particular proceedings you are referring to, and I'm not sure what this sentence is supposed to demonstrate. Nobody doubts that dark matter can annihilate into photons. The point is that it doesn't do so *directly*. The sentence you quote explicitly refers to WIMPS. The "W" in WIMP stands for "weakly" interacting. WIMPS, by definition, do not couple to photons. It's an indirect process, as I said above. The WIMPS couple to some carriers of the weak force and these subsequently decay into other stuff, in the end much of it is photons.
It is correct that dark matter can in principle directly couple to photons if you come up with another explanation for why that happens very rarely. Macro dark matter is an example for this. In this case the reason the interaction is so small is that the stuff is highly clumped.
So I agree to the point that the statement that it doesn't couple is highly simplified. At second order everything that gravitates couples anyway. But this is a blogpost and not a research paper.
6:01 AM, September 17, 2015
"It is correct that dark matter can in principle directly couple to photons if you come up with another explanation for why that happens very rarely. Macro dark matter is an example for this. In this case the reason the interaction is so small is that the stuff is highly clumped."
I would like to understand this little better. Do you think dark matter particles could be closer to each other than atomic, nuclear or quark *radii*?
5:40 PM, September 17, 2015
kashyap,
What I meant wasn't so much a referral to the actual distance, but to the clumping. See, what matters for the interaction probability (or rate) isn't only the strength of the interaction, but also how often the particles meet each other. The more thinly they are dispersed, the more likely they are to meet. If they are very clumped together on the other hand, they meet very rarely, and the probability of interaction can be small even if the interaction strength isn't small.
Consider you have a bucket of water in a large empty room. You will only interact with it if you happen to run into the bucket. But now take the same water and finely sprinkle it in the air. You will find no way to avoid it. It's similar with dark matter. The normal assumption is that it's 'finely sprinkled' (particle dark matter) and in this case the only way you can get a small interaction probability is if the interaction itself is weak. If it is very clumped however (macro dark matter), then the interaction probability might be small even though the interaction, if it happens, is not weak. Best,
B.
1:16 AM, September 18, 2015
"Still it seems like the process would transfer momentum from the heavy solar masses to the dark matter halo. This would heat the halo over time. I don't know how much."
One of the ways that you test a dark matter model is to run a stimulation in which it exists and has certain properties and see what kind of universe that produces in many runs of the simulation with some randomness inserted. If the universe produced often looks like ours, you are on the right track and if it doesn't, you're doing something wrong.
One of the really key factors in devising those simulations is trying to figure out in a simplified model how ordinary matter and dark matter interact, because those interactions via gravity have a huge impact on the overall structure produced. Ordinary matter in isolation tends to produce one kind of structure. Dark matter alone tends to produce another kind of structure. But them together in the same model and you get a kind of structure that is intermediate between the two pulled one way by the dark matter component and another by the ordinary matter component. Earlier simulations modeled pure dark matter thinking that it would be approximately right because most of the stuff in the universe is dark. But, like seasonings in a recipe, the presence of feedback with even modest amounts of ordinary matter in dark matter dominated galaxies dramatically influences the overall structure produced.
8:27 PM, September 18, 2015
2MASS Redshift Survey: "stringy" galaxy distribution in the nearby universe, colors based on redshifts. Filamentary matter and voids distribution may arise from an anisotropic shape of space as the universe expands. Consider Gore-Tex packing and Zetix with negative Poisson's ratios (auxetic materials). They expand in orthogonal diameter when pulled, opening voids amidst strings.
http://physics.aps.org/assets/88e08c5f-b99d-492d-81ad-64ede2698b3b/e90_1.png
Astrophys. J. Suppl. Ser. 199 26 (2012)
arXiv:1108.0669v2
10.1088/0067-0049/199/2/26
http://i.kinja-img.com/gawker-media/image/upload/s--94mUG6R8--/18s0y6owupr3vjpg.jpg
11:17 AM, September 19, 2015
Without reading the paper it is hard to know whether this is applicable:
http://www.sciencedirect.com/science/article/pii/0032063381900131
Quote: Abstract
We show that particles orbiting a central body (e.g., Saturn's rings) can be assembled into one or more dense (e.g., opaque) independent rings without interparticle collisions taking place despite the inevitable particle oscillations about the ring plane. The resultant apparent bulk motion is a slow “rolling” motion of the ring, as it orbits, the individual rings describing a “helical” motion. Such rings would only evolve due to external perturbations or (slow) internal gravitational perturbations, since the particles need never collide. This picture opens up the possibility of having hollow rings, for example. Moreover, it is possible that an initially uniform disk of randomly moving particles may spontaneously separate into a series of such rings. The consequence would be a striated disk having virtually zero internal viscosity.
End quote.
I suppose there is a field of study of the stable structures that can be formed gravitationally from collisionless particles. Surely that would help in trying to figure out where to find dark matter.
10:49 AM, September 20, 2015
Is it possible that dark matter isn't matter at all, but pure gravity in itself, or what might be called "naked gravity"?
11:36 PM, September 22, 2015
Is it possible that dark matter isn't matter at all, but pure gravity in itself, or what might be called "naked gravity"?
Probably not, unless (though I don't think so) you mean a Brill-Hartle geon, in which case Sabine might be able to say whether such objects can be ruled out as dark-matter candidates.
5:03 AM, September 23, 2015
Hi DocG,
I'm not really sure what you mean there. Do you mean gravitons or solitons of some kind? It has been argued that gravitons might be able to condense and appear like dark matter. The problem with this idea is that it only works if gravitons are massive. And that is very difficult to combine with General Relativity. This isn't to say it's impossible. Then there is of course 'modified gravity' which, in some sense, is 'pure gravity' just not the normal gravity. In case you mean it's that we don't really understand general relativity, this is exceedingly unlikely to be an explanation for dark matter and I actually don't know of any such argument. It has been argued though that dark energy might actually be an artifact of misunderstanding spatial averages in non-linear theories. Best,
B.
6:40 AM, September 23, 2015
" It has been argued though that dark energy might actually be an artifact of misunderstanding spatial averages in non-linear theories."
I guess you mean not dark energy itself but rather observations interpreted to indicate dark energy might be due to other effects related to the inhomogeneity of the universe (though on large scales it is still homogeneous enough): the propagation of light is affected so that distances are miscalculated and/or the inhomogeneity influences the local expansion rate (which in turn influences distance calculation).
7:32 AM, September 23, 2015
What about geons? Objects consisting of gravitational waves with an energy density high enough that they are bound objects? The idea goes back to Wheeler. Bizarre, but ruled out as dark-matter candidates?
7:33 AM, September 23, 2015
Phillip,
I might be misremembering this, but I thought it actually appears as an additional source term. As to the geons, aren't these solitons? Best,
B.
8:01 AM, September 23, 2015
I'm no expert on this, but I believe that technically geons are one type of solitons, but there are other types of solitons as well.
I don't know of any mechanism which could produce them in a way to account for dark matter, but I don't know if they are definitively ruled out observationally.
8:53 AM, September 23, 2015