The News From LUX
LUX Rules Out WIMP Dark Matter In The Narrow Sense
In the narrow sense, WIMP dark matter is matter that lacks color charge and electromagnetic charge, but does interact via the weak force, and is not a neutrino, generally with masses of 1 GeV to 1000 GeV.
LUX is the world's most powerful direct dark matter detection experiment and has ruled out the existence of dark matter over a very wide range of plausible dark matter masses to very tiny cross-sections of interaction with ordinary matter. The entire range of exclusions by all other experiments from 4 GeV to about 1000 GeV, except CDMSLite 2015 (which has a bit of an edge in the single digit GeV mass range for a dark matter particle) is simultaneous confirmed by LUX down to more faint cross-sections of interaction. Tweaks to the analysis of the 2013 data announced in December of 2015 imply that:
[W]e have improved the WIMP sensitivity of the 2013 LUX search data, excluding new parameter space. The lowered analysis thresholds and signal model energy cut-off, added exposure, and improved resolution of light and charge over the first LUX result yield a 23% reduction in cross-section limit at high WIMP masses. Reach is significantly extended at low mass where the cut-off has most effect on the predicted event rate: the minimum kinematically-accessible mass is reduced from 5.2 to 3.3 GeV/c2.LUX is getting an upgrade in early 2016 to improve its performance, which means that the exclusion range will be getting even better in a few years.
The LUX exclusions, for the most part, are enough to exclude interactions on the same order of magnitude as that of neutrinos (the cross-section of interaction of a neutrino with a nucleon, which a WIMP with weak force charge comparable to all other weak force interacting particles known to exist would naively be expected to share, is on the order of 4*10^-39 to 8*10^-39 per cm^2/GeV), which means that any interactions of dark matter with ordinary matter via the weak force would have to involve dark matter particles with a weak force charge that is a tiny fraction of that of all other weakly interacting particles (something with no precedent and no good theoretical motivation).
The LUX exclusion will soon be even stronger requiring even tinier fractions of the weak force charge areas where there is effectively already an exclusion and expanding the mass range over which there is an exclusion of particles with ordinary weak force charges.
Different approaches have to be used to attempt to directly detect dark matter particles significantly lower than 4 GeV (e.g. the keV mass range favored for "warm dark matter" or the MeV range favored for "dark photons" in self-interacting dark matter models) because the neutrino background gets to strong to make out a signal. CDMSLite 2015, for example, which uses a methodology similar to the LHC still gets down only to about 1 GeV (with a considerably weaker cross-section of interaction excluded).
The LHC and previous particle accelerator experiments rule out most lighter WIMPS.
The CMS experiment has provided exclusions more strict than LUX as low masses.
Particle accelerator experiments, likewise, strongly disfavor the existence of fundamental particles that have even very slight interaction with any form of Standard Model particles with masses in the 1 eV to hundreds of GeV mass range, overlapping with the direct dark matter detection experiment exclusion range and providing the most strict direct detection limitations at the low end of the mass range.
Astronomy Data Largely Rules Out Simple Non-Self-Interacting Cold Dark Matter
Astronomy data, in general, disfavors cold dark matter (on the order of 10 GeV or more), because it would give rise to far more small scale structure in the universe. This is because, assuming "thermal" dark matter (i.e. dark matter created in the very early universe with a mean lifetime on the order of the age of the universe), the mean velocity of dark matter particles is a function of dark matter particle mass. Mean dark matter particle velocity, in turn, influences the scale at which ordinary matter would be forced by dark matter to have a highly structured distribution at small distance scales (e.g. groups of galaxies or smaller).
For example, colder dark matter would produce more subhalos in large galaxies and more satellite galaxies around larger galaxies. This limitation is even stronger with dark matter predominantly made of particles in the 1000 GeV or more mass range (roughly the mass of four uranium atoms or more).
This robust exclusion applies even if dark matter interacts only via gravity, but could be avoided if dark matter is self-interacting, or is not a thermal relic (i.e. rather than having a tens of billions of year long mean lifetime or more, it is created and destroyed at rates that are basically equal).
It is unlikely that dark matter that interacts via the weak force with ordinary matter exists with particles of more than 1000 GeV although experimental designs can't rule it out for dark matter that has significant self-interactions (which prevent it from acting like ordinary "cold dark matter" particle theories).
It had been hoped that including ordinary matter-dark matter interactions in cold dark matter simulations would solve this problem, but, for the most part, efforts to do this have been insufficient to solve the problem, or to solve other related cold dark matter problems like the cusp-core problem, which notes that inferred dark matter halo shapes differ materially from the shapes inferred to exist from the dynamics observed in galaxies.
This exclusion shouldn't be unduly exaggerated, however. Dark matter does a very good job of explaining observations at a cosmology scale and at explaining phenomena at scales larger than galaxies, while not contradicting solar system scale observations. And, while the simple singlet fermion cold dark matter particle with no non-gravitational interaction is not right on the mark for galaxy scale and smaller structures, it is a pretty decent first order approximation of it. So, the notion that a dark matter self-interaction of some type could fix this real problem isn't far fetched.
Higgs Portal Dark Matter
A particle that interacts via the Higgs force with a Higgs boson in addition to gravity is sometimes called Higgs portal dark matter, because Higgs boson decays in particle accelerators would reveal it providing the only Standard Model connection to the dark sector.
Experiments at the LHC have not yet pinned down the properties of the Higgs boson precisely enough to confirm that it could not possibly have interactions with "dark" particles that have no interactions via the weak, strong, or electromagnetic forces, so they can't rule out "Higgs portal dark matter." But, LHC data in run-2 will greatly narrow this window of the dark matter parameter space, and even tighter boundaries will exist by the time that the LHC has finished its work.
The 750 GeV Anomaly Cannot Itself Be Dark Matter
The 750 GeV anomaly at the LHC announced last December, even if it is real, is not itself a good dark matter candidate, because a dark matter candidate needs to have a mean lifetime on the order of tens of billions of years or more, and needs to be much, much lighter (although dark matter self-interaction models can weaken the particle mass constraints). But, a 750 GeV anomaly, if real, might imply a whole new menagerie of particles which could include a dark matter candidate.
Sterile Neutrinos In The Narrow Sense Are Increasingly Disfavored
Neutrino oscillation data, as it becomes more precise, also increasingly disfavors a "sterile neutrino" in senso stricto that oscillates with other neutrinos despite not having strong, electromagnetic or weak force interactions, although it wouldn't rule out, for example, a particle called a sterile neutrino in the weak sense, which has no strong, electromagnetic or weak force interactions but does have mass and interact via gravity.
Another indirect, but nearly direct, way of detecting dark matter is to see the signature of dark matter-antidark matter annihilation events, if they exist. The Fermi experiment, for example, is of this type. But, these experiments face the fundamental problem that the background is ill understood. It can exclude annihilating dark matter in areas where no signals are seen up to small annihilation cross-sections, but cannot really confirm that potential annihilation signals have a dark matter source.
Also, the notion that dark matter particles which lack electromagnetic charge would produce highly energetic photons in their annihilation, is itself problematic.
The Simplicity Constraint.
It also bears noting that very simple models of dark matter, with dark matter dominated by one kind of fermion and possibly interacting via one kind of boson, tend to be better fits to the data, almost across the board, than more complex models of the dark sector. This doesn't mean that the dark sector is really that simple if it exists (e.g. no one could guess from astronomy data alone, that there were second and third generation fermions, that there were W or Z bosons, that there were eight different kinds of gluons, or that protons and neutrons were composite particles), but it does mean that the dominant particle content of a dark sector must be very simple.
We can also infer this from the fact that modified gravity models can accurately predict the reality that we observe over many order of magnitude of scale, with one degree of freedom at the galactic scale and only about three degrees of freedom at all scales. The minimum number of degrees of freedom in a modified gravity model is an effective cap on the number of particles that contribute to the dominant particle content of the dark matter sector.
Warm dark matter (ca. keV scale matter) with a dominant dark matter singlet fermion, and self-interacting cold or warm dark matter models with a dominant dark matter singlet fermion and a dominant dark matter boson, which in either case do not interact at all with ordinary matter except via gravity, remain the best fits to the data.
Thus, it seems very likely that direct dark matter detection experiments are doomed to not see any dark matter signals and that LUX will merely find nothing and extend the exclusion range in parameter space for dark matter particles.
The exclusion of so much of the parameter space of plausible dark matter candidates is one of the important reasons that gravity modification theories to explain dark matter are more plausible now than they used to be in the 1980s when dark matter theories were formulated and became dominant.
In contrast, in the 1980s, SUSY provided theoretically well motivated dark matter candidates with the right properties in multiple respects to fit what was then known about dark matter from cosmology models and much more crude predictions about dark matter haloes, when small scale structure problems with the cold dark matter paradigm weren't known, and when LUX hadn't excluded so much of the WIMP (in the narrow sense) parameter space. The remaining dark matter parameter space is no longer a good fit to the SUSY particles that had been hypothesized to be dark matter candidates.
For example, even if warm dark matter with keV mass particles is the right solution, there are really no SUSY particles that can serve in this capacity.
Indeed, ultimately, LUX is more of a blow to SUSY, by ruling out light dark matter candidates that would have a signal at the tested dark matter particle masses if SUSY was correct, than it is to the dark matter particle hypothesis in general.
This doesn't mean that the dark sector is really that simple if it exists (e.g. no one could guess from astronomy data alone, that there were second and third generation fermions, that there were W or Z bosons, that there were eight different kinds of gluons, or that protons and neutrons were composite particles), but it does mean that the dominant particle content of a dark sector must be very simple.
Suppose you were living in a dark sector universe, trying to figure out the laws if our universe just through the effects of gravity, how much do you think one could figure out? Do you think the basics of electromagnetism would be within someone's reach? And would electromagnetism be relevant enough to have an effect on the overall structure we observe (ie through its effects on intergalactic and intragalactic gas clouds)?
You could probably discern electromagnetism because it would affect the cross-section of interaction of non-dark matter with itself and would affect, for example, the scattering of interstellar gas. You might not get the form of the force law right, but it would probably be discernible in some respect.
One of the main ways that we know that dark matter, if it exists, is probably not baryonic, is because it does not exhibit the scattering that it would if it was charged under EM.
One of the main ways that we know that dark matter, if it exists, is probably not baryonic, is because it does not exhibit the scattering that it would if it was charged under EM.
Right. I would think the Bullet Cluster suggests that Dark Matter (if it exists) doesn't form gravitationally bound objects past a certain scale - or if it does, those objects are uncommon or easily disrupted. Though the microlensing searches for MACHOs suggests that too I guess. Obviously electromagnetism is essential to star formation for baryonic matter, but I think that's an additional constraint against something like electromagnetism.
How about the Pauli exclusion principle? Could dark matter become trapped in compact objects?
IIRC, there is evidence in the Bullet Cluster that suggests dark matter self-interaction with a force roughly comparable in magnitude to EM.
I don't think that the Bullet Cluster argues against DM not forming gravitationally bound objects beyond a certain scale. It is huge and its collision will probably eventually form a combined two cluster system. Clusters are especially rich in inferred DM.
"I don't think that the Bullet Cluster argues against DM not forming gravitationally bound objects beyond a certain scale."
You're right. I was thinking if the Dark Matter didn't behave like stars that had been subject to gravitational scattering then that would tell us something about their mass distribution, but that's not the case here.
"IIRC, there is evidence in the Bullet Cluster that suggests dark matter self-interaction with a force roughly comparable in magnitude to EM."
Interesting. If you know the source I'd be interested in reading it. I'll dig for it myself.
Any thoughts on the "dark" globular clusters around Centaurus A?
"Any thoughts on the "dark" globular clusters around Centaurus A?"
Some of the bullet cluster self-interaction data is http://arxiv.org/PS_cache/arxiv/pdf/0704/0704.0261v1.pdf although I don't find the comparison to EM which must be somewhere else.
I have been watching too many dark matter and other cosmological mysteries documentaries in the last months and one day, when contemplating the behavior of dark matter in the Bullet Cluster, it dawned to me this hypothesis about dark matter: it behaves as it would the sum of many parallel universes, as it if it would not be "here" at all, yet it is "here" in a way only gravitons leaking between universes can.
What if it doesn't just behave like that but actually is that?
I know Andrew won't like this kind of thinking: parallel universes, extra dimensions, bleh! But in fact, if correct, it would allow us for the first time to get information from outside our universe via "dark matter" analysis, something unthinkable by any other conceivable means.
Maju - that's called brane cosmology. I actually came here just now to ask Andrew how seriously he takes that as an option lol.
I was rescuing that idea from my memory, so only after writing I realized that it is directly related with M Theory, because all other particles are "open strings", attached to our "brane" by the ends but gravitons are not (closed strings instead) and therefore they do leak all the time between universes. I knew that much from M theory but it's not consolidated knowledge, so hard to connect with other stuff, as I'm not physicists nor cosmologist at all, except in rare days.
However until considering "dark matter" in the context of the "Bullet Cluster" (particularly the animations that show how dark matter is totally unaffected by the collision) it didn't dawn to me that it looked like dark matter was traveling in parallel spaces, rather than ours.
So then I loosely linked it with the problem of inflation (also related to dark matter) and early galaxy formation and vague ideas about the Big Bang producing maybe many parallel universes instead of just one (there are theories out there, just not sure which one is it, my knowledge is quite fragmentary), universes separated from us in the "curled up" "higher" dimensions but otherwise sharing space-time proper with us. My idea is that, as dark matter is needed for inflation and galaxy formation, the formation of galaxies in the parallel universes should be roughly the same as in hours: variance may and should exist in detail but not in the overall structure of the universes.
Another issue is that, per Chaos Theory, small differences in the initial conditions can and should produce large unpredictable differences in the long run. But this would probably not affect the general gravitational structure, the big scale of things, because "dark matter", being in my interpretation the "transdimensional" gravity of many universes (~6 per the estimated universal ratio but local rations may be much greater, so uncertain), would keep the structure glued. I may well be wrong on this but it should still work as basic principle of understanding on top of which to describe the local differences and ponder about the implications of the irregularity of DM distribution for the parallel universes and the multiverse set of all them, which is by virtue of all this interconnected ("only" by gravity but that's no petty matter in the big picture).
I'm sure that someone else has described all this better than I can but I haven't really found the reference that talks of all this in a single coherent theory, so I have to use my own words and ideas.
Note: when I wrote "... universes separated from us in the "curled up" "higher" dimensions but otherwise sharing space-time proper with us"... I have the impression that I'm not using the correct words, feel free to correct me.
I just realized that Wikipedia mentions by passing a similar hypothesis to mine, referencing the CERN and T. Siegfred 1999 (but the link does not work properly and I cannot find the paper itself). So I guess I'm 17 years late for the party... but that the hypothesis has never been discarded either.
I just googled for "dark matter gravitons parallel universes" and there seems to be a lot of articles on this issue. I still do not grasp all the implications but it seems that, well, DM doesn't fit cold WIMP models (it does interact with something other than just gravity but we do not know what is it yet, just that some dark matter clusters lag behind their corresponding galaxies) and that it fits well with the brane model of "closed string" gravitons and string-related hypotheses of multiverses or "folded" universe. So I found only support for what I was saying above (the lagging DM cluster could have been affected by non-gravitational interactions affecting only another universe, just as the Bullet Cluster is only affected in ours, so DM keeps going ahead).
A related though or speculation:
The overall ratio of DM seems to be 6:1, implying in my interpretation 7 parallel universes (including ours) in the same "multiverse cluster", maybe originated in the same Big Bang and therefore strongly entangled with each other through gravity since day one. If my maths are correct (very possibly not, I'm awful at that), this fits perfectly with the notion of 11 dimensions that M theory demands: you actually need 7 dimensions to allow for 7 parallel universes, the other four dimensions are "sliced" in each universe (the three spatial ones) and time (which may be essentially the same for all the cluster).
Does this coincidence between the dark:normal matter ratio and the restriction on the number of strictly parallel universes in M theory somehow prove the latter? Or am I imagining things?
And then a fundamental question:
If gravitons do exist, how are they different from space-time itself or rather its curvature? If both explain gravity, then both must be the same or tightly related (function of each other). Thoughts?
I must correct: the dark:normal matter ratio is slightly above 5:1 (not 6:1), so, considering that a small fraction of DM can perfectly be "conventional" invisible matter such as neutrinos or black holes, would make the "local multiverse" sized 6 and not 7. So forget about my previous ranting re. the 11 dimensions of M theory, unless the extra remaining dimension is needed to give curvature (curling) to the others - anyhow we do not know the exact "trans-universal" effect of gravity, so it was very speculative to begin with.
I've found that the theory is known as "parallel dark matter" or PDM and has been floating around for many years now. It does not strictly depend on the existence of gravitons (which would need to be closed strings, what is under debate but would explain the "weakness" of gravity) but only on gravity being weak because it "leaks".
The behavior of DM as dissociated (lagging behind) from its parent galaxy in at least one case (http://www.dailygalaxy.com/my_weblog/2015/09/unknown-nature-of-our-dark-matter-universe-could-contain-rich-physics-and-potentially-complex-behavi.html) is suggestive, as much as the Bullet Cluster but in opposite meaning, of PDM.
One question: do we know the amount of DM in our immediate vicinity (Earth's orbit, Solar System)? I'm curious about how many potential transdimensional neighbors we may have locally, probably none, else DM would have been described earlier in one way or another, as it would affect gravitation in the Solar System itself.
"I know Andrew won't like this kind of thinking: parallel universes, extra dimensions, bleh! But in fact, if correct, it would allow us for the first time to get information from outside our universe via "dark matter" analysis, something unthinkable by any other conceivable means."
You are right, and I'll explain why, but first off, I want to make clear that the general kind of ideas you are having about parallel universes such as the braneworld scenarios of string theory are mostly within the range of mainstream speculation and aren't definitively ruled out.
This said, there is good reason to think that if dark matter is the correct hypothesis that it is simply made up of particles that don't interact except via gravity with other stuff in he same universe. Some of the key points:
1. The very large scale structure of matter in the universe with long filaments of matter rich strands forming cells around large empty areas are exactly what you would expect from a homogeneous Big Bang with exceedingly early post-Big Bang quantum fluctuations accounting for the patterns of varied density. If there were a parallel universe without the same focal Big Bang at exactly the same time one would expect an inhomogeneous distribution of matter clumps in our own universe that wasn't spherically symmetric. In contrast, if there was a parallel universe at exactly the same time and the same place, you would expect the initial quantum fluctuations in each universe to differ producing blurred out filaments of matter rather than rather well defined narrow ones.
2. At the galactic cluster and galaxy level, there are very significant qualitative differences between the particle interactions in the dark sector and the particle interactions in the ordinary matter sector. While there is feedback between them, ordinary matter is much, much more clumpy than dark matter, so if there are parallel universes the laws of physics and kinds of particles in the dark universe(s) is profoundly different from that found in ours. In particular, it would be essentially impossible for the dark sector to have plants, animals or rocks. No solid states of matter would be possible, only diffuse cold gas cloud like fluid structures.
3. Another qualitative difference between our universe and any parallel dark universe is that the degrees of freedom in the dark universe have to be much smaller than in the ordinary matter universe. The entire physics of the dark matter universe as experienced in our world can be summed up with not more than three physical constants. It takes far more physical constants than that to model the gravitational effects that our world has on the dark sector.
Parallel universes aren't strictly impossible, but Occam's Razor strongly disfavors that explanation since it isn't necessary to describe the evidence to date.
Thanks for answering, Andrew.
1. I just discovered the basic ideas of the work of Laura Mersini-Houghton and she actually challenges the supposed homogeneity of the background radiation field at large scales and that's probably why Brian Green doesn't invite her to debates: she would challenge Linden and Guth. However she is not only supportive of the multiverse but also of entangled universes and of course M theory - but also quantum mechanics at its best: she's a radical factualist and that looks very promising. I have to learn more though but it seems a very good lead.
2. Does that objection takes in consideration that we'd be dealing not with one but several parallel universes, whose gravitational effects we perceive only cumulatively (and maybe also filtered by whatever alterations the "gravitational leaking" process may cause)? Anyhow, of course there is no strict need that other universes are like ours at all except at macro-gravitational level, i.e. general organization or clumping of matter at galactic scale (don't need to be galaxies though, it may be different).
However you claim that "no solid states of matter would be possible, only diffuse cold gas cloud like fluid structures". As the organization of matter in the parallel universes is tightly interconnected since day one (big bang or primordial inflation or whatever) that should behave similarly in all the parallel universes... unless the laws of physics (not gravitation, I guess, but maybe others) change between them.
3. Can you explain again this objection in ways I can understand? I'm very much unfamiliar with what you say, sorry. As far as I can discern the interactions between the dark matter (and not "dark sector": dark energy would be something else, totally unrelated to what I say) and conventional matter, i.e. between the parallel universes and ours in my model, are purely gravitational.
"Occam's Razor strongly disfavors that explanation since it isn't necessary to describe the evidence to date".
Listen to Mersini: she argues that the data demands parallel universes, and she's only talking about the background radiation, not even dark matter as far as I can tell. IMO dark matter seems to demand also parallel universes with at least a significant degree of probability, particularly as the solidity of WIMP (and MACHO) tentative pseudo-conventional explanations become less and less likely.
We actually should not talk about "dark matter", as nobody has ever observed it in any way, but about "dark gravity", whatever its source. This source is very possibly some sort of matter, but whatever else that alters the curvature of space-time (or emits gravitons if we go quantum) would do: that's the only constraint, there is no box.
This said, there is good reason to think that if dark matter is the correct hypothesis that it is simply made up of particles that don't interact except via gravity with other stuff in he same universe.
Wouldn't that be a description of matter tat exists on a different brane? Wouldn't the Standard Model Lagrangian (subject to corrections) be more or less the definition of our brane?
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