Did the LUX Dark Matter Experiment Fail to Detect Dark Matter?

[Drakkith]

I'm just looking for extreme examples of how it makes visible matter behave or examples of galaxies without much of it.

You could look at the bullet cluster perhaps, but I'm not sure that's what you were looking for. I'm afraid I can't help you much.

 

[f todd baker]

Although I am no expert in cosmology, I have always thought that "dark matter" is just as likely to be a signal that we do not understand gravity as well as we pretend to than a manifestation of some kind of mysterious new particle. Only on very rare occasions have I seen reputable cosmologists suggest such a thing. In this thread I see some hints of this point of view.

 

[Drakkith]

Although I am no expert in cosmology, I have always thought that it is just as likely that "dark matter" is just as likely to be a signal that we do not understand gravity as well as we pretend to than a manifestation of some kind of mysterious new particle. Only on very rare occasions have I seen reputable cosmologists suggest such a thing. In this thread I see some hints of this point of view.

Modifications to gravity have been considered and extensive investigations have been carried out. That particular possibility is summed up in what is known as "Modified Newtonian Dynamics", or MOND. Unfortunately the evidence is more in favor of dark matter over MOND, though not enough to discount MOND completely.

Edit: Also remember that we already know of particles which are very close to being dark matter: neutrinos. Neutrinos interact only via the weak force and gravity, so it isn't that big of a stretch to think of a particle that interacts solely via gravity. When you look at all the different particles and notice that some interact via all four forces (quarks), some via three (electrons), and some via two (neutrinos), it seems very natural to have a particle that only interacts through a single force. That's my opinion at least.

 

[mfb]

Edit: Also remember that we already know of particles which are very close to being dark matter: neutrinos. Neutrinos interact only via the weak force and gravity, so it isn't that big of a stretch to think of a particle that interacts solely via gravity. When you look at all the different particles and notice that some interact via all four forces (quarks), some via three (electrons), and some via two (neutrinos), it seems very natural to have a particle that only interacts through a single force. That's my opinion at least.

If dark matter does not participate in the weak interaction, then it is unclear how it could have been produced in the right amount.

 

[sharpnova]

If dark matter does not participate in the weak interaction, then it is unclear how it could have been produced in the right amount.

For you to make such a statement implies that you have a deep understanding of how dark matter works.

Please expand on that.

 

[RUTA]

Here is a video from Sean Carroll on DM that you might find interesting

 

[Vanadium 50]

If dark matter does not participate in the weak interaction, then it is unclear how it could have been produced in the right amount.

If dark matter is made of axions, you can produce them in the early universe. Their production mechanisms are not elementary. I bring this up to point out that there are DM candidates that are uncharged under SM interactions.

 

[Drakkith]

If dark matter does not participate in the weak interaction, then it is unclear how it could have been produced in the right amount.

Indeed. Dark matter is quite the conundrum.

 

[Chronos]

The best we can do at present is describe DM in terms of phenomenology. This is not unlike the symptomatic diagnosis of an unknown disease. At first you can only describe its effects. DM is like a persistent cough. A myriad of underlying causes are possible, but, isolation requires deeper examination, which enables us to deduce what it is not, and eventually eliminate enough possibilities to deduce what it is. We are still at that stage of diagnosis. MOND is just another piece in what is not necessarily even the same puzzle. It works pretty decent on galactic scales, but, not so much on larger scales or under peculiar circumstances.

 

[arupel]

I read of one CERN physicist speculating that the gravitational force between matter and anti matter may be repulsive rather than attractive. Could this possibly have any cosmological significance, either as dark matter or dark energy?

Also have there been any thinking about GR and anti matter?

 

[mfb]

We don't have a direct experimental result yet, but everyone expects antimatter to fall down in the same way matter does. The mass of all everyday objects is ~1% from elementary matter particles and ~99% from QCD binding energy, and we know both lead to things falling down in the same way. Antimatter has 1% of its mass from elementary antimatter particles and ~99% from QCD binding energy. Why should the same thing suddenly behave differently?
Any deviation would also break general relativity.

There would be no direct cosmological significance as we don't have relevant amounts of antimatter in our universe. "GR is wrong" would have a huge indirect impact, of course, as we would need a new theory, which could also influence our understanding of the early universe.

 

[Tony Hambro]

We can't see dark matter so it must be completely transparent. If we know it's there because it has mass, which affects our gravitational computations, can we work out what a cubic KM of it weighs? If it has mass, must it have Higgs bosons? Energy and Mass are inter-related/interchangeable (E=MC2) so could it be a unrecognised form of energy? (The thoughts of an amateur, not weighed down with much algebra).

 

[mfb]

The density of dark matter depends on the place, we know the approximate density close to our solar system, roughly 0.5 GeV/cm³ (half the mass of a proton per cubic centimeter).

Its mass could come from interaction with the Higgs field, but other sources are possible as well. "Having Higgs bosons" does not make sense.

Energy and Mass are inter-related/interchangeable (E=MC2)

That is a misleading statement. It is better to say "mass has energy". Dark matter has mass, so it has energy. We did not find dark matter particles yet. Does that count as "unrecognised form of energy"?

 

[Kevin McHugh]

This experiment was not set up to detect a gravitational interaction, it was set up to detect the weak interaction.

But what if gravitation is the only interaction? Why is it assumed that DM has weak interactions? Not all matter spontaneously decays.

 

[ProfChuck]

Ruling out properties can be as useful as confirming them. Both provide constraints on the model. Is dark matter new physics or just ordinary matter that is difficult to observe. The recent discovery of large black hole populations in globular clusters is significant and requires an adjustment to the mass models. It is not clear, yet, if this will influence the DM distribution model.

 

[Chalnoth]

But what if gravitation is the only interaction? Why is it assumed that DM has weak interactions? Not all matter spontaneously decays.

It's difficult to write down a model where the right amount of dark matter is produced in the early universe if there are only gravitational interactions. I'm sure it's not impossible, but it's not easy.

Perhaps more importantly, we don't have any possibility of detecting a dark matter particle that only interacts gravitationally within the forseeable future. It's perfectly sensible to search for possible particles we do have the possibility of detecting in the mean time.

 

[ohwilleke]

It's difficult to write down a model where the right amount of dark matter is produced in the early universe if there are only gravitational interactions. I'm sure it's not impossible, but it's not easy.

Perhaps more importantly, we don't have any possibility of detecting a dark matter particle that only interacts gravitationally within the forseeable future. It's perfectly sensible to search for possible particles we do have the possibility of detecting in the mean time.

The difficulty is not so much in writing down a model where the right amount of dark matter is produced in the early universe, as it is writing down a model where the dark matter that is produced in the early universe gets distributed in that manner that we observe it to be distributed today when we infer its location from gravitational dynamics.

If a particle interacts solely via gravity, you have only two free parameters, its mass and its mean velocity, and in the case of "thermal dark matter" (i.e. models where all dark matter is created shortly after the big bang and then is stable after that), those two parameter are degenerate, because mean velocity is a function of "freeze out temperature" which is a function of particle mass.

On the up side, since we know the total amount of dark matter in the universe, and we can determine mean velocity and the number of dark matter particles in the universe simply by dividing by particle mass, that gives us a nice finite range of singlet dark matter models to investigate. And dark matter researchers have done just that.

We know for certain that the mean velocity of dark matter can't be too high, which is called "hot dark matter" because if it were there would be far less structure in the universe. In the case of thermal dark matter where mass and velocity are degenerate, that corresponds to dark matter particle masses on the order of 1 eV/c^2.

A number of investigators have concluded that "cold dark matter" is also ruled out because in simulations this produces the wrong shaped dark matter halos and too much structure (e.g. sub-halos and satellite galaxies). The masses associated with mean velocities that correspond to cold dark matter in a thermal dark matter model are on the order of 1 GeV/c^2 and up, with 100 GeV/c^2 having been the type specimen of cold dark matter. But, once dark matter is cold, simulations aren't terribly sensitive to just how cold it is over a wide mass range.

The investigators who have concluded that cold dark matter is ruled out favor "warm dark matter" as the key to reproducing the phenomenology that is observed with warm dark matter defined as mean velocities associated with masses in a thermal dark matter scenario on the order of single digit keV/c^2 masses.

In state of the art simulations as of two or three years ago, WDM totally stomps CDM in terms of how well it reproduces what is observed in real life. (N.B., both WDM and CDM qualify as CDM for purposes of the definition used in the lambdaCDM standard model of cosmology which has a quite broad definition of CDM compared to what people trying to pin down particular dark matter particle models use to define those terms.)

But, the last word has yet to be spoken in the CDM v. WDM debate, because the models used to simulate the two kinds of purely gravitational dark matter had some serious flaws and didn't adequately take into account the feedback effects between ordinary baryonic matter such as stars and planets and interstellar gas and dust, and dark matter. This is clearly a problem, because in real life, dark matter halo shapes are tightly correlated with the distribution of ordinary luminous matter in the system.

Considering gravitational feedback between ordinary matter and dark matter reduces the amount of difference between the shape of dark matter halos in CDM models and the shape of dark matter halos in WDM models, although there is also dispute over whether the ordinary matter feedback in the simulations is correctly modeled. There is also dispute over whether any of these models are valid because some of the assumptions made may be wrong or lack of good physical basis -- you have to insert a lot of assumed rules about the non-gravitational interactions (e.g. supernovas and active galactic nuclei) that play a part in the gravitational clumping of matter to make these models work because they are highly oversimplified versions of real life.

However, one real important conclusion that was reached quite a few years ago with these models is that singlet dark matter models (with or without self-interactions between dark matter particles via a Yukawa force carrying boson that only, or predominantly, interacts with dark matter with a strength on the same order of magnitude as the electromagnetic force) more closely reproduce the distribution of dark matter that we observe than models with multiple kinds of dark matter at different masses.

Now, this doesn't mean that there has to be only one possible kind of dark matter, any more than the Standard Model implies that there has to be only two possible kinds of ordinary baryonic matter (protons and neutrons) that are very similar to each other for many purposes (there are in fact, hundreds of possible hadrons, but almost all of them are unstable). But, it does mean that the one kind of dark matter particle, or multiple kinds of dark matter particles with nearly degenerate velocities and masses that can be modeled well as one kind of dark matter particle, must make the predominant contribution to the dark matter phenomena that we observe.

This line of reasoning is also corroborated by the fact that dark matter distributions can be accurately described over many orders of magnitude with toy models like MOND that have just one degree of freedom. This doesn't mean that MOND is correct by a long shot, but it does mean that if you need more degrees of freedom to describe the same data with a dark matter particle model that your model is probably too baroque. So one or two particle models are pretty much the only way to go.

The bottom line then is that the universe of possible dark matter particles that interact only via gravity has been pretty well explored, and that we are reasonably close to pinning down the best fitting singlet only gravitationally interacting dark matter particle model, and to pinning down the best fitting dark matter particle that only has self interactions with a boson of a particular coupling constant and mass model, based upon the empirically observed evidence, and to seeing which of the two is a better fit to the data.

Pretty much the only hold up to solving that problem is finding a way to do a simulation which is accurate enough that a consensus of dark matter theorists agree that it is accurate enough to distinguish between CDM and WDM and self-interacting DM models and between the DM distributions that we actually infer from the dynamics of luminous matter.

The more computing power we can throw at the problem, the less assumptions about the processes involved we have to write into the model and the easier it is for the model to make assumptions that are directly supported by observational evidence or well understood stellar and black hole dynamics. We can also improve the models by directly observing processes that are mere assumptions in the current models to calibrate those assumptions (e.g. what happens when two galaxies of particular relative sizes and shapes collide at particular angles and relative speeds and how common are different scenarios relative to each other). Most importantly, we have to make sure that we are modeling the feedback in gravitational interactions between ordinary matter and dark matter correctly. And, it would also help to have more precise descriptions of the shape of more dark matter halos in a wide variety of circumstances, which is tricky because sometimes a couple of parameters used to describe the shape of a dark matter halo are degenerate in most observations and the degeneracy can only be resolved with a few, particularly difficult to observe, data points that require expensive space telescopes to see.

Unfortunately, there are so many debatable points in current state of the art simulations, that there is a high probability that I will be dead, and a decent probability that my children will be dead, before this can be sorted out definitively.

Eventually, however, one of the three possible gravitation only models will be the winner, or, alternatively all three will be excluded by empirical evidence and we'll have to see if we can either come up with a dark matter model that interacts by some means in addition to gravity that has evidence to support it (such as dark matter annihilation signatures in cosmic rays), or a non-thermal dark matter model where dark matter is routinely created and destroyed and has a characteristic velocity and stable total quantity (like axion dark matter models), or come up with a gravity modification models that can fit the empirical evidence.

Or we might find that none of our models can recreate what we observe, in which case it is back to the drawing board. But, a null result that rules out all plausible models to modify gravity or have particle dark matter is pretty unlikely, because a handful of empirical phenomenological formulas can describe pretty much all observed dark matter phenomena and we just have to figure out how to come up with a model that sews them all together to produce those results.

As they say in the Publisher's Clearing House sweepstakes, one of these theories "may already be a winner" and we just don't know it yet, because we don't have enough data and computational power to confirm this conclusion.

 

[rootone]

Are you familiar with the work of Douglas Adams?
I'm sure what you just said is in the 'Hitch hikers guide to the galaxy' somewhere;

 

[ohwilleke]

Are you familiar with the work of Douglas Adams?

Of course.

I'm sure what you just said is in the 'Hitch hikers guide to the galaxy' somewhere;

I'm going to take that as a compliment and call it good.

 

[ohwilleke]

If dark matter does not participate in the weak interaction, then it is unclear how it could have been produced in the right amount.

Honestly, get creative.

Particles can be created in purely EM and purely strong force interactions, not just weak force interactions, and surely particles can be created by some as yet unknown force associated with dark matter, or by the high energy BSM interactions of one of the four known forces, or by a high energy Higgs portal. Indeed, the optimal theory would create dark matter particles only in the circumstances of the very early universe since it seems that no net new dark matter has been created for billions of years.

Any dark matter particle involves beyond the Standard Model physics which could involve new forces, or properties of existing forces that only manifest at extremely high energy scales that haven't been seen since the earliest moments after the Big Bang. For example, maybe at the GUT scale, when all three Standard Model forces are unified, the unified force boson can decay to multiple dark matter particles, or maybe dark matter particles are produced by an analog to the weak force (e.g. a right handed weak force) that only interacts with particles in the dark sector, or maybe dark matter particles are produced by the decay of one or a pair of extremely high energy gravitons. Any other scenario I've seen is to assume that dark matter arises from the decay of the inflaton, an extremely massive particle associated with cosmological inflation.

Any seriously motivated theoretical physicist can probably come up with three different ways to solve that problem before breakfast.

Also, even if we don't know how it could have been produced in the right amount, so what?

We also have no real clue how the baryon number of the universe or the lepton number of the universe, or the total mass-energy of the universe ended up taking the values that they do. Indeed, we don't even have any real clue what the total lepton number of the universe even is because we don't have any credible estimates of the ratio of neutrinos to antineutrinos in the universe. So, if we don't know how we came to have X many dark matter particles in the universe, that would just be one new problem to add to a heap of similar problems that we also have no answer to at this time.

 

[Orodruin]

If a particle interacts solely via gravity, ... "thermal dark matter" (i.e. models where all dark matter is created shortly after the big bang and then is stable after that)

This is not an accurate description of thermal dark matter. Dark matter is produced shortly after the Big Bang in any viable dark matter model and it does not have to be thermally produced. Axions are a prime example of this.

In addition, I strongly doubt a dark matter candidate with only gravitational interactions would be thermally produced anywhere below the reheating scale.

On the up side, since we know the total amount of dark matter in the universe, and we can determine mean velocity and the number of dark matter particles in the universe simply by dividing by particle mass, that gives us a nice finite range of singlet dark matter models to investigate. And dark matter researchers have done just that.

Reference please. Dividing the density by the mass gives you the number density, not the velocity.

 

[ohwilleke]

This is not an accurate description of thermal dark matter. Dark matter is produced shortly after the Big Bang in any viable dark matter model and it does not have to be thermally produced. Axions are a prime example of this. In addition, I strongly doubt a dark matter candidate with only gravitational interactions would be thermally produced anywhere below the reheating scale.

I have used the terms "thermal relic" and "relic" interchangeably, which is sloppy. A paper suggesting ways to distinguish between thermal and non-thermal DM with experimental constraints is here: https://arxiv.org/pdf/1311.5297.pdf

A rather comprehensive recent review of axion dark matter that explains and/or refers to papers that explain how axions could be non-relativistic despite being produced in the early universe can be found at http://iopscience.iop.org/article/10.1088/1367-2630/11/10/105008 although to be honest, it is not the most readable presentation. A more readable discussion is here: http://web.mit.edu/redingtn/www/netadv/specr/345/node3.html

I had thought, perhaps mistakenly, that axions could also be an example of dark matter that is not just produced shortly after the Big Bang, because it has to be constantly produces in day to day QCD interactions to keep the CP violation constant of the strong force theta, naturally or near zero, which would also allow it to have non-relativistic velocities (as any viable dark matter candidate must) despite having a mass less than that of hot dark matter neutrinos. But, I am too tired to run down a reference for that at the moment.

While I'm at it, a generalized and somewhat outdated case for the WIMP miracle involving thermal relic WIMPs can be found at http://web.mit.edu/redingtn/www/netadv/specr/345/node2.html

FWIW, I am highly unimpressed by the "natural" motivations for both the axions (on the theory that the zero or nearly zero CP violation of the strong force is unnatural) and for SUSY which would naturally solve the "hierarchy problem", both of which have at their foundations a scientists presumption about what physical constants Nature should have that really have no meaningful scientific basis and are mere guesswork, neither of which have born any fruit to date.

Reference please. Dividing the density by the mass gives you the number density, not the velocity.

Many papers do the analysis in a model dependent manner specific to WIMPs but a more general model-independent analysis can be found, for example, in the following 2014 paper. http://arxiv.org/pdf/1309.6971.pdf (Note that I am not citing this paper in support of the claim that everything in it is true, merely because it is an example of many that lays out the basic equations involved in the mass-velocity relationship relevant to dark matter, which it does preliminarily to reach its further conclusions.)

Another fairly general model-independent analysis that focuses on the free streaming length of DM which phenomenologically has the same sort of impacts that could be inferred from velocity is here: http://chalonge.obspm.fr/Dark_Matter.pdf

The relationship between free streaming length and velocity is spelled out here: http://www.thphys.uni-heidelberg.de/~smp/view/Delta09/Slides_rubakov.pdf

 

[Timedial]

Auv = +- Guv = Tuv

Toy theory minor tweek on Einsteins GR gives both dark energy, dark matter like properties

 

[PeterDonis]

Toy theory

Can you give a reference for this? Please bear in mind PF rules regarding personal theories.