A month ago, I had a post about how the search for dark matter was proving to be frustrating with one negative result after another, prompting increased speculation that an alternative theory might be necessary. The hope had been that experiments using more sensitive detectors might prove successful. But the LUX (Large Underground Xenon) experiment in a deep underground mine in South Dakota failed to find evidence of dark matter in the form of WIMPs (Weakly Interacting Massive Particles), the theoretically favored dark matter candidate. The abstract of the paper published on January 11, 2017 in Physical Review Letters says:
With roughly fourfold improvement in sensitivity for high WIMP masses relative to our previous results, this search yields no evidence of WIMP nuclear recoils. At a WIMP mass of 50 GeVc− 2 , WIMP-nucleon spin-independent cross sections above 2.2×10− 46cm2 are excluded at the 90% confidence level.
Negative results from the two most sensitive detectors to date, LUX and PandaX-II in Sichuan, China, has increased the level of skepticism about dark matter, at least in its most common formulation.
If you lost your car keys, you might first check your coat pocket and, if you didn’t find them there, search your kitchen, the car, and so on. Similarly, the null results from LUX and PandaX-II have ruled out some of the parameters (mass, cross section) that could characterize dark matter, telling researchers at direct-detection experiments that they should look elsewhere. They have told us that dark matter interactions are much rarer than suggested by many popular hypotheses. Indeed, with these new null results and the lack of evidence for supersymmetry at the LHC, a number of physicists have started to question the WIMP hypothesis, at least in its simplest form.
Paul Kroupa is a physicist and long-time dark matter skeptic who argues that the failure of the LUX experiment to find anything is a sign that perhaps it may not exist and that we should look for alternative explanations such as MOND (Modified Newtonian Dynamics) for the phenomena whose causes were attributed to it.
What can it be replaced with? The first step is that we need to revisit the validity of Newton’s universal law of gravitation. Starting in the 1980s, Mordehai Milgrom at the Weizmann Institute in Israel showed that a small generalisation of Newton’s laws can yield the observed dynamics of matter in galaxies and in galaxy clusters without dark matter. This approach is broadly known as MOND (MOdified Newtonian Dynamics). Milgrom’s correction allows gravitational attraction to fall off with distance more slowly than expected (rather than falling off with the square of distance as per Newton) when the local gravitational acceleration falls below an extremely low threshold. This threshold could be linked to other cosmological properties such as the ‘dark energy’ that accounts for the accelerating expansion of the Universe.
These links suggest a deeper fundamental theory of space, time and matter, which has not yet been formulated. Few researchers have pursued such an alternative hypothesis, partly because it seems to question the validity of general relativity. However, this need not be the case; additional physical effects related to the quantum physics of empty space and to the nature of mass might be playing a role. MOND also faces its own challenges, both observational and theoretical. Its biggest drawback is that MOND is not yet well-anchored to general relativity. Because of the prevailing dark-matter dogma, few scientists dare to build on Milgrom’s ideas. Young researchers risk not getting a job; senior researchers face losing out on grants.
Many CWRU physicists that included some of my friends and colleagues were part of the LUX team and I am sure that they must be deeply disappointed that their hard work over so many years has failed to detect a positive signal. But that is the way with science. As the Rolling Stones song goes, “You can’t always get what you want”.
“But if you try sometimes well you might find you get what you need….”
Federico Lelli and Stacy McGaugh, both with CWRU, seem to have done a lot of work in this field. McGaugh’s 2014 review of ΛCDM (Λ-Cold Dark Matter, where Λ is the cosmological constant; the standard model of cosmology) versus MOND can be read here.
It’s exciting that nobody’s able to declare “the end of physics”!
The thing is, we have no reason to think that it would be found where they looked. Without any knowledge (or even hints) as to how it would couple to normal matter, I don’t see how a failure to find anything means anything.
ahcuah@#4:
Without any knowledge (or even hints) as to how it would couple to normal matter
I don’t understand the current physics, but isn’t the way scientists detected dark matter based on gravity and relativity? So it does interact with normal matter by having mass.
Or am I missing something? I mean, for dark matter to bend light, it’s there, right?
Marcus,
While dark matter has mass and makes its presence felt via gravity, the force of gravity is so weak that it can only be detected if at least one of the interacting masses is huge. The gravitational effects of individual elementary particles are so small as to be undetectable at our present level of technology. But dark matter particles are currently thought to be WIMPs (Weakly Interacting Massive Particles) and the hope is to detect them via the weakly interacting part instead of the part that depends on the mass. This is what those two experiments were trying to do.
Seconding what Mano said, the only reason we think dark matter might exist is because of the way it interacts gravitationally. However, that is presumably due to zillions of very low mass particles because the gravitational interaction of any fundamental particle is horribly weak. So, to detect them in any other fashion (i.e., to try to confirm their existence) requires that they couple to normal mass in some other way. As Mano says, the thought is that this is through the weak interaction. But without any theoretical knowledge of what they are, we really don’t know the coupling strength. Or they could even possibly couple through some new/different superstring interaction. But it really is all guesswork at this point.
To Marcus Ranum
Thus far, we only know about dark matter from (indirect) cosmological measurements. The best evidence is the Bullet Cluster. IIRC, there’s a few other known examples of colliding galaxies, but the Bullet Cluster is the most famous example.
In the Bullet Cluster, there are these two galaxies which collided about a billion years ago. Being made of stars, the galaxies mostly just went through each other, suffering some deformation from the gravitational effects. Most of the normal mass of a galaxy is the interstellar dust. By using non-visible wavelength light astronomy, we can locate where that dust is, and it’s in between the two galaxies, at the point of collision. The dust is not a bunch of point-like masses that will just pass by each other during collision. The dust will collide, and stay in the middle.
In the case of the Bullet Cluster, there are some galaxies way behind the two collided galaxies. By using fancy computational analysis, we can look at the images of the galaxies that are behind the two colliding galaxies, and using the equations of General Relativity (the matter of the two collided galaxies will bend the light from the other, further away galaxies), we can construct the mass distribution graph for the matter in the two collided galaxies. The results of that computation show that most of the matter is with the stars. But we know most of the normal matter is not with the stars -- it’s in between the two galaxies, with the interstellar dust. Hence, there must be some more matter that we don’t know about -- a lot more -- and using other fancy arguments, we know it’s not normal matter. Hence, dark matter.
So, to get rid of dark matter, you need to invent a theory of gravity where the gravity does not come from the location of most of the normal matter, the interstellar dust. Or you need to show that this story that I just told about the Bullet Cluster is wrong (and you would need to show that a similar story concerning a signal in the cosmic microwave background radiation is wrong). Thus, it seems that we’re stuck with dark matter.
EL @8: Sabine Hossenfelder would like a word;
I barely got out of high school physics alive so I know nothing about any technical facts about Black Matter. It reminds me, a bit, of Joseph Priestley”s Phlogiston which seems to have worked well for several years until Lavosier came up with a better theory.
To Rob
I’m just repeating what I take to be the scientific consensus. I also tried to include language that said that the consensus is less than certain, and there’s room to maneuver. Apparently I fail there, sorry.
Quoting from that link:
First, let me say that I’m in over my head. Let me try to channel Sean Carroll. If I’m reading this right, and if I understand the theory right, the author is suggesting that we can introduce new quantum fields in quantum field theory. Further, if I’m reading this right, this is suggesting that these new fields might have high values where “the matter would be if it didn’t collide”. So, what should we call a new field that couples with gravity in this way? It’s dark matter! This is what dark matter is, a new quantum field that doesn’t interact with electromagnetism, but does couple with gravity.
Apologies if I made any mistakes. Please correct me.
EL @11:
No, it’s dark matter which requires new quantum fields added to the standard model. Modified gravity is adding classical fields to (classical) general relativity. These are the additional fields Hossenfelder is talking about.
An important point is that the bullet cluster also presents problems for a dark matter picture, which seem to be often glossed over. As the authors of this paper point out
To Rob
Ok. So, they’re classical and not quantum. What about my major point? The description of these new fields sounds very much what I would call “dark matter”. It’s a new field throughout space that couples with gravity (I’m sure I butchered that), and which evolves independently of the fields of normal matter. That’s dark matter, isn’t it?
Pedantic: It provides difficulties for a particular model of dark matter, but not the mere dark matter hypothesis.
re #12:
That should be “it’s particle dark matter which requires…”.
To Rob in 14
Ah, I see. An important clarification.
EL @13:
How so? On the one hand (particle dark matter) you’re talking about a quantum field coupling to ordinary matter in the standard model. On the other (modified gravity), you’re talking about a field coupling to gravity which wouldn’t require quantization to explain low-energy behaviour. These are very different beasts.
No, it provides difficulties for the ΛCDM model, which doesn’t (AFAIK) distinguish between particular DM models.
It’s a new field throughout space, which can take on different values throughout space. Places where this new field has high values acts largely indistinguishably from a large collection of normal matter in terms of gravity / bending of spacetime. Otherwise, the field does not interact with other fields (classical or quantum), or the interactions are weak. To my completely untrained ear, as a member of the lay public, this sounds more or less like what has been sold to the lay public as dark matter. The distinction between particle dark matter and this “field” continuous dark matter is important, I suppose, but it doesn’t change what it is: introduction of a lot of new matter. To call this a modification of gravity that removes the need for dark matter is IMHO just wrong. MOND is a modification of gravity. In that theory, gravity still comes from where the mass is, and it’s not adding new mass. In this alternative where we’re adding new fields that couple with gravity, the new field is dark matter -- we’re changing the location of mass by adding more of it.
I’ll add that since modified gravity must reduce to MOND in the non-relativisitic limit, it is not equivalent to dark matter in the senses that any of these terms are used by the practitioners. You can argue that point with them if you like.
EL @17:
You’re not adding more mass. You’re changing the geometric properties of spacetime. If you want to show that this is the same as introducing new matter fields, you have to actually show your work.
And now I admit that I’m in completely over my head. I defer to your judgment for now. No contest.
jkrideau @#10,
The basic idea behind phlogiston, that when something burned, it released a substance and the remainder left behind consisted of more elementary ingredients, was initially proposed as early as the mid-17th century. The idea was made more concrete and the emitted substance given the name phlogiston by the German scientists G. E. Stahl in 1731. Priestly, who worked largely in the second half of the 18th century, was a believer in the phlogiston theory and tried to explain phenomena using it even as it became more difficult to do so. By the early 1780s, though, Lavoisier’s competing oxygen model had become too compelling to ignore.
Thanks for explaining to me. It’s tough when someone doesn’t even have the necessary knowledge to ask the right question -- and giving a useful answer in that case can be challenging. I appreciate it! I took intro physics in 1982 and never went beyond that, aside from listening to a few Feynman lectures. So the whole “what is gravity?” (interactions) was what was going over my head. I get the explanation that dark matter’s interactions appear to be weak, therefore the experiments were set up to attempt to measure weak interactions; that makes sense. It was rather humbling for me to realize, as I was trying to formulate my question, that -- for me, anyway -- gravity may as well be “divine will.” I have no idea what it is or how it works but I (clearly) accept it as some mysterious power.
In this context, I believe “weak interaction” means “interaction via the weak nuclear force”, not “interactions which aren’t very strong”. It’s confusing because gravity is a weak force, but it’s not the weak (nuclear) force. The word “weak” is being heavily overloaded here.
Physicists are terrible at naming things.
Dunc @23:
Definitely. One of my favourites is coupling constant. They’re neither couplings (since they link at least three fields), nor constant (they vary with interaction energy).
It goes to show that even the best informed and most educated can be as dogmatic as the narrowest of mind. All ideas should be encouraged and investigated, not excluded because someone has a pet theory. It took 50 years for old geologists to die before Alfred Wegener’s theory of plate tectonics became widely accepted depite the argument and evidence being the same.