In an email exchange, Dr. Massey, of Durham University in England, explained the story of how the two seemingly contradictory studies came to be and what the results really tell us about dark matter, which has been seen through a decade of research as doing nothing: “Over the past decade, there have been many negative results about dark matter, saying that it interacts less than this … less than a slightly lower number … less still.”
The “does not” and “does” collaborative studies between Massey and Dr. David Harvey, of École Polytechnique Fédérale de Lausanne in Switzerland, delimit both the “does not” interaction level and the now newly known “does” level. Massey explained: “Together, they give a result that dark matter interacts more than this, but less than that. For the first time, we’re squeezing it from top and bottom.”
Collaborative Effort
Massey began chasing dark matter by asking “Where is dark matter?” and proceeding to find it. During his tenure at California Institute of Technology his work resulted in the first 3D map of dark matter. Using Hubble Space Telescope data, he found where dark matter is clustered in the universe. In collaboration with Dr. Harvey, Massey turned his questioning to “What is dark matter?” He and Harvey, then Massey’s doctoral student, sought to find a way of finding the outer edges of what dark matter does, the outer limits of its interactions. To quote Massey from his email:
“David Harvey was my PhD student until last year, so we were very much working together (and we still are, even though we’ve both moved to new cities). The first result was that dark matter interacts very little. The second result is that it interacts very, very little … but not zero.”
With heads together, they tossed notions about till they lit upon the more-than-less-than bookends means to uncover an invisible particle in the dark hearts of what are among the largest structures in the universe as they crash and collide, tearing through each other and, hopefully, disturbing the dark stuff unseen within.
Summaries of Two Studies
Introduction
It’s known that dark matter interacts with gravity. Observations of the effect gravity has upon it are documented through gravitational lensing. One question, leading researchers closer to what dark matter is, is what else does dark matter interact with? We know it does not interact with visible matter, that tiny percentage of visible stuff (including us) that is in the universe. But does it interact with its dark-self in any way? So far the answer has always been that it interacts with itself less than what has been observed so far. In other words, the answer has always been dark matter has not yet been observed to interact with itself.
Harvey’s and Massey’s bookend studies did two things. The first study, Harvey’s, dropped the bar even lower: dark matter interacts with itself even less than previous statistical measurements show. While the second, Massey’s, found some interaction of dark matter with itself. Dark matter does interact with itself. Its self-interaction is not zero. It is tiny but not zero.
72 Clusters of Galaxies in Collision
Premise
The premise behind Harvey’s study of collisions between 72 clusters of galaxies is that “Collisions between galaxy clusters provide a test of the non-gravitational forces acting on dark matter,” as Harvey wrote in his paper published in Science called “The non-gravitational interactions of dark matter in colliding galaxy clusters.” Using both Chandra X-ray and Hubble space telescopes, they observed “72 collisions, including ‘major’ and ‘minor’ [collision] mergers.” They detected that dark matter was present in the galaxy clusters and that it kept in close association with the stars [dark matter kept close, like a halo encircling the stars]. Clusters of galaxies can contain hundreds of galaxies.
Harvey explains in the Science paper that colliding galaxy clusters provide the same observational opportunities that “terrestrial collider experiments” provide, like the Large Hadron Collider (LHC) at CERN. As he explains: “[T]he forces acting on particles can be inferred from the trajectory and quantity of emerging material. Collisions between galaxy clusters, which contain dark matter, provide similar test for dark sector forces.”
In other words, having identified and located dark matter, the trajectories and emergent material from cluster collisions allows for the inference of the forces acting on the clusters and, specifically, on dark matter. Two forces were looked for, a force of minimum momentum and a force of maximum momentum. A minimum momentum force would slow down the progress of dark matter through deceleration and result in a lagging behind of or a drag on dark matter. A maximum momentum force would scatter dark matter away so as to diffuse its concentration around the stars.
Results
The final result was that in the absence of observable drag or scattering, dark matter interacts with itself very little in these astrophysically large particle collider collisions, collisions between galaxy clusters. In fact, the interaction of dark matter between colliding clusters of galaxies is less than previously thought: dark matter interacts with itself even less than this. Consequently, Harvey can determine that the dark matter particle seen in colliding clusters is not one that creates “strong frictional force,” as phrased in Space Telescope, causing a deceleration of dark matter, as would occur with a force of minimum momentum, nor one that causes scattering, like “bouncing billiard balls,” producing a change in the shape of dark matter clumps through dissipation, as would occur with a force of maximum momentum.
Four Galaxies in Collision
Premise
The premise behind Massey’s study of collisions between four unique galaxies in galaxy cluster Abell 3827 is built on the fact that galactic collision mergers reveal fundamental properties of dark matter by how it does or does not react to collision forces (properties can be inferred from what dark matter does not do as well as from what it does do).
The premise is further built on the observation that gas clouds in colliding galaxies — clouds of gas composed of normal matter, or “standard model particles” — lag behind both the star system and dark matter because the gas is effected by the pressure generated in the collision, if dark matter … and this is significant … if dark matter is non-self-interacting.
Massey’s study posits that interacting dark matter — dark matter in one galaxy that does interact with colliding dark matter from another galaxy — will show a similar lag as gas clouds thus will be “offset” from its original galactic halo position. As Massey wrote in his study published in Monthly Notices of the Royal Astronomical Society (MNRAS): “More interestingly still, if dark matter has (even a small) non-zero self-interaction cross-section, infalling [colliding and merging] dark matter will eventually lag behind old stars.”
Galaxy Cluster Abell 3827
Massey chose galaxy cluster Abell 3827 for his study because it has two unique characteristics. Abell 3827 contains four instances of equally bright elliptical galaxies that indicate recent simultaneous mergers resulting from galactic collisions; these are very rare. Abell 3827 is also uniquely lit by a “strong gravitational lens system threaded between its multiple central galaxies … threaded through the [Abell 3827] cluster core.” As explained in Massey’s MNRAS publication:
“[One lensed galaxy] is an almost face-on spiral galaxy, with a bulge and many resolved knots of star formation that can all be used as independent lensed sources. … [Lensing] enables the distribution of its otherwise invisible dark matter to be mapped” when lit by the lensing source; when the lit by the strong gravitational lensing system threaded between the colliding galaxies.
Gravitational lensing is important in studying space because it is an effect of gravity. Since regular matter and even dark matter interact with gravity, gravitational lensing (or simply lensing) allows the mapping of both total star mass and dark matter location and distribution. Lensing reveals the “where” of dark matter.
The exciting part
One colliding, merging galaxy, labeled N.1 (nucleus 1), in Abell 3827 shows evidence of offset of its halo of dark matter (dark matter encircles galaxies like a halo). In galactic collisions, offset in dark matter is similar to lag in galactic gas clouds and can only occur if dark matter is interacting with itself during galactic collision. As Massey wrote in his study: “This is potentially the first detection of non-gravitational forces acting on dark matter.” He further says: “Such offsets are not seen in field galaxies, but are predicted during the long infall [the merge] to a cluster, if dark matter self-interactions generate an extra drag force.”
Using Hubble Space Telescope (HST) imaging and Very Large Telescope (VLT) integral-field spectroscopy to fine-tune measurements of dark matter distribution, Massey mapped the distribution of galaxy associated dark matter and developed integral field spectroscopy of the gravitationally lensed system threaded through the cluster core.
What It All Means
Now that Massey and Harvey have the first indications that dark matter does interact with other dark matter when in collision (like particles in collisions in CERN’s LHC), they can infer from the offset of dark matter in merging N.1 that the dark matter particle collides with a force of minimum momentum: dark matter is decelerated and offset (Harvey did not see this property in collisions between clusters of galaxies). Dark matter in N.1 was not scattered and dissipated as it would be if the colliding dark matter particle had a force of maximum momentum. They now have the first hint of what Harvey and Massey called rich and complex physics.
As Massey described in his email, for particles to interact — in order for particles “to talk to each other” — they need to exchange a particle that is different from their own particle and that is produced as the result of their interaction. Massey used an analogy of tossing a medicine ball about. If we visualize Dark Matter A and Dark Matter B playing medicine ball, we see that they toss a particulate medicine ball back and forth between them — rather like Massey and Harvey tossing ideas back and forth to design their bookend-squeeze-from-top-and-bottom studies.
That ball represents a particle that mediates the interaction and that is different from the dark matter particle. The analogy illustrates how Massey and Harvey can know now that dark matter has more than one, at least two, particles. Physics tells us that every exchange of particles requires a force to govern it. We know from gravitational lensing that dark matter does interact with gravity and we see that interaction (which is different from the offset halo interaction). We also know that dark matter does not interact with the electromagnetic force nor with the weak and strong particle forces. Yet, dark matter has been glimpsed in an interaction, therefore there must be some as yet unknown force governing the action of the mediating particle (the medicine ball). The physics behind dark matter is starting to look rich and complex, with particle, mediator and force to identify. As Massey said in his email: “If two particles of dark matter can talk to each other, the interaction needs a new type of mediator particle. So not all dark matter is identical stuff.”
As Massey explained in his email, the Abell 3827 study shows that dark matter “interacts very, very little” at 0.001 on a particular scale. What this means is that although the interaction approaches zero (no interaction), it is not zero. A not-zero result is very different from a zero result. As Massey said: “[T]here is a HUGE difference between zero interactions (as the inert “Cold Dark Matter” theory would suggest), and tiny-but-not-zero interactions.” He further said:
“Our first result dropped the bar a lot further [even less than this], but all these results were fundamentally saying that we see dark matter being boring [not active]. By contrast, I’m excited about our second result [in MNRAS] because, for the first time, we’ve caught dark matter in the positive act of doing something interesting!”
Parallel World of Dark Matter
The idea of a parallel world of cold dark matter has been proven by Massey’s result to be incorrect. The old theory posited cold dark matter with an interaction value of zero. Massey’s result of a non-zero, a tiny-but-not-zero result proves dark matter is warm and not cold. The final summation of the meaning found through the Massey and Harvey studies of the parallel world of dark matter is best told in Massey’s own words as quoted from his email:
“There is a parallel world of dark matter taking place around us all the time, but it is inert and we are barely aware of it.
“The amount by which dark matter interacts is x. We don’t know what value x is. The old theory hypothesised that dark matter doesn’t interact at all, and x is zero. We made two measurements of what x is in the real world.
“[Harvey’s] paper published in Science showed that the real value of x is less than 0.5. This really does mean that dark matter interacts very little. On its own, this result still allows the possibility that x is zero (which is less than 0.5), and that dark matter might not interact at all.
“Our paper published in MNRAS suggests that the real value of x is higher than 0.001. This would mean that dark matter does interact. It might not interact much at all (if the real value of x is 0.002, say) … [but] the result rules out that dark matter doesn’t not interact (x is not zero). Thus the old theory is incorrect.
“Together, the results say that x is somewhere between 0.001 and 0.5. These values are like bookends on a shelf, and the true value is a single book somewhere between them. We don’t know where the book is – it’s like the book is invisible. The next step is to squeeze the bookends towards each other until the gap between them is only one book wide. At that point, we’ll know where the book must be.
“[W]e didn’t anticipate how well [these results] would fit together when we started — both projects took three or four years from start to finish. When we realised we would finish both at about the same time, we tried to coordinate … to tell the whole story in a single release. Unfortunately, publication schedules wouldn’t allow it.”
To Read the Massey and Harvey Studies
Dr. Richard Massey is a Royal Society Research Fellow, in Durham University’s Institute for Computational Cosmology. His study “The behaviour of dark matter associated with four bright cluster galaxies in the 10 kpc core of Abell 3827” is available in full text online from its publisher, Monthly Notes of the Royal Astronomical Society. Dr. David Harvey’s study “The non-gravitational interactions of dark matter in colliding galaxy clusters” was published in Science but is available in full text online, along with Massey’s, through Research Gate.
