Dark Matter on Earth is Self-Interacting and does not Annihilate
We have been studying dark matter for many decades. It was thought to be composed of a single matter particle that was slow-moving and did not interact very much with anything else. Leading scientists are now proposing that not all dark matter may be so inert and ‘boring’.
Updating on What We Know about Dark Matter
What we popularly think and read of dark matter today is hopelessly outdated. Harvard University physicist Lisa Randall (Time Magazine’s 100 most influential people in the world) and Sean Carroll from Caltech have been proposing that there is a ‘’dark sector’’ with dark matter particles that possess their own ‘’dark’’ charge and their own ‘’dark light’’, over the last 10 years.
Given the complexity of the Standard Model of particle physics…it seems very odd to assume that all of dark matter is composed of only one type of particle.
Lisa Randall, Harvard University, 2015
This thought is not new as this author himself had expressed the same view ten years earlier, noting that that it would be surprising that dark matter, that makes up most of the matter in the universe, could be made up of just one type of particle and ordinary matter, which makes up only a fraction, could have a generous plethora of particles under the physicists’ “Standard Model” (comprising most of the particles that we know and have been detected by Science).
Since dark matter makes up about 85% of all the matter in the universe, it would be simplistic to assume that they would be composed of only one type of particle. There is probably a great diversity of particles and energies [or forces] included in dark matter.
Jay Alfred 2005
Dark Light
Furthermore, the author had also identified that dark matter may have its own ‘light’ which is not detectable by our ordinary matter telescopes. An idea that Randall shares:
Dark matter might also have some of its own forces and interactions not experienced by our matter…I’ll call the force that is experienced by the interacting dark matter component dark light, …this new type of light, which ordinary matter simply doesn’t experience… communicated by an entirely new particle — a dark photon…
Lisa Randall, Harvard University, 2015
Ten years earlier this author noted:
Dark-light or super-light is the counterpart of the light that we are familiar with — composed of super-photons.
Jay Alfred 2005
These ideas are not confined to just one physicist; Sean Carroll also thinks they are worth taking seriously.
I’m referring to the idea that the dark matter isn’t simply a single neutral particle with a negligible rate of interaction in the current universe, but rather a set of one or more kinds of particles with some noticeable forces acting between them.
…I have investigated the idea of dark photons — dark matter being charged under a new “dark force” resembling ordinary electromagnetism. …These ideas, it turns out, are fairly compatible with what we know about the dynamics of dark matter in the real universe.
Sean Carroll, Caltech, 2013
More scientists are now proposing that there could be a ‘’dark sector’’ (like a separate dark universe that is invisible to us) which has its own self-interacting particles and forces; apart from forces that may be felt by both the visible and dark sectors. For example, gravity is “felt” by both the ordinary (light, visible) sector and the dark sector (that is how we know that dark matter exists).
…it is possible that dark matter has a very rich structure. This is especially natural if dark matter resides in a hidden sector with its own gauge interactions. In particular, dark matter might be multicomponent, charged under an unbroken dark U(1)0 gauge interaction, i.e. it interacts with itself via a massless dark photon. It has been suggested that such self-interactions may even go some way toward ameliorating small scale structure problems.
J. D. Clarke and Robert Foot, University of Melbourne, 2015
Self interacting dark matter models can explain better the distribution of dark matter in galaxies. These models imply dark radiation.
…if the [SIDM] model is constrained to explain the dark matter halos inferred for spiral galaxies and galaxy clusters simultaneously, there is a strong indication that dark matter is produced asymmetrically in the early universe. It also implies the presence of dark radiation…
Kaplinghat, Pan and Yu, 2018
Dark Matter has become Interesting — at Least Some of them!
Randall proposed that the same amount of self-interacting dark matter as ordinary matter is possible in the universe. She calculates that if 15% of the the matter in the universe was self-interacting dark matter, it would agree better with dark matter distributions and structures at the sub-galactic (sub-kiloparsec) scale. Randall’s team call the scenario…partially interacting dark matter…
The dominant component interacts only gravitationally and is the conventional cold dark matter that resides in spherical haloes around galaxies and galaxy clusters. The second component interacts gravitationally too, but also through an additional force very similar to electromagnetism.
Lisa Randall, Harvard University, 2015
15% of the universe’s matter is not that insignificant. The ordinary matter which makes up 15% of the matter in the universe, represents all the visible stars and matter in over 2 trillion galaxies! So, we would say that 30% of the universe’s matter is self-interacting (equally divided between ordinary and dark matter).
Dark Charge
Randall, as well as Carroll earlier, proposed that this self-interacting dark matter may have a weak dark charge.
Just as ordinary matter can experience nongravitational forces such as electromagnetism, so too might dark matter…restrictions don’t rule out self-interactions as a possibility…they just provide limits on the allowed strength and form……
Lisa Randall, Harvard University, 2015
These ideas are not completely new. A number of physicists have published peer-reviewed papers to propose that some dark matter particles may have the following interesting features:
- A weak long-range dark charge (analogous to ordinary matter’s long-range electromagnetic force but much weaker) that allows dark matter particles to interact.
- A weak short-range fifth force that allows dark matter particles to interact with ordinary matter particles (similar to what the author had proposed, above). A possible candidate for particles that can carry this force are axions (another class of dark matter particles). These are zero spin bosons, and just like gravitons (which mediates gravity), they will be able to mediate interactions between ordinary and dark matter particles at close range and smaller scales.
Consequences of a Dark Charge — A Dense Inner Halo/Dark Disk
For galaxies with low baryon concentration, dark matter self-interactions thermalize the inner halo and produce a density core over the age of galaxies. While for those with higher baryon concentration, the SIDM thermalization in the presence of the baryonic potential actually leads to a dense inner halo with a smaller core.
Kaplinghat, Ren and Yu, 2019
Kaplinghat has shown and is implying above, that dark matter density follows ordinary matter density. Since our galaxy, the Milky Way, has a dense center with spiral arms, we would expect a dense dark matter inner halo contouring the visible ordinary matter. This author had proposed similar ideas:
…the tables are turned, at least near the centers of galaxies, where ordinary matter is calling the shots and pushing dark matter around. Taking the evidence as a whole, however, it appears that there is a co-operative process between dark and ordinarily visible matter in the construction of visible and invisible structures.
Jay Alfred, 2005.
Randall proposes, consistent with the idea that dark matter density follows ordinary matter density, that, because dark matter is self-interacting, it will lose energy and collapse into a thin flat disk in the center of the galaxy. So, apart from the low density halo of weakly interacting dark matter enveloping the visible galaxy, there is a disc of self-interacting dark matter that sits in the galaxy, coincident with the plane of the spirals in the galaxy.
We would be left with a disk of dark matter and a disk of ordinary matter … — with the dark disk embedded inside the wider disk of the Milky Way plane…
Lisa Randall, Harvard University, 2015
The Solar System revolves (and bobs up and down) around the galaxy within this plane. This means that the dark matter that is passing through the Earth and captured by its gravity would be largely self-interacting dark matter.
Star observations indicate that we left the center of the plane less than a couple of million years ago — a short time on cosmological scales. This tells us that if the double-disk dark matter exists, the Solar System oscillated through the dark disk around that time too, so we weren’t very far (in astrophysical terms). In fact, if the disk turns out to be a little thicker, we might even be inside it…
Lisa Randall, Harvard University, 2015
The thickness of the disk will depend on the interaction strength and mass of the dark matter particle hypothesized, which can be adjusted. So, it could be easily envisaged that the Solar System might be moving within the dark matter disk or dense inner halo.
Asymmetric Self-Interacting Dark (ASID) Matter
Just like ordinary matter, dark matter particles have their anti-matter particles. In a paper published in 2011, some researchers believe that dark anti-matter annihilated ordinary anti-matter.
Antibaryonic dark matter can cause induced nucleon decay by annihilating visible baryons through inelastic scattering.
Davoudiasl, Morrissey, Sigurdson and Tulin, 2011
This explains why we have not found ordinary anti-matter on cosmic scales. It also means that dark matter is asymmetric (i.e., it does not have anti-matter counterparts as the latter have been annihilated in the early universe). That accounts for why, time dark matter annihilations have actually been verified in the universe at the current age. Randall also concludes that dark matter is asymmetric although coming to this conclusion by offering a different perspective.
Asymmetric dark models suggest that since the dark matter energy density is so similar to that of ordinary matter, maybe dark matter too got created in a related process involving a dark matter-dark antimatter asymmetry.
Lisa Randall, Harvard University, 2015
SIDM models predict asymmetric dark matter.
SIDM generically prefers asymmetric DM, and
predicts the existence of dark radiation.Kaplinghat, Pan and Yu, 2018
There are therefore several reasons to conclude that dark matter is generally asymmetric (although we cannot completely rule out any relic presence).
Conclusion
The dark matter in the Solar System and the Earth is largely asymmetric self-interacting dark (ASID) matter. Randall notes that both ordinary and some dark matter would carry charge. Even while obeying similar laws of physics and being in proximity in space, dark and ordinary matter would each occupy their own worlds. Ordinary matter and dark matter could even physically overlap without ever strongly interacting. This has profound consequences and implications which will be explored in future articles.
Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe, Lisa Randall, 2015. Double-Disk Dark Matter, JiJi Fan, Andrey Katz, Lisa Randall, Matthew Reece, Department of Physics, Harvard University, Cambridge, MA 02138, United States.
More Messy Dark Matter, Blog Page, Sean Carroll, 2013. Dark Matter and Dark Radiation, L. Ackerman, M. R. Buckley, S. M. Carroll, M. Kamionkowski, (2008) arXiv: 0810.5126v1 [astro-ph].
Observational Evidence for Self-Interacting Cold Dark Matter, Spergel, David N.; Steinhardt, Paul J. (April 2000). Physical Review Letters.
Baryon destruction by asymmetric dark matter, Hooman Davoudiasl, David E. Morrissey, Kris Sigurdson, and Sean Tulin, Phys. Rev. D 84, 096008, 2011.
Our Invisible Bodies, Jay Alfred, 2005.
