Is Dark Matter a Complex (Dark) Plasma?
Leading physicists are proposing that some dark matter could have a dark charge. If this is so, these dark matter particles could interact to form atoms or more likely, if the charge is weak, plasma.
In the previous article, we explored the possibility of dark matter being self-interacting (while weakly interacting with ordinary matter) in the vicinity of the Earth, and within the Solar System, which is immersed in a dark disk of self-interacting dark matter in the galaxy, theorized by Harvard University physicist Lisa Randall. If so, how would massive particles with a weak dark charge interact? What state or phase of matter would it give arise to? Caltech physicist Sean Carroll says, given these properties, it may very well be in the form of plasma.
…we can’t say with perfect confidence that the dark matter really is effectively non-interacting. If a model like ours is right, and the strength of dark electromagnetism is near the upper bound of its allowed value, there might be very important consequences for the evolution of large-scale structure…What we are proposing is that the dark matter is really a plasma, and to understand how structure forms, one needs to consider dark magnetohydrodynamics.
Sean Carroll, Caltech, Blog, October, 2008
The idea that both self-interacting (weakly charged) dark matter and non-interacting dark matter could could be considered to be in the plasma state was put forward and discussed extensively by this author in 2005. We can argue that even neutral dark matter particles that are (relatively) non-interacting, except through gravity and the weak force (i.e., the bulk of dark matter in the universe), is a pseudo-plasma, in the sense that it is non-atomic and diffused, like a gas. (A pseudo-plasma is one that approximates a real plasma.) This potpourri of neutral non-interacting dark plasma, combined with charged self-interacting dark plasma, results in complex dark plasma.
…All this indicates that dark matter (just like visible matter) is largely in the form of plasma.
Jay Alfred, 2005
Furthermore, dark electromagnetism and dark magnetohydrodynamics have been discussed by this author in the 2005 publication.
…David Spergel of Princeton University says that Natarajan’s findings do not rule out interactions, other than gravitational effects, among dark matter particles colliding at low speeds. Here, perhaps, we have a hint that (dark) electromagnetic effects in (dark) plasma may have a significant role to play…
Jay Alfred, 2005
Several other scientific teams have also speculated that dark matter could be in the plasma state, including researchers from the University of Melbourne.
…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 such a framework, it is possible that the dark matter in the Universe exists primarily in a plasma state, as a macroscopically neutral conductive gas of ions with dark charge, broadly analogous to the state of much of the ordinary matter in the Universe.
J. D. Clarke and R. Foot, University of Melbourne, 2015
The notion of charged dark matter easily brings to the fore plasma physics.
Plasma physics should be the starting point for understanding the phenomenology of charged dark matter. DM charged under an unbroken U(1) interaction should be treated primarily as a plasma that develops collisionless shocks at relatively small scales. Consequently, at larger scales it behaves effectively as a collisional fluid…
Matti Heikinheimo et al, 2015
This finding was discussed at the 60th Annual Meeting of the APS Division of Plasma Physics. One of the implications is that dark matter can be in the state of a cold collisionless (dark) plasma.
DM (dark matter) can behave like a cold collisionless plasma of self-interacting DM particles, and exhibit plasma-like instabilities with observational consequences…
Nitin Shukla et al, 2018
There are also some popular opinions which support that dark matter could be in the form of plasma.
Plasmas are not just the ‘fourth state of matter’ — they are really the first state in modern cosmology, and they continue to be, by far, the dominant state of visible matter in the universe; perhaps also of invisible matter as well if so-called ‘dark matter’ continues to remain unobserved and unexplained.
Timothy Eastman, President, Plasmas International
What is Plasma?
More than 99% of the (ordinary matter) universe is composed of plasma. Plasmas are like gases where the sub-atomic particles are no longer bound in an atom. Think of a hydrogen atom, which consists of basically a positively charged proton in the nucleus with a neutralizing negatively charged electron forming a cloud around the nucleus. When enough energy is given to the electrons, they break free from atoms and form complex soups of negative electrons and positive ions (consisting of protons and neutrons). The plasma will be overall neutral but the particles will exhibit collective behaviour because of the long-range electromagnetic force and other reasons.
Examples of plasma in everyday objects include fluorescent lamps and neon lights. In nature, the aurora borealis (or northern lights) is another example of plasma. The Sun and stars are balls of plasma. Also, the hot gas which makes up the bulk of ordinary matter in the universe is also in the plasma state. Only a negligible amount of the matter in the visible universe is atomic matter, which makes up the cold celestial bodies such as the Earth and other planets, the moons, asteroids, comets, meteoroids, and dust grains.
Why is Self-Interacting Dark Matter largely in the Plasma state?
A complex plasma is one where there are not only charged particles but also large neutral (usually dust) particles mixed in. The equivalent of ‘dust particles’ in dark matter are the large massive uncharged particles such as WIMPs, which make up the bulk of dark matter (in the currently measurable universe). Just like WIMPs, dust particles in ordinary complex plasma are heavy, move slowly and are generally inert.
These WIMPs are mixed into the weakly charged self-interacting dark sector, currently estimated to be about 15% of all the matter in the universe, i.e. the same proportion as ordinary matter (based on the Randall-Reece model).
There are many reasons to believe that the bulk of self-interacting dark matter (within the theorized dark matter disk in the galaxy in which the Solar System inhabits) is in the state of a cold complex (dark) plasma, as follows:
- The dark charge is weak
A weak dark charge for dark matter has been theorized by several physicists, including Sean Carroll (and others at Caltech) and Lisa Randall (at Harvard University). As the charge is weak (approximately one hundred times weaker than ordinary electromagnetism), the particles will not interact readily or strongly with other particles in the vicinity to form atoms (although there will be long-range correlations, as in a plasma). Due to the large inter-particle distance in tenuous plasma and the weaker dark electromagnetic interactions , recombinations would be less likely to occur.
Furthermore, because the charge is weak, it will take less energy for the particles to free themselves from any binding force within atoms, making the formation and maintenance of long-lived plasma more probable. It logically follows from this that even if the temperature of the dark sector was equal or less than the temperature of the light sector (which is more than 99% plasma), it could still provide conditions for plasma to be the dominant state of matter in the dark sector.
2. Dark matter particles move slowly, reducing collisions
Dark matter particles that allow structure to form move slowly. A large class of dark matter particles are thought to be WIMPs, which are theorized to be massive particles, and hence move slowly. Axions, another class of dark matter particles, are much lighter and also move slowly due to other reasons (which are beyond the scope of this article to explain). In both cases, because they move slowly, the frequency of collisions with other particles is low, reducing the frequency of interaction to form atoms.
3. Neutral dark matter particles shield, reducing interactions
Included in the complex dark plasma is a large number of neutral particles (analogous to dust particles in ordinary plasma), such as WIMPs, which significantly shield the charged particles from interacting. These neutral particles number 4 times more than the charged particles.
4. Collective behaviour will still be one of a plasma
Although some (self-interacting) dark matter could form atoms (if a dark charge exists), the bulk would still behave as a complex plasma. Using ordinary plasma as an analogy, this is because even if 1% of a gas was ionized, the whole collection would have features associated with plasma. Hence, even if dark matter atoms form, the collective behaviour would be one of a cold dark plasma if a small proportion is ionized.
5. Signature features of complex (dusty) plasma found in dark matter
Signature features of plasma can be found in dark matter haloes and related phenomena, including concentric shells and Mach cones. (Sean Carroll et al had predicted another feature, a Weibel instability, characteristic of plasma, which has not yet been reported.)
On a cosmological scale, it could be envisaged that the dark sector did not undergo the recombination epoch that the visible sector underwent about 380,000 years after the Big Bang but remained largely in the plasma state.
Signature Features of Plasma in Dark Matter
H Thomas and his colleagues have generated plasma crystals in the laboratory, in the form of assemblies of particles which were held in a crystal-like array by a plasma of weakly ionized gas. When the assembly of microscopic particles was contained between two electrodes and illuminated by a laser beam, it could be seen, even with the naked eye, that the particles naturally arranged themselves regularly into as many as 18 planes parallel to the electrodes. In another experiment, the particles in a plasma crystal arranged themselves into neat concentric shells, to a total ball diameter of several millimeters. These orderly Coulomb balls, consisting of aligned, concentric shells of dust particles, survived for long periods. Dark matter halos constructed for elliptical galaxies also reveal the presence of faint shells. These shells extend out to two or three times further than the bulk of the starlight. As many as 20 shells have been discovered around one bright galaxy. Shells have also been found in other galaxies.
The presence of shells in dark matter around galaxies suggests its (complex) plasma state. It is well-known that in interstellar plasmas and dust clouds, in comets, in accretion disks around stars, and in planetary ring systems in our physical universe, the interaction between standard plasmas and dust plays an important role. Dark matter appears to show similar interactions. This is strong evidence that super (symmetric) dark plasma is involved in interactions which can be modelled by using standard plasma.
Jay Alfred, 2005
In 2017, researchers simulated the mergers of the Abell 520 cluster and the Bullet Cluster under idealized conditions in the context of a two-component dark matter model (similar to the Randall-Reece partially interacting dark matter model), in which a fraction of dark matter is composed of particles with long range interactions mediated by a dark photon, while the rest remains relatively non-interacting. Here, we also see signature features of plasma, including shock-fronts and Mach cones, as reported by the researchers:
The fact that the position of the dark core coincides with the X-ray emitting gas is the main motivation of our hypothesis that a subcomponent of dark matter might be dynamically similar to the baryonic plasma. The new DM component, dubbed dark plasma, is capable of forming shock-fronts that dissipate energy, thereby behaving similarly to the baryonic plasma in the intra-cluster medium during cluster mergers. The detailed control simulation shows that the interacting plasma component then forms Mach cones…
Christian Spethmann et al, 2017
These signature features of plasma are not just predicted, it has actually been observed in (gravitational) lensing reconstructions.
…a generic prediction of the dark plasma model is the existence of the bow-shaped dark matter shock fronts, that should be visible in the weak lensing reconstructions of the cluster merger events, given sufficient angular resolution. Our suggestion is that these effects might have already been discovered in the Abell 520 and 3827 clusters.
Matti Heikinheimo et al, 2015
The behaviour of dark plasma is compatible with the dark disk, theorized by Lisa Randall and her colleagues.
The possible observation of DM plasma is compatible with recently proposed models of a dark disk within our own Galaxy. In these models, the galactic dark plasma collapses into a thin disk through radiative cooling. In order for this collapse to occur within the lifetime of the galaxy, a light “dark electron” is required…
Christian Spethmann et al, 2015
Conclusion
There is sufficient support from a number of scientific teams that dark matter, which makes up 85% of the matter in the universe, is largely a complex dark plasma of (relatively) non-interacting neutral particles, such as WIMPs, and self-interacting weakly charged dark matter particles. This dark plasma interacts very weakly with ordinary matter. This conclusion was reached by this author more than a decade ago.
It is postulated by Dark Plasma Theory that supersymmetric bulk matter or dark matter consists of non-standard (or super) plasma…
Jay Alfred, June 2008
In a Nutshell
Based on these series of articles, it can be surmised that dark matter in the vicinity of the Earth and Solar System has the following properties:
- Asymmetric — there are no significant amounts of anti-dark matter particles in the vicinity of the Solar System.
- Complex Plasma — they are a non-atomic mix of both self-interacting weakly charged particles and (relatively) non-interacting neutral particles.
Dark Matter and Dark Radiation, Lotty Ackerman, Matthew R. Buckley, Sean M. Carroll, and Marc Kamionkowski, California Institute of Technology, Pasadena, CA 91125, USA, December 2008
Simulations of galaxy cluster collisions with a dark plasma component, Christian Spethmann, Hardi Veermäe, Tiit Sepp, Matti Heikinheimo, Boris Deshev, Andi Hektor, and Martti Raidal, 2017.
Dark matter self-interactions via collisionless shocks in cluster mergers, Matti Heikinheimoa, Martti Raidala, Christian Spethmann, Hardi Veermäe, 2015.
60th Annual Meeting of the APS Division of Plasma Physics, Volume 63, Number 11, Nitin Shukla et al., 2018.
Plasma dark matter direct detection, J. D. Clarke and R. Foot, ARC Centre of Excellence for Particle Physics at the Terascale, School of Physics, University of Melbourne, Victoria 3010 Australia.
Our Invisible Bodies, Jay Alfred, 2005.
