Galactic Cluster Collision Observations Disfavor Heavy Particle Dark Matter

El Gordo is the largest known example of two galaxies colliding.  Comparing the X-ray spectrum emissions of hot colliding gas with the visible light from the stars are the cores of the galaxies in the galactic clusters, provides an observational foundation from which the nature of dark matter phenomena in the clusters can be inferred.

The New Result

Scientists who have done this find that a heavy particle model of dark matter is a poor fit to this data.

This distinctive configuration has allowed the researchers to establish the relative speed of the collision, which is extreme (~2200km/second), as it puts it at the limit of what is allowed by current theory for dark matter.

These rare, extreme examples of clusters caught in the act of colliding seem to be challenging the accepted view that dark matter is made up of heavy particles, since no such particles have actually been detected yet, despite the efforts being made to find them by means of the LHC (Large Hadron Particle Collider) accelerator in Geneva and the LUX (Large Underground Xenon Experiment), an underground dark matter detector in the United States. In Tom Broadhurst's opinion, "it's all the more important to find a new model that will enable the mysterious dark matter to be understood better." Broadhurst is one of the authors of a wave-dark-matter model published in Nature Physics last year.

This new piece of research has entailed interpreting the gas observed and the dark matter of El Gordo "hydrodynamically" through the development of an in-house computational model that includes the dark matter, which comprises most of the mass, and which can be observed in the Xray region of the visible spectrum because of its extremely high temperature (100 million kelvin). Dr Broadhurst and Dr Molnar have managed to obtain a unique computational solution for this collision because of the comet-like shape of the hot gas, and the locations and the masses of the two dark matter cores that have passed through each other at an oblique angle at a relative speed of about 2200 km/s. This means that the total energy release is bigger than that of any other known phenomenon, with the exception of the Big Bang.

The underlying study to this interpretative description from a university PR office is here (preprint here). The abstract (with a few mathematical symbols converted to words) states that:

The distinctive cometary X-ray morphology of the recently discovered massive galaxy cluster "El Gordo" (ACT-CT J0102–4915; z = 0.87) indicates that an unusually high-speed collision is ongoing between two massive galaxy clusters. A bright X-ray "bullet" leads a "twin-tailed" wake, with the Sunyaev-Zel'dovich (SZ) centroid at the end of the northern tail. 

We show how the physical properties of this system can be determined using our FLASH-based, N-body/hydrodynamic model, constrained by detailed X-ray, SZ, and Hubble lensing and dynamical data. 

The X-ray morphology and the location of the two dark matter components and the SZ peak are accurately described by a simple binary collision viewed about 480 million years after the first core passage. We derive an impact parameter of about 300 kpc, and a relative initial infall velocity of about 2250 km s–1 when separated by the sum of the two virial radii assuming an initial total mass of 2.15 × 1015 M ☉ and a mass ratio of 1.9. 

Our model demonstrates that tidally stretched gas accounts for the northern X-ray tail along the collision axis between the mass peaks, and that the southern tail lies off axis, comprising compressed and shock heated gas generated as the less massive component plunges through the main cluster. 

The challenge for ΛCDM will be to find out if this physically extreme event can be plausibly accommodated when combined with the similarly massive, high-infall-velocity case of the Bullet cluster and other such cases being uncovered in new SZ based surveys.

In the pertinent part of the conclusion the paper states that:

This massive merging cluster with an infall velocity of 2250 km s−1 at the the time the Universe was half of its age (at the redshift of z = 0.87) is apparently very unusual in the standard ΛCDM models, for which extensive simulations have been performed to determine the expected probability distribution of relative velocities (Thompson & Nagamine 2012). At face value, such extreme cases present a challenge to our understanding of structure formation and may lead to a better understanding of dark matter and/or alternative theories of gravity.

Basically,  then, the example is notable because structure formation of such a huge scale tends to come much later in standard ΛCDM models than is observed.  Thompson, Dave & Nagamine 2014, however, finds that the deficit of merging clusters of this type may have simply been an artifact of an inadequate simulation program, and reproduce the observed number of large cluster combinations from  the standard ΛCDM models in a simulation using a different methodology.

Wave, Scalar Field and Boson Star Dark Matter

More background on wave dark matter is found here and here.  A key point is that "In these wave dark matter mechanisms, the substructure of the dark matter (the perturbations from spherical symmetry) is what drives the substructure in the regular matter (spiral patterns and shells)."  The observed texture of luminous matter distributions in elliptical galaxies (which often have "shells") and spiral galaxies (which often have spiral arms rather than being smooth disks) is a major motivation for this theory.

Another notable observation about wave dark matter theories in these links is this one:

[T]his is where our discussion intersects with the fascinating works of many others who have studied “scalar field dark matter” and “boson stars.” In the boson star case, the motivation is quantum mechanical, so the above scalar field f is supposed to represent the overall wave function for a very large number of very tiny bosons with masses on the order of 10−23 eV. We relate our constant Υ to the mass of the Klein-Gordon equation by noting that the Compton wavelength in both cases is λ = 2π Υ = h m ≈ 13 light years if we take m ≈ 10−23eV , or equivalently, Υ ≈ 1/(2 light years). These other motivations are interesting as well but should be distinguished from the purely geometric motivations provided in this paper.

The related scalar field dark matter theory is also described here and was originally conceived in 1994 by Ji and Sin.

A recent analysis of cosmological data in the context of the theory concludes that the right boson mass is actually about 10 orders of magnitude smaller than previously assumed.  My own intuition is to note that the bosons of scalar field dark matter are suspiciously close in mass-energy magnitude to the hypothetical graviton.  And, indeed, the mass predicted in this recent analysis is precisely the mass a graviton would need to have in order to reproduce the observed cosmology without the cosmological constant.  This mass also corresponds to a Compton wave length of approximately the same length of the size of the universe (i.e. about 13 billion light years).

I suspect that a lot of the discussion of the massless graviton confounds particle rest mass, which an infinite range force boson should not have, and mass-energy which is something which gravity couples to and is present even in massless bosons like the photon.  It is not at all obvious to me that "massive gravity" theories are in any way distinguishable from "mass-energyful gravity" theories, in which case "massive gravity" is not inconsistent with a graviton that has zero rest mass as expected (but I would seem to be wrong).  A hypothetical graviton, while it lacks rest mass, should have mass-energy of some small, finite amount.

Some blog level discussion of massive gravity theories and their history is found here.  The criticism is a mass-energyful gravity expressed in the comments is that: 

There is no such thing as the energy density of the curvature. This is an old result from GR --- one cannot define the concept of energy for a gravitational field, locally. It can be done only in a suitable global sense (spacetime with asymptotic global time-translation symmetry).  

It is indeed an old result, but I continue to think that it is almost certainly wrong and probably the main cause of the deviation between GR and what is observed in Nature. Even if this is true of GR, that may just mean that GR is wrong.

As an aside, an excellent pdf defining the key terms and concepts in GR and the Einstein-Gordon-Klein equation (which describes the gravitational force generated by a scalar field in GR) is found here.  The Einstein-Gordon-Klein equation is intimately related to wave dark matter models and scalar field dark matter models.