Dark Matter Phenomena Is More Wave-Like Than Particle-Like

Overview

A new paper uses a novel approach to show that dark matter phenomena is more wave-like than particle-like. It is important because it generically rules out huge swaths of the dark matter parameter space with a single set of observations.

The authors argue that this favors axion-like particle dark matter over WIMP-like dark matter without considering gravitational based sources of dark matter phenomena or other dark  matter candidates of intermediate masses. But gravitational based sources of dark matter would also be more wave-like than particle-like. 

Since the paper doesn't consider middle ground between 10^-22 eV mass axion-like dark matter particles which it models, and WIMPs of more than 10 GeV, it doesn't map out the parameter space bounds that its Einstein Ring lensing observations themselves impose, instead merely comparing two hypotheses.

My analysis below supposes a cutoff of 10 keV common associated with warm dark matter theories, because warm dark matter particle theories claim to start to overcome observational failures to cold dark matter particle theories precisely because it is at that mass scale that wave-like behavior starts to emerge.

Dark Matter Candidates More Massive Than 10 keV Are Ruled Out

This result generically disfavors dark matter candidates more massive than warm dark matter (in the single digit keV mass range), which is the most massive dark matter candidate to exhibit significant wave-like behavior. It also tilts the balance towards even lighter masses than warm dark matter, while disfavoring cold dark matter candidates in the WIMP mass range of about 1 GeV to 1000 GeV, let alone even more massive dark matter candidates that aren't effectively screened with direct dark matter detection experiments or primordial black holes. 

As the introduction to the paper notes, WIMPs (defined in the paper as having at least a 10 GeV particle mass), were already in trouble as a hypothesis:

The lightest among the stable WIMPs has long been heralded as the most likely candidate for CDM. Laboratory searches, however, have failed to detect WIMPs through direct-detection or in collider experiments. Cosmological simulations employing massive bodies to stand in for WIMPs have been highly successful at predicting the large scale structure of the universe, but face enduring problems on galactic or sub-galactic scales (< 10 kpc), the best documented of which are the “missing satellite” (along with the related “too big to fail”) and “cusp versus core” problems.

This result even largely rules out already non-viable dark matter candidates like the hypothetical X17 boson which at 17 MeV would be far too massive to have significantly wave-like behavior, in addition to being far too short lived.

Thermal Freeze Out Candidates Other Than Warm Dark Matter Are Ruled Out

Dark matter particle candidates lighter than warm dark matter (i.e. basically below 1 keV or so), since they can't be thermal freeze out dark matter particle candidates, need to be in some sort of dynamic equilibrium in which particles with very slight kinetic energy per particle are created and destroyed at the same rate. This is necessary to reflect the apparently roughly constant amount of dark matter in the universe suggested by the approximate fit of the LambdaCDM model to the astronomy data.

Since dark matter particle candidates in a thermal freeze out scenario get too "hot" (i.e. have too fast of a mean velocity) to be consistent with the amount of galaxy scale structure in the universe, any thermal freeze out dark matter candidate other than warm dark matter is essentially ruled out. 

Colder thermal freeze out dark matter candidates are ruled out because they are insufficiently wave-like.

There are other tensions with observation for warm dark matter candidates as well, many of which are shared with cold dark matter candidates, even though warm dark matter particle candidates can overcome some challenges including this one.

New Higher Energy Colliders Won't Find Dark Matter Candidates

Another important consequence of this paper is that higher energy particle colliders, which could potentially reveal beyond the Standard Model particles more massive than hundreds of GeVs, almost certainly won't reveal any new viable dark matter candidates, even if new particles are discovered in these colliders. 

Existing particle colliders are plenty powerful enough to comb the dark matter particle mass range that this study leaves in the running from zero rest mass to about 10 keV. 

The issue in this light particle mass range is escaping noise in experiments from the neutrino background to be able to see a hypothetical dark matter particle signal, not insufficient particle accelerator energy. 

Prospects For Further Research

Presumably, however, a deeper and more generalized analysis of the same data could discern whether some of the higher end of the mass range below 10 keV is also ruled out by the Einstein Ring observations.

In particular, if the Einstein Ring observations ruled out dark matter particle masses of 10 eV or more, all of the viable parameter space for sterile neutrino dark matter and warm dark matter hypotheses discussed below could be ruled out. An analysis along these lines would be a very valuable scientific contribution in service of the cause of narrowing the observationally permitted dark matter parameter space, that could extend the results of this paper with essentially the same data. This result, if established, would also completely rule out the thermal dark matter parameter space.

If this mass range was excluded, it would effectively push the parameter space for dark matter particles to below the meV order of magnitude mass range inhabited by Standard Model neutrinos. And, since we know that dark matter cannot be "hot" (i.e. have high mean particle velocity) based upon the scale of galactic structure, these particles would not only have to have a low rest mass, but would also have to have, unlike most observed neutrinos, exceeding low amounts of kinetic energy per particle (i.e. many orders of magnitude lower in mass-energy than the rest mass of the particle itself).

Standard Model Dark Matter Candidates Are Ruled Out

Hadronic Dark Matter Is Ruled Out

But high energy physics experiments have pretty well scoured nature for subatomic particles with masses of less than hundred of GeVs. The lightest hadron, the pion, is about 139 MeV (far to massive to be wave-like in addition to being too short lived). Particles with strong force color charge (i.e. quarks and gluons) are always confined in hadrons at temperatures below those that were last present in nature shortly after the Big Bang. Even the lightest quark, at about 2.5 MeV, is too massive to pass this test. While gluons theoretically have zero rest mass, their confinement into hadrons at the temperatures that have been present for the last 99.999999999999% of the universe or more, including all astronomy observations supporting the dark matter hypothesis also rule out free gluons. Also, of course, no hadrons but protons and bound neutrons are stable.

The motivation to look for ultra-heavy stable hadrons in some "island of stability" contrary to all existing indications that any heavier hadrons than those seen to date would be extremely unstable, is also undermined by the fact that any newly discovered heavy hadrons would be too massive to have predominantly wave-like behavior.

Non-Neutrino Standard Model Dark Matter Candidates Are Ruled Out

The W boson, Z boson and Higgs boson are all, of course, far too short lived and far too massive to fit the bill.

Charged leptons are obviously not dark matter candidates. Even electrons at 511 keV, in addition to being ruled out for being charged particles, are also perhaps fifty times too massive to be wave-like in the sense of this new paper's astronomy observations. The heavier muons (about 105 MeV) and tau leptons (about 1776 MeV) are also charged, are an even more poor fit due to their greater mass, and also aren't stable.

The only Standard Model particle less massive than a neutrino is the photon. A photon is stable and has zero rest mass, but we know cannot be dark matter because it is not, of course, "dark" and the aggregate amount of radiation in the universe (i.e. energy tied up in photons) is far too small to account for observed dark matter phenomena. 

Standard Model Neutrino Dark Matter Is Ruled Out

We know that dark matter is not due to Standard Model neutrinos (because it would be too "hot" and because there aren't enough neutrinos in the universe to fit the model). Also, robust cosmology estimates establish that there are only the three Standard Model  neutrino species with masses of 10 eV or less mass. 

Sterile Neutrino Dark Matter Has A Narrow Window And Is Disfavored

The experimental neutrino physics data and the observations of dark matter phenomena dynamics tends to disfavor "sterile" neutrinos, although there could be a window for sterile neutrino dark matter with particle masses greater than 10 eV and less than about 10 keV, considering this paper alone, and oscillations with ordinary neutrinos could provide a low energy dynamic process to maintain a constant aggregate mass of sterile neutrinos in the universe since a thermal freeze out mechanism wouldn't work in that mass range. There is, however, basically no credible and replicated experimental evidence for sterile neutrinos in this mass range.

If the sterile neutrino mass were below about 10 eV, this would show up in cosmology parameter estimations as addition effective numbers of neutrino species, which are robustly limited by astronomy observations to exactly three (plus an adjustment that leaves N(eff) as a non-integer value). So, light sterile neutrino species are ruled out.

But, if the sterile neutrino mass were more than 10 keV, the non-wave-like nature of the particle would rule it out based upon the Einstein Ring data.

Conjectures About Gravitons And Axions

The paper basically lumps all dark matter particle candidates with masses of much less than 1 eV into the axion-like particle category with preference for a particle mass of about 10^-22 eV. 

The introduction sums up evidence about the possible mass of axion-like particle dark matter:

While cosmological simulations involving both ϱDM and ψDM predict the same large-scale structure for the universe, ψDM naturally gives rise to characteristics observed among galaxies that pose as problems for ϱDM. 

For instance, ϱDM halos are predicted to increase dramatically in abundance toward lower masses until the Jeans limit at roughly 10^3 M, thus giving rise to the missing satellite problem. By contrast, ψDM halos are predicted to be increasingly suppressed below masses of ∼ 10^10(mψ/10^−22 eV)^−4/3 M until a cutoff at ∼ 10^7 (mψ/10^−22 eV)^−3/2 M. 

The suppression of relatively low-mass ψDM halos in the early universe provide an explanation for the apparent turnover in the abundance of galaxies toward lower luminosities at high redshifts (large cosmological distances). For mφ ∼ 10^−22 eV, λdB ranges from ∼100 pc in massive galaxies (Mh ∼ 10^11–10^12 M) to ∼1 kpc in dwarf galaxies (Mh ∼ 10^9 M), thus imposing a sizeable ψDM solitonic core in, especially, low-mass galaxies. 

The presence of solitonic cores explains why dwarf galaxies do not exhibit a cusp, a problem that can be circumvented in ϱDM only for relatively massive galaxies by appealing to feedback from star formation. 

In addition, the recently reported transition in stellar density at a radius of ∼1 kpc in local dwarf galaxies provides direct evidence for a solitonic core in these galaxies. 

Finally, a ψDM halo featuring a solitonic core can reproduce the flat stellar velocity dispersion of the ultra-diffuse galaxy DF44 that extends to about 3 kpc.

There is one almost Standard Model hypothetical particle that has strongly wave-like behavior which isn't ruled out, which is the zero rest mass graviton. 

The combined mass-energy of a graviton has been estimated, and is strikingly similar to the sweet spot for an axion-like particle dark matter mass.

The graviton's Compton wavelength is at least 1.6×10^16 m, or about 1.6 light-years, corresponding to a graviton mass of no more than 7.7×10^−23 eV/c^2.

One could say, in a quantum gravity based gravitational explanation for dark matter phenomena that dark matter (and dark energy) are basically just due to gravitons because gravity is just due to gravitons. 

Perhaps there are non-perturbative quantum gravity effects that Einstein's field equations don't capture, or the conventional GR scholarship has overlooked that could reproduce this result.

My intuition is that axion-like particle dark matter theories (which is not well motivated because the original axion motivated by a desire to explain the lack of CP-violation in the strong force has many other good explanations and doesn't really even require an explanation), to the extent that they can fit the data, do so because they have per particle mass-energy in the same ballpark as hypothetical gravitons. 

The Paper

The paper and its abstract are as follows:

Unveiling the true nature of dark matter, which manifests itself only through gravity, is one of the principal quests in physics. Leading candidates for dark matter are weakly interacting massive particles or ultralight bosons (axions), at opposite extremes in mass scales, that have been postulated by competing theories to solve deficiencies in the Standard Model of particle physics. Whereas dark matter weakly interacting massive particles behave like discrete particles (ϱDM), quantum interference between dark matter axions is manifested as waves (ψDM). 

Here, we show that gravitational lensing leaves signatures in multiply lensed images of background galaxies that reveal whether the foreground lensing galaxy inhabits a ϱDM or ψDM halo. Whereas ϱDM lens models leave well documented anomalies between the predicted and observed brightnesses and positions of multiply lensed images, ψDM lens models correctly predict the level of anomalies remaining with ϱDM lens models. 

More challengingly, when subjected to a battery of tests for reproducing the quadruply lensed triplet images in the system HS 0810+2554, ψDM is able to reproduce all aspects of this system whereas ϱDM often fails. The ability of ψDM to resolve lensing anomalies even in demanding cases such as HS 0810+2554, together with its success in reproducing other astrophysical observations, tilt the balance toward new physics invoking axions.

Alfred Amruth, "Einstein rings modulated by wavelike dark matter from anomalies in gravitationally lensed images" Nature Astronomy (April 20, 2023) https://doi.org/10.1038/s41550-023-01943-9 (Open access copy available at arxiv).