Cold dark matter models fail to match observational evidence once again. How long will it take before the astrophysics community acknowledges that their model has been falsified?
The large mass of a galaxy cluster deflects light from background objects, a phenomenon known as gravitational lensing. The large-scale gravitational lens caused by the whole cluster can be modified by smaller-scale mass concentrations within the cluster, such as individual galaxies.
Meneghetti et al. examined these small-scale gravitational lenses in observations of 11 galaxy clusters. They found small lenses that were an order of magnitude smaller than would be expected from cosmological simulations. The authors conclude that there is an unidentified problem with either prevailing simulation methods or standard cosmology.
From Science. The abstract of the article states that:
Cold dark matter (CDM) constitutes most of the matter in the Universe. The interplay between dark and luminous matter in dense cosmic environments, such as galaxy clusters, is studied theoretically using cosmological simulations. Observations of gravitational lensing are used to characterize the properties of substructures—the small-scale distribution of dark matter—in clusters.
We derive a metric, the probability of strong lensing events produced by dark-matter substructure, and compute it for 11 galaxy clusters. The observed cluster substructures are more efficient lenses than predicted by CDM simulations, by more than an order of magnitude. We suggest that systematic issues with simulations or incorrect assumptions about the properties of dark matter could explain our results.
We performed several tests to investigate potential sources for this discrepancy. The results remain unchanged even when one key ingredient - energy feedback from active-galacticnuclei powered by SMBH accretion - which alters the internal structure of halos is disabled in the simulations. This feedback suppresses star formation in substructures, altering the slope of their inner density profiles, making them less centrally concentrated and, hence, weaker gravitational lenses. Even without feedback, we are unable to bridge the gap between simulations and observations completely. In addition, simulations without feedback would be grossly discrepant from observations for other well measured quantities like the total fraction of baryons in clusters converted into stars. The mass and spatial resolutions of our simulations are sufficiently high to resolve the typical substructures included in the lensing mass models. We also exclude the possibility that the computed GGSL probability could be enhanced by unassociated halos along the line-of-sight (LOS) to these clusters. Including multiple lens planes in the models generated using cosmological simulations, we find that the substructure critical lines and caustics are negligibly affected by halos along the LOS. The observationally constrained lens models reproduce the shapes and sizes of the observed GGSL events. For instance, the model predicted image positions match within ∼ 0.5 arc sec with what is seen.
The discrepancy between observations and simulations may be due to issues with either the CDM paradigm or simulation methods. Gravitational lensing has previously been used to probe detailed properties of dark-matter halos associated with individual cluster galaxies. Simulations show that the mass and radial distributions of sub-halos are nearly universal. Varying results have been reported for the level of agreement between lens model predictions and simulations for other derived quantities, e.g. the mass distribution functions of substructure derived from lensing data agree with simulations but their radial distributions are more centrally concentrated in observed clusters than seen in simulations. Strong lensing clusters also contain more high-circular-velocity sub-halos (i.e. sub-halos with maximum circular velocities Vcirc > 100 km s−1 ) compared with simulations. The maximum circular velocity is given by
Vcirc = maxs GM(r) r , (1)
where G is the gravitational constant, M(r) is the galaxy mass profile and r is the distance from the galaxy center. Fig. 4 shows that, in our lens models, observed galaxies have larger circular velocities than their simulated analogs at a fixed mass. This implies that dark matter sub-halos associated with observed galaxies are more compact than theoretically expected. Observed substructures also appear to be in closer proximity to the larger scale cluster critical lines. Explaining this difference requires the existence of a larger number of compact substructures in the inner regions of simulated clusters. Baryons and dark matter are expected to couple in the dense inner regions of sub-halos, leading to alterations in the small-scale density profile of dark matter, it could be that our current understanding of this interplay is incorrect. Alternatively, the difference could arise from incorrect assumptions about the nature of dark matter.
Previous discrepancies between the predictions of the standard cosmological model and data on small scales have arisen from observations of dwarf galaxies and of satellites of the Milky Way, namely the so called missing satellite, cusp-core, too-big-to-fail problems, and planes of satellite galaxies. The discrepancy that we report is unrelated to these other issues. Previous studies revealed that observed small satellite galaxies were fewer in number and were less compact than expected from simulations; here, we find the opposite for cluster substructures. The GGSL events that we observe show that subhalos are more centrally concentrated than predicted by simulations i.e. there is an excess not a deficit. Hypotheses advocated to solve previous controversies on dwarf galaxy scales would only serve to exacerbate the discrepancy in GGSL event numbers that we report here.
Our results therefore require alternative explanations. One possibility is numerical effects arising from the resolution limits of simulations. However, currently known numerical artefacts are not effective enough at disrupting satellites. We investigated this issue thoroughly and found that it can change the predicted GGSL event rate at most by a factor of 2, which is insufficient to explain the nearly order of magnitude discrepancy that we find. These numerical artefacts would also appear on galactic scales, where they would in turn worsen the missing satellite problem.