While the idea of Dark Matter was originally proposed to explain the structure of galaxies, one of its great successes was to explain the nature of the universe. The characteristics of the cosmic microwave background can be explained by the presence of dark matter. And early universe models build galaxies and galaxy clusters by building on structures created by Dark Matter. The fact that these models get the bigger picture is why a strong argument has been made in their favor.
But a new study suggests that similar models yield inaccurate details – by an entire order of magnitude. Those behind the study suggest that either there is something wrong with the model, or that our understanding of the dark matter may need adjustment.
Under a lens
The new study by an international team of researchers took advantage of a phenomenon called gravitational lensing. Gravity warps itself into space, and it can do so in a way that tilts light, corresponding to a lens. If a massive object – say, a galaxy – sits between us and a distant object, it can create a gravitational lens that magnifies or distorts the distant object. Depending on the exact description of how the objects are arranged, the results can range from a simple magnification to a circular ring or the object appears multiple times.
Because the effect of dark matter can be detected through gravity, we can “see” the presence of dark matter through the gravitational-lensing effect. In some cases, we have also detected lensing where little substance is present. This is one of many evidences in favor of dark matter.
Researchers used gravitational lensing to set up a test, which was at least conceptually very simple. We have created models of the early Universe that indicate how dark matter helped structure the first galaxies and draw them into clusters of galaxies. These models, when going forward, provide details of what dark color distribution should be presently seen at various points in the history of the universe. The researchers therefore decided to use gravitational lensing to determine whether the dark matter distribution observed in the models matches where we see it through gravitational lensing.
According to these models, the Universe was constructed hierarchically. Through gravitational interaction with itself, Dark Matter produced filaments that are embedded in a complex, three-dimensional meshwork. The extra gravitational pull at the points where the fibers are pulled regularly leads to the first galaxies. Over time, the constant draw of gravity drew galaxies together, forming large clusters. By examining the output of these models, we can have a look at the expected distribution of dark matter around the bunches. And by zooming in, we can see how dark matter should be distributed over the region of individual galaxies.
The distribution of dark matter can be viewed as a predictor of models.
Meanwhile, in the real universe …
To test those predictions, researchers used images from the Hubble Space Telescope and mapped out all the objects in a large collection of galaxy clusters. Follow-up imaging using the Very Large Telescope helped identify the distances of those objects by how much their light shifted from the expanses of the universe to the red end of the spectrum – the larger the redshift, the farther away the object. This allowed researchers to determine which objects should be behind the galaxy cluster and thus a potential candidate for gravitational lensing.
A software package used the data to create a mass distribution for each galaxy cluster. This included overall lensing effects of the entire cluster, as well as sub-lensing driven by individual galaxies within the cluster. Researchers found a strong agreement between the presence of lensed objects and the location of individual galaxies, allowing them to validate their mass-distribution calculations.
The researchers then used the Universe simulator to create 25 simulated clusters and performed a similar analysis with the cluster. They did this to identify the locations of possible lensing and the locations that could cause the greatest distortions.
The two did not match. Areas generating higher deformation were more important in the real-universe universe than the model. This will occur when the distribution of dark matter is slightly thicker than the model, which would lead to the supposition that – the estimate of dark mantle around galaxies was more compact than the model.
This is not the first such discrepancy that we have seen. Dark Matter models also speculate that there should be more dwarf satellite galaxies around the Milky Way and to spread more about what they are. But if we were to adjust our model to spread these galaxies more, we would be less likely to see more compact structures in galaxy clusters. So instead of finding two problems that both can be solved by making one adjustment, the two issues seem to require adjustment in opposite directions.
Researchers suggest that there are two possible explanations for this discrepancy: either we do not appreciate all the properties of dark matter or something is missing from our simulations of the evolution of the Universe. As both of them get the larger picture of the Universe at large, however, the issue is going to be a subtle one and consequently difficult to identify, should these results get an independent confirmation. One possibility is that the problems appear to be in the region of galaxies, where there will be a lot of case – dark matter interactions. If something more complex is going on there, it can easily throw away the model.
For now, though, there are likely to be teams with additional data already on hand that can do similar analysis, so we’ll have to wait for those. Theoretical cosmologists, being of the impatient type, will undoubtedly test the Dark Matter variants long before any additional reanalyses are out.
Science, 2020. DOI: 10.1126 / science .ax 5164 (About DOI).