While the idea of dark matter was suggested to explain the formation of the original galaxies, one of its great successes was giving an understanding of the nature of the universe. The features of the cosmic microwave background can be explained by the presence of dark matter. And models of the early universe produce galaxies and galaxy clusters by building on structures formed by dark matter. The fact that these models get such a big picture is a strong argument in their favor.
But a new study suggests that similar models can get the details wrong – by absolute command. The people behind the study suggest that either there is something wrong with the models, or that our understanding of dark matter may need to be adjusted.
Under a lens
A new study by an international team of researchers took advantage of a phenomenon called gravitational lensing. Gravity surrounds itself in space, and it can do so in a way that bends light, similar to a lens. If a giant object – say, a galaxy sits between us and a distant object object, it could create a gravitational lens that magnifies or distorts the distant object. Depending on the exact details of how the objects are arranged, the results can be anything from simple increments to rounded rings or multiple multiple displays.
Because the effects of dark matter can be detected by gravity, we can “see” the presence of dark matter through its gravitational-lensing effects. In some cases, we even find lensing where there is less matter. That is one of the many pieces of evidence in favor of dark matter.
Researchers used gravitational lensing to establish a test that was, at least conceptually, very simple. We have created models of the early universe that show how dark matter helped form the first galaxies and drew them into clusters of galaxies. These models, when advanced, provide a description of what the distribution of the dark matter should be on various points in the history of the universe. So the researchers use gravitational lensing to determine if the dark matter distribution seen in the models matches whether we look at it through gravitational lensing.
According to these models, the universe was created hierarchically. By gravitational interaction itself, dark matter forms filaments that intersect in a complex, three-dimensional meshwork. The extra gravitational pull at the points where the filaments intersect will lead to a regular matter, leading to the first galaxies. Over time, a continuous draw of gravity pulled the galaxy together, forming large clusters at once. By examining the output of these models, we can get a look at the expected distribution of dark matter around the clusters. And by zooming in, we can see how the dark matter should be distributed in the realm of individual galaxies.
That distribution of dark matter can be seen as a prediction of models.
Meanwhile, in the real universe …
To test those predictions, researchers used images from the Hubble Space Telescope to map a vast collection of galaxy clusters and everything around them. Follow-up imaging using a very large telescope helped to identify the distances of objects based on how much of their light was moved to the red end of the spectrum by the expansion of the universe – the larger the redsheft, the more ant objects. These researchers were allowed to determine what objects should be behind the galaxy cluster and thus potential candidates for gravitational lensing.
The software package then used the data to create a mass distribution for each Galaxy cluster. This includes the overall lensing effects of the entire cluster, as well as the sub-lensing driven by the individual galaxies within the cluster. The researchers found a strong agreement between the appearance of objects with lenses and the location of individual galaxies, which led them to validate their mass-distribution calculations.
The researchers then used a universe simulator to create 25 simulated clusters and performed a similar analysis with the clusters. They did this to identify possible lensing sites and places that could cause the greatest distortion.
Both do not match. There were many significant areas that produced higher distortions than the model in the real-universe galaxy. This would be the case if the distribution of dark matter was a little more confusing than the predictions of the models. The dark matter around the galaxies was more compact than the predictions of the halo models.
This is not the first discrepancy in the sort we have seen. Dals of Dark Matter also predicts that there should be more dwarf satellite galaxies around the galaxy and that they should be more scattered than they are. But if we adjust our model delo to spread these galaxies further, we will be less likely to see more compact structures in galaxy clusters. So, instead of finding two problems that can be solved by adjusting one, it seems that both issues need to be adjusted in opposite directions.
Two possibilities
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 simulation of the evolution of the universe. Since they both get a bigger picture of the universe, however, the issue will become a subtle one and as a result it is difficult to identify, should these results be independently supported? One possibility is that the problems seem to be in the realm of galaxies, where there will be lots of subject-dark interactions. If there’s something more complicated going on, he can easily throw out the models.
For now, though there are likely to be teams with additional data at hand that can do similar analysis, so we’ll have to wait for that to happen. The theoretical cosmologist, being an impatient sort, will no doubt be testing different types of dark matter before any additional renaissance occurs.
Science, 2020. DOI: 10.1126 / Science.X5164 (About DOI).
