The new gravity-lensing study points to problems for dark matter models

The new gravity-lensing study points to problems for dark matter models
Image of a large group of galaxies, magnified views of some of them.
Zoom in / The giant galaxy cluster MACSJ 1206. Distorted images of distant background galaxies embedded within the cluster, which are seen as curves and smear features.

Although the idea of ​​dark matter was first proposed to explain the structure of galaxies, one of its greatest achievements was to explain the nature of the universe. The presence of dark matter can explain the features of the cosmic microwave background. Models of the early universe formed galaxies and galaxies by creating structures made of dark matter. It has been a strong argument in their favor that these models get the big picture very accurately.

But a new study shows that the same models get the details wrong — with a full-scale sequence. Those behind the study suggest that something is wrong with the model, or that our understanding of the dark matter may need adjustment.

Under a lens

A new study by a team of international researchers uses a phenomenon called gravitational lensing. Gravity wraps itself around space, and it can do this by bending light similar to a lens. A massive object – If a galaxy sits between us and a distant object, it can form a gravitational lens that magnifies or distorts the distant object. Depending on the exact details of how the objects are arranged, the results can range from simple magnification to circular rings or the object appearing multiple times.

Since the effects of a dark object can be detected by gravity, we can “see” the existence of a dark object through its gravitational-lensing effects. In some cases, we have also found lensing where the little thing is. This is one of the many sources that support the dark side.

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The researchers used gravity lensing to set up an experiment that was at least theoretically very simple. We have created models of the early universe, which refers to how the dark matter helped to form the first galaxies and attracted them into clusters of galaxies. These models, as they move forward, provide an explanation of how the distribution of that dark object should have been at different points in the history of the Universe up to the present. Therefore, the researchers decided to use gravity lensing to determine whether the distribution of the dark matter found in the samples is applicable to the locations we see through gravity lensing.

According to these models, the Universe was built step by step. Through gravitational interactions with itself, the dark matter formed intersecting fibers in a complex, three-dimensional meshwork. The extra gravitational force at the points where the fibers intersect will be drawn in the regular case, which will lead to the first galaxies. Over time, the continuous equilibrium of gravity pulled the galaxies together, forming large clusters. By examining the output of these samples, one can see the expected distribution of dark matter around the clusters. By zooming in, you can see how the dark matter should be distributed over the area of ​​the individual galaxies.

The distribution of dark matter can be viewed as a prediction of models.

Meanwhile, in the real universe …

To test those predictions, the researchers used images from the Hubble Space Telescope to map a large collection of galaxies and all objects around them. Follow-up imaging, using a very large telescope, helped to identify the distances of those objects based on how much their light was transferred to the red edge of the spectrum by the expansion of the universe – the larger redshift, and the more distant the object. This allowed researchers to determine which objects were behind the galaxy cluster, thus potential candidates for gravitational lensing.

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A software package then used the data to create mass distribution for each Galaxy cluster. This includes the overall lensing effects of the entire cluster and the sub-lensing driven by the individual galaxies within the cluster. The researchers found a strong agreement between the lenses’ appearance of objects and the location of individual galaxies, which allowed them to verify their mass-distribution calculations.

The researchers created 25 simulated clusters using the Universe simulator and performed a similar analysis with the clusters. They did so in order to identify potential lensing sites and areas that could produce the largest distortions.

The two do not match. Areas that make up more distortion in the real-universe galaxy were significantly larger than those in the model. This is if the distribution of the dark matter is slightly higher than the models predict — models of dark objects around galaxies are much smaller than the models predict.

This is not the first contradiction of the type we have seen. Dark matter models predict that there should be more dwarf satellite galaxies around the Milky Way and that they should be wider than they are. But if we adjust our models to further propagate these galaxies, we are less likely to see even smaller structures in galaxies. So it seems that both problems require adjustment in opposite directions, rather than finding two problems that can be solved by one adjustment.

Two possibilities

Researchers suggest that there are two explanations for this discrepancy: we do not appreciate all the properties of the dark matter, nor do we find any in the simulations of the evolution of the universe. As these two get the bigger picture of the universe, the issue will be a more subtle one, and the result will be difficult to identify, and whether these results will receive an independent confirmation. One possibility is that there seems to be problems in the galaxy, where there will be a lot of object-dark matter interactions. If there is something more complicated, it could easily throw out the models.

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However, for now, there may already be teams with additional data that can do a similar analysis, so we’ll have to wait for them to be done. Theoretical cosmologists, being of the impatient type, will undoubtedly test the variations of dark matter before any further restorations appear.

Science, 2020. DOI: 10.1126 / Science.Ax5164 (About DOIs).

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About the Author: Cary Douglas

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