New gravitational lens research hints for the dark matter model problem

New gravitational lens research hints for the dark matter model problem
Enlarge / Massive galaxy cluster MACSJ 1206. The cluster contains distorted images of distant background galaxies, visible as arcs and speckled features.

The concept of dark matter was originally proposed to explain the structure of galaxies, but one of its great successes was to explain the nature of the universe itself. The characteristic of the cosmic microwave background can be explained by the presence of dark matter. And models of the early universe create galaxies and clusters of galaxies by building structures formed by dark matter. The fact that this model gets the big picture so right was their favor.

However, a new study suggests that the same model misunderstands the details at scale. Those involved in the study suggest that there may be problems with the model or that your understanding of dark matter may need adjustments.

Under the lens

A new study conducted by an international research team used a phenomenon called the gravitational lens phenomenon. Gravity distorts space itself and can do so by refracting light like a lens. If a huge object (such as a galaxy) is between us and a distant object, we can create a gravitational lens that magnifies or distorts the distant object. Depending on the exact details of how the objects are arranged, the result can be anything from a simple magnification to a circular ring, or an object appearing multiple times.

Since the effects of dark matter can be detected through gravity, you can “see” the presence of dark matter through the effect of a gravitational lens. In some cases, I even detected lenses with little problems. That’s one of the many evidences in favor of dark matter.

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The researchers set up a very simple test, at least conceptually, using a gravity lens. We created a model of the early universe showing how dark matter helped structure the first galaxies and attract them into the cluster. Moving forward, this model provides an explanation of what the dark matter distribution looks like at various points in cosmic history to this day. So the researchers decided to use a gravitational lens to make sure that the dark matter distribution seen in the model matches where we see it through the gravitational lens.

According to this model, the universe was built hierarchically. Through gravitational interaction with itself, dark matter formed filaments intersecting into a complex three-dimensional mesh. At the point where the filaments intersect, additional gravity pulls the regular matter, leading to the first galaxy. Over time, constant gravity brought the galaxies together to form a large star cluster. By examining the output of these models, you can see the expected distribution of dark matter around the cluster. And if you zoom in, you can see how dark matter should be distributed over the regions of individual galaxies.

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

Meanwhile, in the real universe…

To test these predictions, the researchers used images from the Hubble Space Telescope to map all objects inside and around a large cluster of galaxies. Subsequent imaging using a very large telescope helped to identify the distance of an object based on how much light traveled to the red end of the spectrum by the expansion of the universe. The greater the redshift, the farther the object is. This allowed researchers to determine what objects should be behind the cluster and potential candidates for gravitational lenses.

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The software package then used the data to generate mass distributions for each cluster. This included the overall lens effect of the entire cluster and sub-lenses driven by individual galaxies within the cluster. Researchers were able to validate the mass distribution calculations by finding a strong match between the shape of the lensed object and the location of individual galaxies.

The researchers then built 25 simulated clusters using the space simulator and performed similar analyzes on the clusters. They did so to identify possible lens locations and where they could create the greatest distortion.

The two did not match. In real space galaxies, much more than the model caused distortion. This is the case when the distribution of dark matter is slightly more bumpy than the model predicts. The dark matter halo around the galaxy was denser than the model predicted.

This is not the first kind of discrepancy we’ve seen. The dark matter model also predicts that there should be more dwarf satellite galaxies around the Milky Way and more diffuse than they exist. However, if we tweak the model to make these galaxies more diffuse, the chances of seeing more dense structures in the cluster will be less. So, instead of finding two problems that can be solved with a single adjustment, it seems that you need to tweak both problems in opposite directions.

Two possibilities

Researchers suggest that there are two possible explanations for this discrepancy. We don’t recognize all the properties of dark matter, or something is missing from our cosmic evolution simulation. However, since the big picture of the universe is largely correct for both, the problem would be subtle and consequently difficult to identify if these results were confirmed independently. One possibility is that the problem appears to be in galactic regions with a lot of matter-dark matter interactions. When more complex things happen, you can easily throw away the model.

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However, for now, it is possible that there may already be teams with additional data capable of performing similar analyzes, so you will have to wait for these tasks to complete. Impatient theoretical cosmologists will undoubtedly test dark matter transformations long before further reanalysis emerges.

Science, 2020.DOI: 10.1126/science.aax5164 (DOI information).

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About the Author: Max Grant

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