Ingredient missing from current Dark Matter theories

Pablo Tucker
September 13, 2020

Embedded within the cluster are the distorted images of distant background galaxies, seen as arcs and smeared features.

Dark matter accounts for approximately 85% of the matter in the Universe.

These formations also contain a lot of dark matter - not only because it binds them together, but because the individual galaxies they're comprised of contain dark matter, too.

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. 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.

The discovery of this disparity should help astronomers design better computer simulation models and thus develop a better understanding of how dark matter clusters.

Hubble researchers used a technique called gravitational lensing, in which distant objects are observed by looking at the way light is bent by the gravity of closer objects, with the closer objects acting like a magnifying glass. A handsome example of this is an Einstein ring, where a single object appears multiple times forming a ring-like arrangement. The effect of the gravity distorts light as it travels through space, and that light can be seen and used to measure the otherwise invisible dark matter. If the lensing star is accompanied by a planet, one can (potentially) observe not only the principal effect from the star, but also a secondary, smaller effect resulting from perturbation by the planet.

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.

And they double-checked their distance calculations, because that can make a crucial difference to dark matter calculations. Therefore, the researchers made a decision 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. Through gravitational interactions with itself, the dark matter formed intersecting fibers in a complex, three-dimensional meshwork.

The first is to plot out all the gravitational lenses that can be seen. Over time, the continuous equilibrium of gravity pulled the galaxies together, forming large clusters. The higher the concentration of dark matter in a cluster, the more dramatic its light bending.

To check their findings, the team conducted spectroscopic observations of the galaxies, using the shift of light to calculate the velocity of the orbiting stars - a classic tool for measuring dark matter.

Meanwhile, in the real universe ...

Hubble Space Telescope image of the lensing cluster MACSJ1206.

There's a problem in galaxy cluster MACS J1206, though. This allowed researchers to determine what objects should be behind the cluster and potential candidates for gravitational lenses. This included the overall lensing effects of the entire cluster, as well as the sub-lensing driven by individual galaxies within the cluster. This allowed them to assemble a well-calibrated, high-resolution map of the mass distribution of dark matter in each cluster.

The researchers then built 25 simulated clusters using the space simulator and performed similar analyzes on the clusters.

The two didn't match. In real space galaxies, much more than the model caused distortion.

Scientists are sure it is there though, because galaxies need vast amounts of gravity, which is generated from matter, to stop themselves from tearing apart.

This isn't the first discrepancy of the sort we've 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. "But we don't yet know whether this is telling us something about our computations and simulations, or whether it's telling us something fundamental about dark matter". Since both of those get the big picture of the Universe largely right, however, the issue is going to be a subtle one and consequently hard to identify, should these results get an independent confirmation. If there's something more complicated going on there, it could easily throw off the models.

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.

Bob Jacobsen, a physicist as UC Berkeley who wasn't involved in the new research, said the two maps - one produced by Hubble data and the other by current dark matter theories - look like they're in conflict.

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