Scientists Investigate Dark Matter’s Missing Ingredient

New Hubble data suggests there is an ingredient missing from current dark matter theories

A few decades ago, astronomers realized they had a problem: They could not find enough matter in the universe to account for all the space in between stars and galaxies. This missing stuff is called dark matter, and, despite constituting the vast majority of matter in the universe, it doesn't emit, absorb, or reflect light. It has been a strong argument in their favor that these models get the big picture very accurately.

However, a new study suggests that the same model misunderstands the details at scale. As researchers around the world investigate the nature and composition of this mysterious substance, a study published in the journal Science suggests dark matter theories may be missing an important component. Otherwise known as gravitational lensing it is commonly seen when a background object-which could be as small as a star or as large as a galaxy- moves in front of a foreground object and curves light from it giving it an apparent location in the sky. Gravity wraps itself around space, and it can do this by bending light similar to a lens. Depending on the precise details of how the objects are arranged, the results can be anything from a simple magnification to circular rings or having the object appear multiple times. But they also found smaller lensing effects nested inside. An object, such as a star, crossing our line of sight to a more distant source star will affect the light from that star just like a lens, producing two close images whose total brightness is enhanced.

A new study conducted by an global research team used a phenomenon called the gravitational lens phenomenon.

There is an unusual mismatch between our theoretical models of how dark matter should be spread across galaxy clusters, and the way it actually seems to be behaving in those clusters, the researchers said.

And they double-checked their distance calculations, because that can make a crucial difference to dark matter calculations.

According to these models, the Universe was built step by step. So, the researchers made a decision to use gravitational lensing to determine whether the dark matter distribution seen in the models matched where we see it via gravitational lensing. The rest of the matter out there is invisible. Once again, subtract the visible matter, et voila - a map of the dark matter within that lensing cluster. So they turned to galaxy clusters, which hide a huge amount of dark matter. Moving forward, this model provides an explanation of what the dark matter distribution looks like at various points in cosmic history to this day.

Meanwhile, in the real universe ...

To the surprise of the team, the Hubble images also revealed smaller arcs and distorted images intertwined within the wide-range lens distortions at the core of each cluster, where the largest galaxies are. Follow-up imaging using the Very Large Telescope helped identify the distance of those objects based on how much their light was shifted to the red end of the spectrum by the expansion of the Universe-the larger the redshift, the more distant the object. Over time, the continued draw of gravity pulled galaxies together, forming large clusters. This allowed researchers to determine what objects should be behind the cluster and potential candidates for gravitational lenses.

When the team sat down to analyse the data, they found the large-scale lensing effects as expected to be produced by the galaxy as a whole. This allowed them to assemble a well-calibrated, high-resolution map of the mass distribution of dark matter in each cluster.

For now, however, there are already likely to be teams with additional data in hand that could perform similar analysis, so we'll have to wait for those to be done.

The two did not match. In real space galaxies, much more than the model caused distortion. However, the dark matter used to interact with the visible matter in the universe via gravity.

This is not the first kind of discrepancy 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. So, rather than finding two problems that could both be solved by making one adjustment, the two issues appear to need adjusting in opposite directions. This missing ingredient may explain why researchers have uncovered an unexpected discrepancy between observations of the dark matter concentrations in a sample of massive galaxy clusters and theoretical computer simulations of how dark matter should be distributed in clusters. 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. This occurs when the gravity of dark matter in a galaxy cluster acts like a magnifying glass. If there's something more complicated going on there, it could easily throw off the models. Astronomers measured the amount of gravitational lensing caused by this cluster to produce a detailed map of the distribution of dark matter in it.

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