An international team of researchers claim that the internal structure of vast galaxy clusters, containing thousands of individual galaxies, may be shaped by the dark matter environments in which they evolve. The study also provides evidence for a theoretical notion known as assembly bias.

Our understanding of dark matter is still relatively infantile, owing to the fact that it can only be observed through the influence it asserts over visible matter. Despite this difficulty, breakthroughs in the field of astronomy have not only provided strong evidence for its existence, but have actually allowed astronomers to estimate that dark matter makes up around 27 percent of all matter and energy in the universe.


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"Galaxy clusters are remarkable windows into the mysteries of the universe," says Hironao Miyatake of NASA's Jet Propulsion Laboratory, and lead researcher for the study. "By studying them, we can learn more about the evolution of large-scale structure of the universe, and its early history, as well as dark matter and dark energy."

For the study, Miyatake and his team drew on data covering 9,000 of the enormous clusters, collected as part of the Sloan Digital Sky Survey DR8 galaxy catalog. The masses of the clusters were determined by observing the extent to which the structures warp the light emitted from more distant objects through a phenomenon known as gravitational lensing.

The researchers divided the clusters into two categories based on their internal structure. The first category contained clusters with relatively tightly clumped structures, while the galaxies in the others were more sparsely distributed.

According to the researchers, the dark matter environment in which the clusters evolve has a significant influence in shaping their internal structures, and that the root of the phenomenon dates back to the first moments of the universe.

In the first trillionth of a second following the Big Bang, mass was first spread across the cosmos via an event known as cosmic inflation. However, mass was not spread evenly due to energy shifts in this formative period known as quantum fluctuations.

Images of a sparsely-packed cluster (left) and densely-packed galaxy cluster (right) (Credit: Sloan Digital Sky Survey)

These quantum fluctuations are potentially responsible for creating an uneven distribution of matter, resulting in natural peaks of material in the cosmos – described by the researchers as "peaky" regions – that contain a higher than average density of matter.

The current model for mass distribution throughout the universe is predicated on Einstein's theory of general relativity. Astronomers believe that matter is clustered in a filamentary web-like manner – a theory that has been largely supported by observation.

This model has been slightly complicated by a notion known as assembly bias, which, at its most basic level. says that the distribution of galaxies and galaxy clusters are subject to not only the total mass of the structures, but also the process by which they formed.

In the "peaky" regions of space, galaxy clusters are able to form without the aid of a dark matter environment. The resultant clusters are believed to form comparatively slowly with the many thousands of galaxies that form the cluster taking on a densely packed structure

Conversely, Miyatake and his team discovered that outside of these peaks, galaxy clusters require a dark matter-rich environment in order to coalesce. The dark matter-saturated environment causes the cluster to form significantly faster than its "peaky" cousins, albeit with a more sparsely distributed internal structure.

The results of the new study highlight a relationship between the dark matter environment in which the clusters develop, and their internal structure, while also providing evidence for the notion of assembly bias. However, the bias signal detected by the researchers was significantly higher than the theoretical expectation.

This could simply be a rare exception to the theoretical norm, however it may also indicate a problem with the theory. Further studies will be needed to settle the discrepancy.

A paper on the study has been published online in the online journal Physical Review Letters.

Source: NASA

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