Astronomers recently made a major discovery by observing a spectacular collision between two galaxy clusters. This revealed the presence of dark matter, an invisible substance that makes up a large part of the universe. Through this observation, we gain valuable insight into how this mysterious substance interacts with ordinary matter in order to further understand its role in the structure of our cosmos.
Understanding dark matter
Dark matter is a form of matter that, unlike normal matter, does not emit or reflect light. This makes it invisible to our traditional detection instruments. However, its presence is known thanks to its gravitational effect on visible objects like stars and galaxies. Indeed, scientists have discovered that dark matter exerts enough gravitational force to influence the way galaxies form and behave. Without it, galaxies would scatter and would not form the large structures we observe today.
According to estimates, we also now know that dark matter constitutes approximately 85% of the total matter in the Universe and 27% of its total mass-energy. However, the nature of this mysterious form of matter remains elusive. So, any observation is worth taking in order to try to understand it.
A spectacular galactic collision
Astronomers have observed a collision between two galaxy clusters, called MACS J0018.5+1626. These clusters are located approximately five billion light years of the Earth. To put this into perspective, this means that the light from these clusters traveled for five billion years before reaching us.
To study this event, scientists used a variety of telescopes and observatories. Among them are NASA's Hubble and Chandra space telescopes, as well as other instruments like the Caltech Submillimeter Observatory, the WM Keck Observatory, and the Planck Observatory. These tools provided crucial data by capturing images and measuring light and gas emissions.
The data was collected over several years and required complex analysis. Astronomers have looked at changes in light from stars and the behavior of hot gas to determine how dark matter moves and interacts during such collisions. They used phenomena like the Sunyaev-Zel'dovich (SZ) effect, which measures shifts in cosmic light caused by hot moving gas, to assess the speed and direction of movement of different types of matter.
-Surprising behavior
The researchers observed that despite the violence of the collision between the galaxy clusters, the individual galaxies themselves remained relatively intact. This phenomenon can be explained by the vastness of the space between the galaxies which is large enough that they do not collide directly, even during such cosmic collisions. In contrast, debris generated by the collision, such as hot gas and normal matter, were heavily disrupted by the impact. The gas, heated to extreme temperatures, was ejected and dispersed, and normal matter structures were distorted by the force of the collision.
What makes this observation particularly fascinating is the behavior of dark matter during the event. Unlike normal matter, dark matter indeed seems having passed through the debris of the collision almost unaffected. This invisibility in a collision situation is analogous to that of a ghost passing through physical objects without interacting with them. This phenomenon is crucial, because it demonstrates that dark matter does not interact with normal matter in the same way. She actually seems not subject to the friction and pressure forces that affect conventional particleswhich allows it to move independently of the disturbances caused by the collision.
A breakthrough in cosmic understanding
This discovery marks a significant step in the study of dark matter, reinforcing the idea that it could be made up of as yet unknown exotic particles, insensitive to electromagnetic forces but responsive to gravity. By observing how dark matter passes through these cataclysmic events without directly interacting with normal matter, scientists can refine their models of the composition and properties of this mysterious substance. These new data also allow us to deepen our knowledge of the evolution of the Universe, particularly on the formation of large structures such as galaxy clusters. Ultimately, they could offer crucial clues to elucidate one of the greatest mysteries of modern cosmology.
The study is published in The Astrophysical Journal.
