The Bullet Cluster is one of the most famous objects in astronomy.
For years, many scientists have considered it one of the strongest pieces of evidence for the existence of dark matter, the mysterious substance thought to make up most of the matter in the universe.
However, a new study suggests there may be another way to explain what astronomers observe.
About 4 billion years ago, two enormous galaxy clusters collided at speeds exceeding 2,500 kilometers per second.
Each galaxy cluster contains hundreds of galaxies and billions of stars. However, most of their visible matter is not found in stars but in vast clouds of hot gas that exist between the stars.
During the collision, these gas clouds crashed into each other and slowed down because of friction.
The impact heated the gas to extremely high temperatures, making it visible today as two glowing clouds that can be detected by X-ray telescopes.
The galaxies themselves behaved differently. Because stars are separated by enormous distances, most of them simply passed by one another without colliding.
As a result, the galaxies moved ahead while the gas lagged behind, creating the unusual structure now known as the Bullet Cluster.
The Bullet Cluster became famous because of another effect called gravitational lensing. According to Albert Einstein’s theory of general relativity, massive objects bend space and can deflect light traveling nearby. Astronomers can measure this light bending to estimate where mass is located.
Surprisingly, the strongest gravitational lensing in the Bullet Cluster does not occur where the hot gas clouds are located, even though they contain most of the visible matter.
Instead, the strongest lensing appears around the galaxies on either side of the gas clouds. This observation led many scientists to conclude that a large amount of invisible dark matter must also be present around the galaxies.
However, dark matter has never been directly detected despite decades of searching.
The new study revisits this puzzle using fresh observations from the James Webb Space Telescope and improved measurements of the Bullet Cluster. The researchers found that the clusters contain more stars than previously estimated and also contain large amounts of heavy elements such as iron and oxygen.
These elements are produced inside massive stars. When such stars reach the end of their lives, they can collapse into neutron stars or black holes. These objects are essentially invisible but still exert powerful gravitational forces.
The researchers argue that these hidden stellar remnants may account for much of the gravitational effect observed in the Bullet Cluster. They also show that the observations are compatible with an alternative theory called Modified Newtonian Dynamics, or MOND.
MOND was proposed about forty years ago as an alternative to dark matter. Instead of introducing invisible matter, it suggests that gravity may behave differently under certain conditions. For many years, scientists believed the Bullet Cluster could not be explained by MOND. However, the new analysis suggests that the famous cosmic collision may actually fit the theory surprisingly well.
Even if dark matter does exist, the researchers conclude that its quantity in the Bullet Cluster may be much smaller than previously thought. The findings reopen an important debate and remind scientists that some of the universe’s biggest mysteries remain unsolved.
Source: KSR.
