Why Are Superclusters the Largest Objects in the Observable Universe?

In the vast expanse of the observable universe, there are still many mysteries to uncover. However, galaxies tend to form groups and clusters for some reason. The latest discovery of a protocluster 11 billion light-years away from Earth is on its way to becoming a supercluster and could shed new light on this fascinating topic.

What Are Galaxy Clusters?

A galaxy cluster is a large gravitationally bound group of galaxies, typically containing hundreds or even thousands of galaxies, along with vast amounts of hot gas and dark matter. The superclusters, on the other hand, are more amorphous groups of these clusters. Often, they are not dense enough to be gravitationally bound, but some indeed are. Along with supervoids, superclusters are among the largest 'structures' in the universe, with supervoids being regions that contain very few galaxies.

The Shapley Supercluster: The Largest Known Supercluster

Named after the astronomer who first discovered it, the Shapley Supercluster is the largest known supercluster, containing some two dozen clusters with a mass of approximately 10,000 trillion solar masses. It is located roughly 2/3 of a billion light-years away and was first uncovered by Harlow Shapley 90 years ago. It is thought to be gravitationally bound overall, with a density roughly 15 times the average for matter in the universe, or 5 times the critical density required for binding.

Exploring the Scale of Cosmic Structures

We can observe the entire universe minus the part blocked by our Milky Way. This allows us to catalog galaxies, clusters of galaxies (clusters), and clusters of clusters (superclusters). Essentially, the universe is a cluster of superclusters. Thus, the largest things we observe (excluding the universe itself) are superclusters.

The Distinction Between Superclusters and Other Structures

While superclusters are indeed massive formations, they are not the biggest structures in the universe. Walls, filaments, and sheets, which are larger, are just collections of superclusters. The one important fact that distinguishes superclusters from mere clusters of galaxies is that superclusters are not gravitationally bound, contrary to one misconception. Due to their large size and low density, superclusters expand with the Hubble expansion of the universe. By contrast, clusters of galaxies remain relatively stationary in their local cosmic environment.

The Role of Hubble Expansion in the Formation of Superclusters

A supercluster expands as the universe expands, causing its constituent clusters to be pushed further apart from one another. However, the individual clusters themselves do not expand. Instead, they act as dense regions within an expanding, cosmic web of structures. This dynamic interplay between expansion and matter density is crucial in shaping the large-scale structure of the universe as we observe it today.

Conclusion: The Complexity of Cosmic Structures

The study of superclusters and other cosmic structures is far from a simple endeavor. The ongoing discovery and cataloging of these massive formations represent a significant step in understanding the complex and interwoven nature of our universe. Further research into these structures could reveal deeper insights into the origins and evolution of the cosmos.