1131
Can we understand the universe without understanding the largest structures in it? In principle, probably not. In practice? Absolutely not. Extremely large objects can distort our understanding of the cosmos. Astronomers have found the largest structure in the universe to date, named Quipu after the Inca system of measurement. It contains a shocking 200 quadrillion solar masses.
Astronomy is a science where extremely large numbers are part of everyday discourse. But even in astronomy, 200 quadrillion is such a large number that it's rarely encountered. And if the extremely large mass of the Kipu doesn't draw attention, its size certainly does. The object, called the superstructure, is over 400 megaparsecs long. That's more than 1,3 billion light-years.
Such a large structure simply has to affect its surroundings, and understanding these effects is crucial to understanding the cosmos. According to new research, studying Kipu and its siblings could help us understand how galaxies evolve, improve our cosmological models, and increase the accuracy of our cosmological measurements.
The study, titled "Discovering the Largest Structures in the Nearby Universe: Discovery of the Kipu Superstructure," has been accepted for publication in the journal Astronomy and Astrophysics Hans Boehringer of the Max Planck Institute is the lead author.
“To accurately determine cosmological parameters, we need to understand the effects of the local large-scale structure of the Universe on measurements,” the authors write. “These include modifications of the cosmic microwave background, distortions of sky images by large-scale gravitational lensing, and the effects of large-scale streaming motions on measurements of the Hubble constant.”
Superstructures are extremely large structures that contain groups of clusters and superclusters of galaxies. They are so large that they challenge our understanding of how our universe evolved. Some are so massive that they disrupt our models of cosmological evolution.
Quipu is the largest structure we've ever found in the universe. It and four other superstructures the researchers discovered contain 45 percent of galaxy clusters, 30 percent of galaxies, 25 percent of matter, and occupy 13 percent of the volume. The image below helps explain why they called it Quipu. Quipu are recording devices made of knotted cords, in which the knots hold information by color, order, and quantity.
“This view gives the best impression of the superstructure as a long thread with small side threads, which initiated the name Kipu,” the authors explain in their article.
In their work, Boehringer and his co-researchers discovered Kipa and four other superstructures at distances between 130 and 250 Mpc. They used X-ray galaxy clusters to identify and analyze the superstructures in their Cluster Survey of Cosmic Large-Scale Structure in X-rays (CLASSIX).
X-ray galaxy clusters can contain thousands of galaxies and lots of very hot, X-ray-emitting gas within the clusters. These emissions are key to mapping the mass of the superclusters. The X-rays trace the densest regions of matter and the underlying cosmic web. The emissions act like pointers to identify the superclusters.
The authors note that “the difference in galaxy density around field clusters and members of superstructures is striking.” This may be because field clusters are populated by less massive clusters than those in the superstructure, rather than because field clusters have a lower density of galaxies. Whatever the reason, the mass of these superstructures has a huge impact on our attempts to observe, measure, and understand the cosmos. “These large structures leave their mark on cosmological observations,” the authors write.
The superstructures leave an imprint on the cosmic microwave background (CMB), which is the remnant radiation of the Big Bang and the key evidence that supports it. The properties of the CMB match our theoretical predictions with almost surgical precision. The gravitational pull of the superstructures alters the CMB as it passes through them according to the Integrated Sachs-Wolf (ISW) effect, causing CMB oscillations. These oscillations are foreground artifacts that are difficult to filter out, interfering with our understanding of the CMB and, therefore, the Big Bang.
The superstructures can also affect measurements of the Hubble constant, a fundamental quantity in cosmology that describes the rate of expansion of the universe. While galaxies are moving away from each other due to expansion, they also have local velocities called peculiar velocities or streamwise motions. These must be separated from the expansion to understand the expansion clearly. The large mass of these superstructures affects these streamwise motions and distorts our measurements of the Hubble constant.
The study also notes that these massive structures can alter and distort the image of our sky through large-scale gravitational lensing. This can lead to errors in our measurements. On the other hand, Lambda CDM simulations produce superstructures like Quipu and four others. Lambda CDM is our standard model of Big Bang cosmology, which explains much of what we see in the universe, such as its large-scale structure.
“We find superstructures with similar properties in models based on the Lambda-CDM cosmological models,” the authors write.
It is clear that these superstructures are critically important to understanding the Universe. They store a significant portion of its matter and fundamentally influence the environment. More research is needed to understand them and their effects.
“Interesting further studies of our findings include, for example, investigating the effects of these environments on the population and evolution of galaxies,” the authors write in their conclusion.
According to the study, these superstructures will not exist forever. "In the future cosmic evolution, these superstructures will inevitably break up into several collapsing blocks. Thus, they are temporary configurations," Boehringer and his co-researchers explain.
"But at present they are special physical entities with characteristic properties and a special cosmic environment that deserve special attention."
