The prediction of structures in our cosmological model

In theory, numerical models predict the existence of cosmic webs, structures existing between different objects of the cosmos. These structures result from the fact that the Universe, on a smaller scale, is not homogenous. Objects are not uniformly distributed throughout space. Over the course of billions of years, they groupe and create denser volumes in the distribution of matter in the Universe. On a very large scale, the structure of the Universe appears like a web composed of interconnected filaments. These filaments mark the boundaries of “voids”, where the density of galaxies or clusters of galaxies is smaller than in the filaments. Up to a certain scale, observations confirm and support this theory. “This is the limit of our models”, explains Vincent Pelgrims, a doctoral student at AGO and coauthor of the publication. “They predict that galaxies can organize themselves into filaments that look like neuronal connections but are much bigger of course. “These cosmic webs depend on gravitational force”, continues Dominique Sluse, a researcher at the Argelander Institute of Astronomy in Bonn and the University of Liege. “The models show that, under the influence of gravity, the angular momentum, therefore the rotational axes of galaxies, are linked to the orientation of the structure to which they belong”.

The standard model predicts structures of size up to 350 megaparsecs; a parsec is a unit of measure corresponding to 3.26 light years. “In the case of structures of galaxies whose scale is about 100 megaparsecs, and on scales that are smaller and nearer to us, the Universe is clearly not homogeneous”, explains Damien Hutsemékers. “But on larger scales, we consider that it has to be homogeneous . In particular, calculations show that the Universe had not enough time to form bigger structures. In summary, according to the standard model, a structure much bigger than 350 megaparsecs cannot exist”.

The suspected existence of a quasar structure

In this context, the observation of these 93 quasars has become interesting. Such a high density of these objects in the same region of the sky raised suspicions. “The recent discovery of this group was already surprising”, explains Dominique Sluse. “Observing this overdensity of quasars was a first step which allowed astronomers to speculate that it was probably a structure that was bigger than one gigaparsec, that is to say, more than 3.26 billion light years, at a time when the Universe was only 5 billion years old. This is ten times bigger than the structures that are usually observed and three times bigger than the predicted limit of the standard model. But was this the result of chance or not? Was this really a single individual structure or were we missing something? It was difficult to say”.    

In order to establish the facts once and for all, it was necessary to pay more attention to this structure. The answer was to be found in the emission of light from the accretion disc. As a reminder, quasars are too far for their internal structures to be directly observed. Only the light they emit can be studied. Another important point to remember is that the orientation of the rotation axis of an object in the Universe without the influence of another force is completely random. Statistically, the alignment of the rotation axes of different objects therefore has little chance of being random, and is rather an indication that they belong to the same structure.  By being able to determine the orientation of the rotation axes of the quasars observed, it now becomes possible to verify whether these axes are aligned or not. In the affirmative case, the theory of the presence of a structure is reinforced.