Our current understanding of the evolution and structure of the cosmos relies on the assumption that gravity is described by Einstein’s general theory of relativity, and that only about 5% of the mass in the whole Universe is composed of ordinary matter.
The remaining 95% is thought to be composed of dark matter and dark energy. Dark matter, comprising one third of the total mass, is responsible for the formation and growth of galaxies, while dark energy, which accounts for more than two-thirds of the mass, is thought to drive the accelerated expansion of the Universe.
This understanding falls under a model known as ΛCDM, where CDM stands for “cold dark matter” and Λ is the widely used symbol for dark energy. In it, dark matter particles are considered to be heavy and slow moving, much like particles that make up a cold gas.
While the ΛCDM provides an excellent description of astronomical observations, it still has some conceptual problems. For example, although there should be much more dark matter than ordinary matter in the Universe, this mysterious entity has never been directly observed and so scientists still don’t know what it is made or whether it exists at all. Dark energy has proven even more elusive, with no plausible hypothesis regarding its composition and nature put forth to date.
A modified theory
These issues have prompted researchers to look for alternatives to ΛCDM that might not require these strange, undetectable entities. One of the most popular of these is a cosmological theory based on two postulates, where the first claims that gravity should actually be described by a modification of Einstein’s theory that does not require the introduction of dark energy to explain the accelerated expansion of the Universe. The second is the assertion that dark matter is not composed of heavy, unknown particles, but of tiny, light particles called neutrinos.
“This model is particularly interesting because it yields almost identical growth and expansion histories to the ΛCDM, unlike the other alternative models,” said Jounghun Lee of the Seoul National University in an email. “In other words, this model satisfies all the constraints put by the standard diagnostics like the cosmic microwave background radiation temperature power spectrum.”
The fact that this model provides such an accurate description of observational data prompted Lee and his colleagues from Italy and Korea to conduct a detailed analysis of the model’s predictions, which could help determine whether the ΛCDM or the new approach considered by the authors of the study is correct.
In their study published in The Astrophysical Journal, they analyzed both cosmological models through theoretical and numerical computations to determine the orientation of galaxy clusters — which have an ellipsoidal shape — within galactic superclusters, the largest gravitationally bound objects in the Universe. This analysis can help determine which cosmological model best describes the real world, because the results of calculating the orientation of clusters depend both on which theory describes gravity and on what particles dark matter is made of.
“In our work, we have come up with an efficient diagnostics based on the shape alignments of galaxy clusters, with which this model can be discriminated from the ΛCDM model,” explained Lee.
Future experiments will provide proof
What they found was quite surprising. It turns out that despite the fact that for many observed and measured quantities both theories give almost identical predictions. However, in terms of the orientation of clusters inside a supercluster, the predictions diverge — the orientation of individual clusters tend to be quite different, which in principle should make it possible to find out which description is correct through astronomical observation.
To actually study the orientation of galactic clusters, it is necessary to improve observational instruments, since the current level of development does not allow scientists to probe these objects with the required accuracy to be able to provide evidence for either of these theories.
“To use our method in practice and to truly test the model we considered, however, it will be necessary to come up with a method to determine the three dimensional shapes of galaxy clusters in real space from observations,” concluded Lee. “That is the main difficulty.”
Hopefully, with the advent of next-generation telescopes the theories explored in this study can be put to the test, potentially leading to a breakthrough in our understanding of the mysteries surrounding dark matter and dark energy. This would be a huge leap forward in our understanding of how the Universe is organized.
Reference: Jounghun Lee, Suho Ryu, and Marco Baldi, Disentangling Modified Gravity and Massive Neutrinos with Intrinsic Shape Alignments of Massive Halos, The Astrophysical Journal (2023). DOI: 10.3847/1538-4357/acabbc
Feature image credit: NASA