Large cosmic structures are predicted to expand at a specific rate as the universe expands, with galaxy clusters and other dense regions expanding faster than empty space.
The rate of growth of these substantial structures, however, is slower than expected by Einstein’s Theory of General Relativity, according to research from the University of Michigan.
Additionally, they demonstrated that the suppression of cosmic structure growth observed in the data is considerably more pronounced than predicted by the hypothesis as dark energy speeds up the universe’s overall expansion. Physical Review Letters publish their findings.
Our universe is woven with galaxies like a vast cosmic spider web. They are not distributed randomly. Instead, they assemble in groups. In reality, the early universe’s cosmic web began as tiny aggregates of matter that later developed into individual galaxies, galactic clusters, and filaments.
An initially modest mass draws and gathers more and more matter from its local area through gravitational interaction over the course of cosmic time. The region finally collapses under its own gravity as it gets thicker and denser, according to Minh Nguyen, a postdoctoral research fellow at the University of Michigan’s Department of Physics and the study’s lead author.
Therefore, the clumps become denser as they crumble. Growth in this sense is what we mean. One-, two-, and three-dimensional collapses resemble a sheet, a filament, and a node, similar to a fabric loom. Galaxies live along the filaments while galaxy clusters—groups of hundreds of galaxies and the most massive, gravitationally linked objects in our universe—sit at the nodes in reality, which is a mixture of all three scenarios.
There is more than just matter in the universe. Dark energy, a mystery component, is also probably present. The global expansion of the universe is accelerated by dark energy. Dark energy has the opposite effect on big structures than it has on the universe’s expansion.
Dark energy functions as an attenuator, dampening these perturbations and limiting the evolution of structure, according to Nguyen. If gravity operates as an amplifier it enables matter perturbations to build into large-scale structures. We can attempt to comprehend the nature of gravity and dark energy by looking at how the cosmic structure has been expanding and gathering.
Nguyen, Dragan Huterer, a professor of physics at the University of Michigan, and graduate student Yuewei Wen used a variety of cosmological probes to study the temporal development of large-scale structures over cosmic time.
The team started by using something known as the cosmic microwave background. The cosmic microwave background, or CMB, is made up of photons that were released immediately following the Big Bang. These photons offer a glimpse into the very beginning of the universe. Large-scale structures in their path may distort or gravitationally lens the photons’ path as they approach our telescopes. The distribution of structure and matter between our planet and the cosmic microwave background can be deduced by the researchers by looking at them.
The weak gravitational lensing of galaxy forms was a similar phenomenon that Nguyen and colleagues exploited. Through gravitational interactions with the foreground matter and galaxies, background galaxies’ light is bent. The distribution of the intermediate matter is subsequently determined by the cosmologists by decoding these distortions.
Crucially, galaxy-weak gravitational lensing often probes matter distributions at a later time than what is probed by CMB-weak gravitational lensing since the CMB and background galaxies are positioned at different distances from us and our telescopes, according to Nguyen.
The researchers also used the motions of galaxies in the local universe to chart the emergence of the structure until an even later epoch. Their motions directly follow the development of the underlying cosmic structures as galaxies fall into their gravity wells.
As we get closer to the present, the disparity in these growth rates that we may have uncovered becomes more obvious, according to Nguyen. “Both separately and in combination, these various probes show a growth suppression. Either we are missing some systematic flaws in each of these probes, or our standard model is lacking some fresh, contemporary physics.
The results might address cosmology’s so-called S8 tension. The parameter S8 describes how a structure grows. Scientists are at odds when attempting to calculate the value of S8 using two separate methodologies. A greater S8 value is predicted by the first technique, which makes use of photons from the cosmic microwave background, than by the values deduced from observations of galaxy weak gravitational lensing and galaxy clustering.
These two probes don’t track the present-day expansion of the structure. Assuming the usual model, they instead examine structures from previous times and extrapolate those measurements to the present. While galaxy-weak gravitational lensing and clustering examine structures in the late universe, cosmic microwave background examines structures in the early universe.
Nguyen believes that the researchers’ discovery of a late-time suppression of growth would bring the two S8 values into complete agreement.
The substantial statistical significance of the unusual growth suppression “surprised us,” according to Huterer. “To be honest, I think the cosmos is trying to communicate with us. Now it is up to us cosmologists to explain these results.
“We want to make the statistical support for the growth suppression stronger. We also want to know why structures develop more slowly than predicted by the conventional model with dark matter and dark energy, which is a trickier topic to solve. The origin of this effect may be related to unique dark energy and dark matter features, or to an unanticipated extension of general relativity and the standard model.