Cosmic structure growth is the production and evolution of large-scale structures in the universe, such as galaxies, galaxy clusters, and cosmic filaments, driven by the gravitational interactions of dark matter and ordinary matter. The expansion of cosmic structures is a basic feature of cosmology that is best understood within the frameworks of the Big Bang theory and the notion of cosmic inflation. These theories describe how the cosmos evolved and how structures emerged over billions of years.
Scientists expect enormous cosmic structures to grow at a particular rate as the universe evolves: dense places such as galaxy clusters will grow denser, while the nothingness of space will grow emptier. However, researchers at the University of Michigan revealed that the rate at which these massive structures expand is slower than predicted by Einstein’s Theory of General Relativity.
They also demonstrated that, as dark energy drives the universe’s global expansion, the suppression of cosmic structure growth shown in the researchers’ data is considerably stronger than predicted by the hypothesis. Their findings have been published in Physical Review Letters.
Galaxies are woven like a gigantic cosmic spider web throughout our cosmos. Their distribution is not haphazard. Instead, they tend to congregate. In fact, the entire cosmic web began in the early cosmos as tiny clumps of matter that developed into individual galaxies, and then galactic clusters and filaments.
If gravity acts as an amplifier, amplifying matter perturbations as they grow into large-scale structure, then dark energy acts as an attenuator, dampening these perturbations and slowing structure growth. By examining how cosmic structure has been clustering and growing, we can try to understand the nature of gravity and dark energy.
Minh Nguyen
“Through gravitational interaction, an initially small clump of mass attracts and accumulates more and more matter from its local region throughout cosmic time. As the region becomes denser and denser, it eventually collapses under its own gravity,” said Minh Nguyen, the study’s principal author and postdoctoral research researcher in the University of Michigan’s Department of Physics.
“So as they collapse, the clumps grow denser. That is what we mean by growth. It’s like a fabric loom where one-, two- and three-dimensional collapses look like a sheet, a filament and a node. The reality is a mixture of all three cases, and you have galaxies living along the filaments while galaxy clusters – groups of thousands of galaxies, the most massive objects in our universe bounded by gravity – sit at the nodes.”
The universe is made up of more than just matter. It is also believed to include a mystery component known as dark energy. On a global scale, dark energy drives the expansion of the cosmos. Dark energy has the opposite effect on massive structures since it accelerates the expansion of the universe.
“If gravity acts as an amplifier, amplifying matter perturbations as they grow into large-scale structure, then dark energy acts as an attenuator, dampening these perturbations and slowing structure growth,” Nguyen explained. “By examining how cosmic structure has been clustering and growing, we can try to understand the nature of gravity and dark energy.”
Nguyen, U-M physics professor Dragan Huterer, and U-M graduate student Yuewei Wen used numerous cosmological probes to study the temporal evolution of large-scale structures over cosmic time.
First, the scientists employed a technique known as the cosmic microwave background. The cosmic microwave background, or CMB, is made up of photons that were emitted shortly after the Big Bang. These photons offer a glimpse into the very early universe. Large-scale structures along the route can distort or gravitationally lens photons as they travel to our telescopes. The researchers can infer how structure and substance are dispersed between us and the cosmic microwave background by examining them.
Nguyen and colleagues took advantage of a similar phenomenon with weak gravitational lensing of galaxy shapes. Light from background galaxies is distorted through gravitational interactions with foreground matter and galaxies. The cosmologists then decode these distortions to determine how the intervening matter is distributed.
“Crucially, as the CMB and background galaxies are located at different distances from us and our telescopes, galaxy weak gravitational lensing typically probes matter distributions at a later time compared to what is probed by CMB weak gravitational lensing,” Nguyen said.
To track the growth of structure to an even later time, the researchers further used motions of galaxies in the local universe. As galaxies fall into the gravity wells of the underlying cosmic structures, their motions directly track structure growth.
“The difference in these growth rates that we have potentially discovered becomes more prominent as we approach the present day,” Nguyen added. “These various probes, both individually and collectively, indicate growth suppression.” Either we are missing some systematic mistakes in each of these probes, or our conventional model is missing some novel, late-time physics.”
The findings could help to resolve the so-called S8 tension in cosmology. S8 is a parameter that describes structural growth. The conflict emerges when scientists employ two distinct ways to calculate S8, and they do not agree. The first approach, which uses photons from the cosmic microwave background, yields a larger S8 value than observations of galaxy-weak gravitational lensing and galaxy clustering.
Neither of these probes now assesses the expansion of the structure. Instead, they examine structures in the past and extrapolate those measurements to the present, adopting the standard model. The cosmic microwave background examines early universe structure, whereas galaxy weak gravitational lensing and clustering probe late universe structure.
According to Nguyen, the researchers’ discovery of a late-time inhibition of growth would bring the two S8 values into complete agreement.
