Universe slows cosmic growth defying the theory of relativity

Dark energy is believed to have a negative impact on big structures, limiting the formation of such particles.
Jijo Malayil
An artist’s representation of matter in the early universe
An artist’s representation of matter in the early universe

University of Michigan 

Large cosmic structures are predicted to expand at a certain rate as the universe expands, with galaxy clusters and other dense areas expanding faster than empty space. 

Contrary to earlier understandings based on Einstein's theory of general relativity, research from the University of Michigan has now found that the pace of growth of these substantial structures is slower than expected.

A look into the anatomy of these structures shows that our cosmos 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 small aggregates of matter that later developed into individual galaxies, galactic clusters, and filaments.

Their study has been published in the journal APS.

Dark energy effect

An initially modest mass draws and gathers more and more matter from its local area during the course of the universe through gravitational interaction. The area gradually collapses under its own gravity as it gets denser and denser.

According to the team, these clumps become denser as they crumble, which is described as growth in this reference. "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," said Minh Nguyen, lead author of the study and postdoctoral research fellow in the University's Department of Physics.

The researchers claim that growth suppression is supported by both the Planck data on the cosmic microwave background and the data on the large-scale structure of galaxies, clusters of galaxies, and cosmic velocities.

Scientists highlight the relevance of dark energy in the universe's expansion. This mystery component aids in the expansion of the cosmos. However, dark energy has the opposite impact on big structures than it has on the universe's expansion.

Dark energy works as an attenuator, dampening these perturbations and limiting the formation of structure. In contrast, gravity acts as an amplifier enabling matter disturbances to build into large-scale structures. “By examining how cosmic structure has been clustering and growing, we can try to understand the nature of gravity and dark energy, said Nguyen.

Cosmological probes

The team utilized various probes to analyze the temporal growth of large-scale structures, starting with what’s termed as cosmic microwave background (CMB). This process is based on photons emitted just after the Big Bang, which provide a snapshot of the very early universe. Large-scale structures in their route may bend or gravitate the photons' course as they approach our telescopes. The distribution of structure and substance between our planet and the cosmic microwave background may be deduced by the researchers by looking at them.

“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,” said Nguyen.

Researchers further probed the movements of galaxies in the local universe to chart the emergence of the structure until an even later epoch. Their movements directly follow the development of the underlying cosmic structures as galaxies fall into their gravity wells.

The team found that the differences became more as time passed. “These different probes individually and collectively indicate a growth suppression. Either we are missing some systematic errors in each of these probes, or we are missing some new, late-time physics in our standard model, said Nguyen.

Moving forward, the group now wants to improve the statistical support for the growth suppression. Additionally, spend time understanding why structures develop more slowly than anticipated in the conventional model with dark matter and dark energy. "The cause of this effect may be due to novel properties of dark energy and dark matter, or some other extension of General Relativity and the standard model that we have not yet thought of, said Nguyen.

Study abstract

We present evidence for a suppressed growth rate of large-scale structures during the dark-energy-dominated era. Modeling the growth rate of perturbations with the “growth index” γ, we find that current cosmological data strongly prefer a higher growth index than the value γ=0.55 predicted by general relativity in a flat Lambda cold dark matter cosmology. Both the cosmic microwave background data from Planck and the large-scale structure data from weak lensing, galaxy clustering, and cosmic velocities separately favor growth suppression. When combined, they yield γ=0.633+0.025−0.024, excluding γ=0.55 at a statistical significance of 3.7σ. The combination of fσ8 and Planck measurements prefers an even higher growth index of γ=0.639+0.024−0.025, corresponding to a 4.2σ tension with the concordance model. In Planck data, the suppressed growth rate offsets the preference for nonzero curvature and fits the data equally well as the latter model. A higher γ leads to a higher matter fluctuation amplitude S8 inferred from galaxy clustering and weak lensing measurements, and a lower S8 from Planck data, effectively resolving the S8 tension.

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