An artistic representation of matter in the early universe slowly coalescing into larger cosmic structures in the late universe. Credit: Minh Nguyen, University of Michigan and Thanh Nguyen (husband)
Scientists have discovered that cosmic structures grow more slowly than predicted by Einstein’s theory of general relativity, with dark energy playing a more dominant inhibitory role than previously thought. This discovery may reshape our understanding of dark matter, dark energy and fundamental cosmological theories.
As the universe evolves, scientists expect large cosmic structures to grow at a certain rate: dense regions such as galaxy clusters will grow denser, while the vacuum of space will become emptier.
However, researchers from the University of Michigan have discovered that the growth rate of these large structures is slower than predicted by Einstein’s theory of general relativity.
They also showed that while dark energy is accelerating the global expansion of the universe, the suppression of the growth of cosmic structure that the researchers see in their data is more pronounced than theory predicts. Their results were published on September 11 in the journal Physical review letters.
The cosmic web
Galaxies are interconnected throughout our universe like a giant cosmic spider’s web. Their distribution is not random. Instead, they tend to cluster together. In fact, the entire cosmic web started out as small clumps of matter in the early universe, which gradually grew into individual galaxies, and eventually galaxy clusters and filaments.
“Over the course of cosmic time, a small lump of mass attracts and accumulates more and more matter from its local region through gravitational interaction. As the region becomes denser, “It eventually collapses under its own gravity.”
“So, as they collapse, the density of the clumps increases. That’s what we mean by growth. It’s like a loom where the one-, two-, and three-dimensional collapses look like a leaf, a thread, and a knot. The reality is a combination of the three states, and you have galaxies living along the threads while galaxy clusters — clusters of Thousands of galaxies – the most massive objects in our universe bound by gravity – sit in the nodes.
Dark energy and cosmic expansion
The universe is not made only of matter. It also likely contains a mysterious component called dark energy. Dark energy is accelerating the expansion of the universe on a global scale. While dark energy accelerates the expansion of the universe, it has the opposite effect on larger structures.
“If gravity acts as an amplifier that promotes perturbations of matter to grow into large-scale structure, then dark energy acts as a dampener on these perturbations and slows the growth of structure,” Nguyen said. “By studying how cosmic structure assembles and grows, we can try to understand the nature of gravity and dark energy.”
Methodology and probes
Nguyen, University of Maryland physics professor Dragan Huterer, and University of Maryland graduate student Yuyu Wen examined the temporal growth of large-scale structure throughout cosmic time using several cosmological probes.
First, the team used what is called the cosmic microwave background. The cosmic microwave background, or CMB, consists of photons emitted directly after… the great explosion. These photons provide a snapshot of the very early universe. As photons travel to our telescopes, their path can become distorted, or gravitationally affected, by extensive structure along the way. By examining them, researchers can infer how structure and matter are distributed between us and the cosmic microwave background.
Nguyen and his colleagues took advantage of a similar phenomenon of weak gravitational lensing of galaxy shapes. Light from background galaxies is distorted by gravitational interactions with foreground matter and galaxies. Cosmologists then decode these distortions to determine how the intervening matter is distributed.
“Crucially, since the CMB and background galaxies are at different distances from us and our telescopes, weak gravitational lensing of galaxies typically explores matter distributions later than they do through weak gravitational lensing of the CMB,” Nguyen said. .
To trace the growth of the structure to later times, the researchers also used the motions of galaxies in the local universe. When galaxies fall into the gravitational wells of underlying cosmic structures, their motions directly follow the growth of the structure.
“The difference in growth rates that we are likely to detect becomes more pronounced as we get closer to the present day,” Nguyen said. “Individually and collectively, these different investigations point to growth inhibition. Either we’re missing some systematic error in each of these probes, or we’re missing some new late-stage physics in our standard model.”
Stress handling S8
The results potentially address the so-called S8 tension in cosmology. S8 is a parameter describing the growth of the structure. Tension arises when scientists use two different methods to determine the value of S8, but they do not agree. The first method, using photons from the cosmic microwave background, indicates a higher S8 value than the value inferred from weak galaxy gravitational lensing and galaxy cluster measurements.
Neither of these probes measures structure growth today. Instead, they examined the structure at earlier times, then extrapolated those measurements to present time, assuming the Standard Model. The structure of cosmic microwave background probes in the early Universe, while weak galactic gravitational lensing and the structure of cluster probes in the late Universe.
The researchers’ findings about late growth suppression would put the two S8 values in complete agreement, according to Nguyen.
“We were surprised by the high statistical significance of the suppression of dysplasia,” Hutterer said. “Honestly, I feel like the universe is trying to tell us something. Our job now, as cosmologists, is to interpret these results.”
“We would like to strengthen the statistical evidence for growth suppression. We would also like to understand the answer to the more difficult question of why structures grow slower than expected in the Standard Model with dark matter and dark energy. This effect may be caused by new properties of dark energy and dark matter, or some other extension.” For general relativity and the Standard Model we haven’t thought about it yet.
Reference: “Evidence for the suppression of structure growth in the conformal cosmological model” by Nhat Minh Nguyen, Dragan Hutterer and Yue Wen, 11 September 2023, Physical review letters.
doi: 10.1103/PhysRevLett.131.111001
