Artistic representations of matter in the early universe slowly coalesce 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 general theory of relativity, and dark energy plays a more dominant inhibitory role than previously thought. This discovery may change our understanding of dark matter, dark energy and fundamental cosmological theories.
As the universe expands, scientists expect large cosmic structures to grow at a certain rate: dense regions such as galaxy clusters will become denser, while empty space will become increasingly empty.
However, researchers from the University of Michigan have discovered that the growth rate of these massive structures is slower than Einstein’s general theory of relativity predicted.
They also show that although 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 greater than theory predicts. The results were published Sept. 11 in the journal Physical review letter.
Cosmic network
Galaxies are interconnected throughout our universe like a giant cosmic spider’s web. The distribution is not random. Instead, they tend to flock together. In fact, the entire cosmic web began as tiny clumps of matter in the early universe, which gradually grew into individual galaxies, and eventually into clusters and filaments of galaxies.
“Throughout cosmic time, a small blob of mass attracts and collects more matter from its local region through gravitational interactions. As the region becomes denser, “It eventually collapses under its own gravity.”
“So, as it collapses, the density of the lump increases. That’s what we mean by growth. It’s like a loom whose one-, two-, and three-dimensional collapses look like leaves, threads, and knots. The reality is a combination of all three states, and you have galaxies living along the strings, while clusters of galaxies — clusters of Thousands of galaxies – the most massive objects in our universe bound by gravity – are at the point -the node point.
Dark energy and cosmic expansion
The universe is not only made of matter. It may also contain a mysterious component called dark energy. Dark energy accelerates the expansion of the universe on a global scale. Although dark energy accelerates the expansion of the universe, it has the opposite effect on the larger structure.
“If gravity acts as an amplifier that encourages material disturbances to grow into large-scale structures, then dark energy acts as a damper for those disturbances and slows down the growth of structures,” Nguyen said. “By studying how cosmic structures assemble and evolve, we can try to understand the nature of gravity and dark energy.”
Methodology and investigation
Nguyen, University of Maryland physics professor Dragan Huterer, and University of Maryland graduate student Yuyu Wen researched the temporal growth of large-scale structures through cosmic time using several cosmological probes.
First, the team used what’s called a cosmic microwave background. The cosmic microwave background, or CMB, consists of photons emitted directly after… the big bang. These photons provide an early picture of the universe. As photons travel toward our telescopes, their paths can be distorted, or affected by gravity, by extensive structures along those paths. By studying them, researchers can deduce how structure and matter are distributed between us and the cosmic microwave background.
Nguyen and his colleagues exploited a similar phenomenon, namely weak gravitational lensing on the shape of galaxies. Light from background galaxies is distorted by gravitational interactions with foreground matter and galaxies. Cosmologists then decode these distortions to determine how the material within them is distributed.
“Most importantly, because the CMB and the background galaxy are at different distances from us and our telescopes, weak gravitational lensing of the galaxy typically explores the distribution of matter more slowly than through weak gravitational lensing of the CMB,” Nguyen said.
To trace the growth of these structures into later times, the researchers also used the movement of galaxies in the local universe. When a galaxy falls into the gravitational well of an underlying cosmic structure, its movement directly follows the growth of that structure.
“The differences in growth rates that we might detect become more pronounced as we get closer to current conditions,” Nguyen said. “Individually and collectively, these investigations suggest growth inhibition. Either we missed some systematic error in each of these investigations, or we missed some late stage of new physics in our standard model.”
S8 stress handling
The results have the potential to resolve the so-called S8 tension in cosmology. S8 is a parameter that describes structure growth. Tensions arose when scientists used two different methods to determine the value of S8, but they did not agree. The first method, using photons from the cosmic microwave background, shows higher S8 values than values inferred from weak galactic gravitational lensing and galaxy cluster measurements.
None of these studies measured current structural growth. Instead, they examine structures in the past, then extrapolate those measurements to the present, with the assumptions of the Standard Model. The probe structure of the cosmic microwave background in the early universe, while the gravitational lensing of weak galaxies and the probe structure of clusters in the late universe.
The researchers’ finding of late growth suppression would put the two S8 grades in complete agreement, according to Nguyen.
“We were surprised by the high statistical significance in dysplasia suppression,” Hutterer said. “Honestly, I feel like the universe is trying to tell us something. Our task now, as cosmologists, is to interpret these results.”
“We want to strengthen statistical evidence to suppress growth. We also want to understand the answer to the more difficult question of why the structure grows more slowly than predicted 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 other expansions.” For general relativity and the Standard Model, we haven’t thought about it yet.
Reference: “Evidence of structure growth suppression in conformal cosmological models” by Nhat Minh Nguyen, Dragan Hutterer and Yue Wen, 11 September 2023, Physical review letter.
doi: 10.1103/PhysRevLett.131.111001
2023-09-14 11:42:51
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