Scientists have discovered that structure in the universe is growing more slowly than Einstein’s theory of general relativity predicted, and that dark energy plays a more dominant suppressive role than previously thought. This discovery could reshape our understanding of dark matter, dark energy, and fundamental cosmological theory.
As the universe evolves, scientists expect large cosmic structures to grow at a constant rate. This means that dense regions such as galaxy clusters will grow denser, while the voids in the universe will become emptier.
But researchers at the University of Michigan found that these large structures grow more slowly than Einstein’s theory of general relativity predicted.
They also showed that as dark energy accelerates the global expansion of the universe, the suppression of cosmic structure growth that the researchers see in their data is even more pronounced than theory predicts. Their results were published in the journal September 11th. physical review letter.
cosmic web
Galaxies are stretched throughout our universe like a giant cosmic spider web. Their distribution is not random. Instead, they tend to cluster together. In fact, the entire cosmic web began as a small clump of matter in the early Universe, gradually growing into individual galaxies, and finally into galaxy clusters and filaments.
“Throughout cosmic time, an initially small clump of mass attracts and accumulates more and more matter from its local region through gravitational interactions. As this region becomes more and more dense, it eventually It collapses under the force of gravity,” said Minh Nguyen, lead author of the study and a postdoctoral researcher in UM’s Department of Physics.
“So as they collapse, the clumps become denser. That’s what we mean by growth. It’s like a loom, where the collapse of one, two and three dimensions creates sheets, filaments. , which look like nodes. In reality, all three cases are mixed, with galaxies along filaments, while clusters (groups of thousands of galaxies, bounded by gravity) The heaviest objects in the universe) reside at the nodes.”
Dark energy and the expansion of the universe
The universe is not made only of matter. It may also contain a mysterious ingredient called dark energy. Dark energy accelerates the expansion of the universe on a global scale. Because dark energy accelerates the expansion of the universe, it has the opposite effect on large structures.
“If gravity acts like an amplifier that increases perturbations in matter to grow into large-scale structures, dark energy acts like an attenuator that dampens these perturbations and slows the growth of structures.” Nguyen said. “By studying how the structure of the universe assembled and grew, we can understand the nature of gravity and dark energy.”
Methodology and probes
Nguyen, University of Massachusetts physics professor Dragan Huterer, and University of Massachusetts graduate student Yuewei Wen will use several cosmological probes to examine the temporal growth of large-scale structures over cosmic time. Ta.
First, the team used something called the cosmic microwave background radiation. The cosmic microwave background radiation (CMB) is made up of photons emitted shortly after the cosmos. big bang. These photons provide a snapshot of the very early universe. As photons reach the telescope, their paths can be distorted by large structures along the way or gravitational lensing. By studying them, researchers can infer how structures and matter are distributed between us and the cosmic microwave background radiation.
Nguyen and colleagues exploited a similar phenomenon due to weak gravitational lensing in the shape of galaxies. Light from background galaxies is distorted by gravitational interactions with foreground matter and galaxies. Cosmologists then decipher these distortions to determine how the intervening matter is distributed.
“Importantly, because CMB galaxies and background galaxies are located at different distances from us and the telescope, weak gravitational lensing in galaxies typically slows down the distribution of matter compared to what weak gravitational lensing in CMB probes. Time will tell,” Nguyen said.
To track the growth of the structure to even later points in time, the researchers further exploited the motion of galaxies in the local universe. When a galaxy falls into the gravitational well of the underlying cosmic structure, its motion directly tracks the growth of the structure.
“These growth rate differences that we are potentially finding become more pronounced as we move closer to the present,” Nguyen said. “These various probes individually and collectively exhibit growth suppression. Either we are missing systematic errors in each of these probes, or we are missing new modern physics in the standard model. That’s it.”
S8 Dealing with tension
This discovery could address the so-called S8 tension in cosmology. S8 is a parameter representing the growth of the structure. Tension arises when a scientist uses two different methods to determine the value of his S8 and their opinions disagree. The first method, which uses photons from the cosmic microwave background, shows higher S8 values than those estimated from measurements of the weak gravitational lensing of galaxies and galaxy clustering.
None of these probes measure the growth of today’s structures. Instead, they examine the structure at previous times and extrapolate those measurements to the current time, assuming a standard model. The Cosmic Microwave Background probes the structure of the early Universe, while the late Universe probes for weak gravitational lensing and clustering of galaxies.
Nguyen said the researchers’ findings about late-stage growth suppression would bring the two S8 values into perfect agreement.
“We were surprised by the high statistical significance of the abnormal growth suppression,” Hutterer said. “To be honest, it feels like the universe is trying to tell us something. It’s now up to us cosmologists to interpret these discoveries.
“We would like to further strengthen the statistical evidence for growth suppression. We would also like to know the answer to a more difficult question: why is structure growth slower than expected in standard models that include dark matter and dark energy?” We believe that the cause of this effect could be due to new properties of dark energy and dark matter, or to other extensions of general relativity and the Standard Model that we have not yet considered.”
References: Nhat-Minh Nguyen, Dragan Huterer, and Yuewei Wen, “Evidence for Suppression of Structural Growth in Coincident Cosmology Models,” September 11, 2023. physical review letter.
DOI: 10.1103/PhysRevLett.131.111001