Quantum-gravitational mechanism could explain the universe's homogeneity
Our universe is known to be remarkably homogeneous and isotropic. This essentially means that matter is distributed evenly throughout the universe and that it looks almost the same in all directions.
Physics theories, however, predict that in its early days, the universe may have been far less orderly, with different regions expanding at varying rates. Yet how the universe could have evolved from this potentially uneven beginning into the smoothness we observe today remains unclear.
Researchers at Baylor University, Jiangxi Normal University, State University of Rio de Janeiro and Universidade Federal Fluminense recently delineated a mechanism that could explain how the universe shifted from early unevenness (i.e., anisotropy) to its current homogeneity. Their theoretical paper, published in Physical Review Letters, models the evolution of the early universe using a framework known as the modified loop quantum cosmology (mLQC-I) model.
"Einstein's general theory of relativity famously predicts an initial singularity—a point of infinite density—at the beginning of the universe," Anzhong Wang, senior author of the paper, told Phys.org.
"Cosmic bounce models, such as those arising from loop quantum cosmology (LQC), offer a compelling way to avoid this singularity. In LQC, our expanding universe emerges from a prior contracting phase, passing smoothly through a high-density quantum bounce."
The universe's homogeneity and isotropy were confirmed by numerous cosmological observations. While existing physical theories with a cosmic bounce describe the underpinnings of the universe's current homogeneity, they do not convincingly explain how early, directionally dependent irregularities (i.e., anisotropies) might have been suppressed.
"While this smooth state is well explained by standard cosmological inflation, bounce models face a major hurdle known as the anisotropy problem," said Wang.
"Any tiny deviations from isotropy in the contracting phase tend to grow drastically as the universe approaches the bounce. As a result, the universe could emerge from the bounce highly deformed and anisotropic, which would lead to a universe entirely different from the one we observe today."
A quantum 'bounce' that smooths the early universe
Wang and his colleagues wanted to identify a robust mechanism that could resolve the long-standing uncertainty about how the universe became more homogeneous and isotropic. To do this, they specifically explored the evolution of a so-called Bianchi I universe, theorized to expand at different rates in different directions.
They relied on a framework called mLQC-I, which uses principles of quantum mechanics to describe gravity and the evolution of the universe. This framework essentially predicts the occurrence of a so-called "quantum bounce," an event during which a previously contracting universe reaches a minimum nonzero size and starts expanding again.
"We demonstrate that a specific modification in how loop quantum cosmology is implemented, the mLQC-I model, can naturally suppress these disruptive anisotropies," explained Wang. "Through analytical calculations and numerical modeling of early cosmic dynamics, we found that even if the universe is highly anisotropic before reaching the bounce, quantum geometry corrections alter the evolutionary equations near the Planck scale."
The researchers' theoretical model predicts that as a universe emerges from the quantum bounce and starts expanding again, effects that arise solely due to the quantum properties of the geometry of spacetime can dampen asymmetries. This ultimately forces the universe to become isotropic and smooth almost instantly.
"The most notable contribution of our work is the discovery of this self-isotropization mechanism, which is driven entirely by non-perturbative quantum effects without requiring exotic matter fields," said Wang. "This mechanism resolves a long-standing conceptual vulnerability in bouncing cosmologies by proving that a smooth, symmetric universe can reliably emerge while still in the deep quantum regime."
A promising framework for understanding the early universe
This recent study offers a new perspective on a long-standing cosmological mystery: why the universe is so homogeneous if it started off with numerous anisotropies. It also highlights the potential of the mLQC-I framework for studying the early evolution of the universe.
"Structurally, our work provides a pristine, isotropic starting point for subsequent cosmic inflation, bridging the gap between quantum gravity scenarios and verifiable cosmological observations," said Wang.
In the future, other theorists could further investigate the newly proposed mechanism or try to identify signatures that could be observed experimentally to confirm the team's predictions. Meanwhile, Wang and his colleagues plan to extend their framework to explore the theorized quantum-mechanically smoothed transition to a classical universe in greater detail.
"Specifically, we want to calculate the evolution of the primordial cosmological perturbations generated during this isotropic phase and see how this post-bounce state can be connected with our current universe," added Wang. "In addition, we shall also seek distinct, measurable signatures in the cosmic microwave background (CMB) or primordial gravitational waves, allowing us to observationally test this version of LQC."
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Publication details
Wen-Cong Gan et al, Quantum Damping of Cosmological Shear: A New Prediction from Loop Quantum Cosmologies, Physical Review Letters (2026). DOI: 10.1103/f8tr-bq61.
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Citation: Quantum-gravitational mechanism could explain the universe's homogeneity (2026, July 13) retrieved 13 July 2026 from https://phys.org/news/2026-07-quantum-gravitational-mechanism-universe-homogeneity.html
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