On the largest scales, matter in the universe is distributed remarkably evenly, and the universe itself looks almost the same in all directions. However, cosmological models suggest that in the earliest stages, different regions of space may have expanded at different rates. Now, physicists have proposed a mechanism that explains how this initial anisotropy could have smoothed out so quickly.
The weak point of bounce models
If we extrapolate the equations of general relativity back in time, they lead to a cosmological singularity—a state of infinite density where the usual description of space and time ceases to work. Loop quantum cosmology proposes a different scenario. In it, the current expansion phase was preceded by a contraction phase, and the transition between the two occurred through a quantum bounce (the Big Bounce), during which the universe reached a minimal but non-zero size.
Such scenarios face a serious obstacle known as the anisotropy problem. Tiny early deformations grow rapidly as the moment of the bounce approaches. As a result, after this event, the Universe would have remained strongly distorted and completely unlike the one we observe today.
Quantum smoothing
A team from Baylor University in the United States, together with colleagues from China and Brazil, modeled the behavior of a so-called Bianchi type I universe, in which the rate of expansion depends on direction. For their calculations, the scientists used modified loop quantum cosmology, or mLQC-I. This approach combines the principles of the microworld with the description of gravity.
The calculations showed that near Planck-scale regimes, quantum corrections to the geometry of spacetime change the evolution equations. Even if the asymmetries before the bounce are enormous, they decay exponentially fast immediately after it.
According to the study’s senior author, Anzhong Wang, the effect works regardless of how strong the initial deviations were. A homogeneous and isotropic Universe reliably emerges while still in a deeply quantum regime, Phys.org reports.
Alignment without exotic matter
The main result of the work is the proposed mechanism of self-isotropy, which is based solely on the quantum nature of space-time and does not require the introduction of exotic matter fields. If confirmed, this mechanism could eliminate one of the main conceptual problems of cosmological models with quantum rebound. The paper presenting the findings was published in the peer-reviewed journal Physical Review Letters.
Observations of the cosmic microwave background, the oldest light accessible to our instruments, show that the temperatures of different regions of the sky match to an accuracy of about one part in one hundred thousand. Theories of the early Universe are meant to explain precisely this striking uniformity. In addition, the described process creates ordered initial conditions for subsequent inflation, the era of rapid expansion of space.
Next steps
The authors next plan to calculate the evolution of cosmological perturbations that arise during this symmetric phase. This will help connect the state immediately after the bounce with the modern Universe.
At the same time, the team will search for measurable traces of the predicted effect. They hope to find them in the cosmic microwave background and in primordial gravitational waves.
