Recently, Phys. Rev. Lett., an internationally renowned physics journal, published online the research result "Large Anisotropies of the Stochastic Gravitational Wave Background from Cosmic Domain Walls" by Liu Jing, a postdoctoral fellow at HIAS, UCAS, his supervisor Research Fellow Cai Ronggen, and Research Fellow Guo Zongkuan, which proposes a new mechanism of the generation of strong anisotropiesin the stochastic gravitational wave background (SGWB).
Gravitational waves open a new window for human beings to explore the universe. In some of the intense processes of the early universe, including phase transitions, preheating, and generation and evolution of topological defects, there are often strong perturbations of energy density within the Hubble horizon that will lead to considerable gravitational waves due to their non-zero transverse-traceless components. Gravitational waves travel through the universe with little interaction with other matter and little energy attenuation, which makes them ideal probes for direct exploration of early cosmic physics and provides unique clues for new physics. The study proposes a new mechanism is proposed a new mechanism of the generation of strong anisotropies in the SGWB, and obtains the gravitational wave energy spectrum and its anisotropic angular power spectrum via a semi-analytical method. It is found that the SGWB anisotropies generated under this mechanism are very strong, and the predicted angular power spectrum is greater than 0.01, which is expected to be observed by future SKA and other detectors to identify the energy scale of inflation.
Considering the existence of a light scalar field during inflation, its potential function has multiple non-degenerate vacuums, as shown in Figure 1, and the effective mass of the scalar field is less than the Hubble parameter during inflation. The scalar field crosses the potential energy barrier due to vacuum quantum perturbations during inflation. After inflation, the Hubble parameter gradually decreases below the effective mass of the scalar field, the scalar field stabilizes near different vacuums, and the domain wall (DW) forms. Although the proportion of the DW energy density increases with time, the DW will collapse before dominating the universe due to the non-degeneracy of the vacuums, and gravitational waves are mainly generated around the collapse of the DW. Since the scalar field is a light field during inflation, its large scale perturbations remain almost constant after leaving the Hubble horizon, leading to large scale perturbations of DW energy density and large anisotropies in the SGWB. The energy spectrum and anisotropies of predicted gravitational waves are obtained via the semi-analytical method, as shown in Figure 2. Unlike previous studies, the model in this study shows strong anisotropies, and the angular power spectrum is greater than 0.01, which is likely to be observed by SKA and other detectors in recent days. At the same time, the angular power spectrum of SGWB puts a stricter limit on the energy scale of inflation.
Figure 1: Potential Function of a Typical Non-degenerate Scalar Field
Figure 2: SGWB Intensity and Strong Anisotropies Generated by DW Collapse
This study was jointly completed by Liu Jing, a postdoctoral fellow at HIAS School of Fundamental Physics and Mathematical Sciences, his supervisor Research Fellow Cai Ronggen, and Research Fellow Guo Zongkuan, and was funded by the Major Program of the National Natural Science Foundation of China, the National Key R&D Program, and the CAS Strategic Priority Research Program. The HIAS School of Fundamental Physics and Mathematical Sciences is the first completion unit.
Article link: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.141303
