# Researchers propose a type of random gravitational wave background model that produces strong anisotropy

Similar to CMB, the gravitational waves generated after inflation are superimposed in the propagation process to form a directionrandom gravitational wave background (SGWB), which is also in anisotropy Contains the key information of the inflation model. During some violent processes in the early universe, such as phase transition, preheating, and the generation and evolution of topological defects, Hubble< /span>There are often large fluctuations in energy density in the horizon, due to its non-zero horizontal untracked component , These disturbances will produce observable gravitational waves. Gravitational waves hardly interact with other matter when propagating in the universe, and their energy hardly attenuates. This makes gravitational waves an ideal probe for directly detecting the evolution of the early universe and will provide unique clues to new physics. The frequency of these random gravitational waves can fall within the sensitive range of various detectors, such as ground-based gravitational waves Detectors LIGO, space gravitational wave detectors LISA and Taiji, pulsar timing array SKA, etc. In addition, SGWB can also explain the random totality discovered by the NANOGrav Cooperative Group of North America Spectrum process.

SGWB and CMB are also random backgrounds. From the history of CMB observation, it can be found that with the advancement of detection methods, it is a development trend from measuring energy spectrum to observing anisotropy and improving angular resolution. The research of SGWB anisotropy has received more and more attention. Recently, Cai Ronggen and Guo Zongkuan, researchers from the Institute of Theoretical Physics of the Chinese Academy of Sciences, and Liu Jing, a postdoctoral fellow (graduated) from the Institute of Theoretical Studies, explored the universe’s domain Collapse generated SGWB, and propose a new mechanism to generate strong anisotropy of SGWB, and then observing this strong anisotropic SGWB can limit the energy scale of inflation. The domain wall is a sheet-like topological defect, which is produced in the particle physics model with discrete symmetry spontaneously broken. The movement and collapse of domain walls can generate considerable gravitational waves. Researchers consider a type of light scalar field during inflationary periods. This kind of light scalar field can be Higgs field, axis subfield, etc., and its potential function has many A non-degenerate vacuum (Figure 1), the effective mass of the scalar field is smaller than the Hubble parameter during inflation . The vacuum quantum perturbation during the inflationary period caused the scalar field to cross the barrier. After the end of inflation, the Hubble parameter gradually decreases below the effective mass of the scalar field, the scalar field is stabilized near a different vacuum, and domain walls are formed. Although the energy density of the domain wall increases with time, due to the non-degenerate vacuum, the domain wall will collapse before its energy dominates the universe, and gravitational waves are mainly generated before and after the domain wall collapses. Since the scalar field is a light field during the inflation period, its large-scale perturbation remains unchanged after going out of the horizon, resulting in large-scale perturbation of domain wall energy density and anisotropy of SGWB.

This study uses a semi-analytical method to obtain the predicted gravitational wave energy spectrum and anisotropy (Figure 2). The difference from previous studies is that the background anisotropy of random gravitational waves generated by this type of model is large, and the angular power spectrum can reach greater than 0.01 on a large scale, which is expected to be observed by detectors such as SKA. Through the observation of the angular power spectrum of the SWB, it is possible to provide more stringent limits on the inflation energy scale. The model can also explain the 12 and a half years of observations made by the NANOGrav cooperative group.

Related research results were published on *Physical Review Letters *126.141303(2021). The research work was funded by the National Natural Science Foundation of China, the Ministry of Science and Technology, and Chinese Academy of Sciences.

*Figure 1. A typical non-degenerate scalar field potential function*

*Figure 2. Random gravitation generated by domain wall collapse Wave background intensity and strong anisotropy*

**Source:Institute of Theoretical Physics, Chinese Academy of Sciences**