The new study limits the parameters of gravitational waves induced by scalar perturbations in the early universe

Recently, Professor Cai Yifu’s research group at the University of Science and Technology of China, Dr. Yan Shengfeng from the Italian National Institute of Nuclear Physics and Dr. Yuan Guanwen from the Institute of High Energy Physics of the Chinese Academy of Sciences have used the latest observed nanohertz gravitational wave stochastic background signal to limit the theoretical image of gravitational waves induced by scalar perturbation in the early universe, which will help deepen people’s understanding of the extremely early universe. On October 29, the study was published online in Science Bulletin.

The propagation in the universe of a random background of nanohertz gravitational waves induced by scalar perturbations in the very early days of the universe, which can be observed by pulsar timing arrays Courtesy of the interviewee

In 2023, the world’s four major pulsar timing array (PTA) research teams, NANOGrav in the United States, CPTA in China, EPTA in Europe and PPTA in Australia, independently published the latest observations of nanohertz gravitational wave stochastic background signals, which is the key evidence for the existence of nanohertz gravitational wave stochastic background.

On the basis of observational facts and theoretical research, the researchers propose to parameterize the energy spectrum of possible scalar perturbation-induced gravitational waves by using a class of inflected power-law spectra, and compare them with the latest observed nanohertz gravitational wave random background signal data. The model fitting shows that the existing observational data almost fall in the infrared segment of the theoretical gravitational wave spectrum, and the infrared segment satisfies the general prediction of scalar disturbance-induced gravitational waves in theory, and the near-infrared segment is more inclined to the amplitude of the original scalar disturbance with a large range of gain at a small scale.

“There is a large uncertainty in the ultraviolet segment of the energy spectrum due to the lack of observations, and in the future, this can be compensated by space gravitational wave detection experiments such as the European LISA and the Chinese Tianqin and Taiji. Cai Yifu said that the energy spectrum limited by the data also indicates that the possible mass of the primordial black hole should be less than 0.1 times the mass of the sun, and that there is a slight nonlinear effect on the primordial scalar perturbation.

“This study establishes a framework for interpreting and understanding the observed data of nanohertz gravitational waves in the stochastic background using the energy spectrum of scalar perturbation-induced gravitational waves, and combines it with the properties of primordial black holes and primordial perturbations, which is helpful for understanding the physical processes of the very early universe,” said Cai. (Source: China Science Daily Liu Runan)

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