How Can We Detect Gravitational Waves Generated By Axion-Like Particle? (Cosmology)

Wei Cheng and colleagues investigated the gravitational wave (GW) generated by the Axion-like Particle inflation, by applying the multi-nature inflation model, where the end of inflation is achieved through the phase transition (PT). They showed that, the GW will consist of two parts, i.e., GW is generated by quantum fluctuations (QF) during inflation and by phase transition (PT) during the end of the inflation, and it will bring a sizable GW that may be detected by the future GW detectors. Their study recently appeared in Arxiv.

In the inflationary models with broken slow-roll conditions, the primary GW only comes from the quantum fluctuations (QF) during inflation, and its intensity is related to the tensor-to-scalar ratio ‘r’ of inflation. However, the ‘r’ predicted by the theory, in this case, is so small that a weak GW will be generated, which brings great challenges for experimental detection.

However, there is a new way to end inflation, i.e., the inflationary end of the bubble nucleation method. This method also has the natural function of reheating the universe through the collision of true vacuum bubble walls and convert it to radiation. Here are two scalar fields in this inflationary models, one of the fields is responsible for driving the inflation of the universe, whereas the other field can indeed percolate the true vacuum from the meta-stable one, and complete the phase transition (PT) and end inflation simultaneously. In this case, the source of GW includes quantum fluctuations during inflation as well as the PT that will produce considerable GW. This makes it possible to detect GW during inflation.

Now, Wei Cheng and colleagues investigated the Axion-like Particle (ALP) inflation by applying the multi-natural inflation model, where the end of inflation is achieved through the phase transition. They also investigated the gravitational wave generated by the ALP inflation.

“The multi-natural inflation (MNI) based on natural inflation (NI) can increase the feasible parameter space of the NI that is shrink by the latest CMB data. The inflation of MNI has a good property, shift symmetry, allows an ALP to act as the inflaton. Besides, it also keeps the flatness of potential energy, which is essential for driving the expansion of the universe and generating density perturbations. Moreover, MNI can provide multiple false vacuums due to the multiple cosine functions in the potential of MNI. With the slow-roll inflation proceeds, the inflation first falls into the false vacuum, the vacuum bubbles then collide with each other, and the inflation finally transits into the true vacuum. This provides the possibility for MNI of ALP to complete cosmic inflation and to end inflation through PT simultaneously.”

Finally, they calculated the gravitational wave for the period of ALP inflation that will consist of two parts, namely, the QF GW and the PT GW.

“As the PT events need more than 200, therefore, the model parameter ‘n’ should not less than 404. We investigate two scenarios (n = 404, 2000), and find that generated GW is very insensitive to n, which can be obtained from all the GW figures.”

FIG. 1: ALP inflation predictions in (ns, r) planes with n = 404 and n = 2000. Gray, red and blue shadows stand for [TT,TE,EE+lowE+lensing], [TT,TE,EE+lowE+lensing+BK15] and [TT,TE,EE+lowE+lensing+BK15+BAO] costraints © Wei Cheng et al

They showed that, the GW generated by quantum fluctuations (QF) dominates the low-frequency part/regions, which can be detected at f ∼ 10¯1 Hz, by DECi-hertz Interferometer Gravitational wave Observatory (or DECIGO), a Japanese, space-based, gravitational wave observatory.

FIG. 2: The spectrum of GW from the quantum fluctuations during the ALP inflation. © Wei Cheng et al.

While, the GW generated by PT dominates the high-frequency part/regions, which is expected to be detected by 3DSR designed by Chongqing University.

Fig 3: The spectrum of PT GW from the ALP inflation © Wei Cheng et al.

Their work has been supported by the National Science Foundation of China under Grant No.12047564, the Fundamental Research Funds for the Central Universities under Grant No. 2020CDJQY-Z003, and the China Postdoctoral Science Foundation under Grant No. 2019TQ0329 and No. 2020M670476.

Featured image: Schematic diagram of ALP inflation and PT with n = 20. © Wei Cheng et al.

Reference: Wei Cheng, Tao Qian, Qing Yu, Hua Zhou, Rui-Yu Zhou, “Gravitational Wave From Axion-like Particle Inflation”, Arxiv, pp. 1-7, 2021.

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