| 研究実績の概要 |
In FY 2020 I have published 5 papers in journals, and 1 paper in submission. Some interesting achievements are listed below.
(1)With collaborators, we discussed a version of swampland conjecture, the Trans-Planckian Censorship Conjecture (TCC). We found that when applying this conjecture to inflationary universe, the post-inflationary thermal history becomes very important. We have derived a universal upper bound of the tensor-to-scalar ratio from TCC due to our findings. This paper is recognized as the most general bound on the primordial gravitational waves, if TCC is true. (Mizuno, Mukohyama, SP, and Zhang, Phys.Rev.D 102 (2020) 2, 021301.)
(2)We derived a universal infrared power of the gravitational-wave energy density spectrum. This universality does not depend on the source, but only depend on the thermal history of the universe. (Cai, SP, and Sasaki, Phys.Rev.D 102 (2020) 8, 083528.) This paper is important for checking the universal characteristic features of stochastic gravitational wave background (SGWB) in the early universe. We also found the infrared scaling of the SGWB can be used to probe the thermal history of the universe when the peak wavelength is still far outside the hubble horizon. (Domenech, SP, and Sasaki, JCAP08(2020)017.)
(3)We discussed the gravitational waves induced by the scalar perturbation with a lognormal peak, and derived an analytical expression for the induced SGWB. This result will be helpful for the signal searching in the future experiments. (SP and Sasaki, JCAP 09 (2020)037.)
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| 現在までの達成度 (区分) |
現在までの達成度 (区分)
2: おおむね順調に進展している
理由
As planed, in the first fiscal year I will continue my study in the general feature of the energy density spectrum of the stochastic gravitational wave background (SGWB). I found a universal infrared power-law of the spectrum of SGWB. Also, I planed to study some characteristic features of the SGWB, which is done by analytically studying the induced GWs from a lognormal peak of the scalar perturbations.
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| 今後の研究の推進方策 |
In the next fiscal year I would like to proceed my research in the following ways.
(1) I would like to study more mechanisms to enhance the primordial black holes (PBHs), and check the induced gravitational waves (IGWs). Especially, I would like to make clear the dependence on the non-Gaussianity of the PBHs and IGWs. This includes a thorough study of the impact from non-Gaussianity on the formation of PBHs and IGWs, and also depends on which specific model we choose.
(2) I would like to expand my interest to some other kinds of stochastic gravitational wave background (SGWB), especially that from first-order phase transition and cosmic strings.
(3) Recently, the pulsar timing array result from NANOGrav implied there might be a SGWB at nanohertz with amplitude (in energy density spectrum) 10^{-9}. I would like to continue working on this phenomenon, and try to find our what kind of model can generate such SGWB, if the signal is indeed from gravitational waves.
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| 次年度使用額が生じた理由 |
Because of the pandemic of COVID-19, I was unable to travel for most of FY2020. Therefore the budget in FY2020 is difficult to be used due to the cancellation of almost all the trips.
In the next fiscal year as the pandemic is gradually under control, I plan to visit Japan for a few months and spend the grant appropriately according to my travel plan.
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