Seminar on Early Universe and General Relativity

Time: Monday 14:00-16:00
Location:1st floor at the 4th Bldg of Science faculty

J.Yokoyama→S.Yokoyama→Yamaguchi→Suyama→Yamauchi→Chiba→Itoh→Watanabe→Kuroyanagi→Kimura→Kobayashi→Saikawa→Gao→Miyamoto→Fujita→Kunimitsu→Nakama→Eda→IPMU postdocs→Mukohyama

Seminar archive


2020/10/26 (月)


Topic 1: Baryon isocurvature perturbations from non-helical magnetic fields

Topic 2: Saddle-point solutions from gradient flow

Fumio Uchida

The first topic: The origin of the baryon asymmetry of the universe is a fundamental problem in cosmology. Among many proposed mechanisms that could solve it, baryogenesis from helical magnetic fields is characteristic in that it relates baryon asymmetry directly to the cosmic magnetic fields. In the scenario, the chiral anomaly in the Standard Model converts hypermagnetic helicity into baryon number. The observed baryon-to-entropy ratio gives conditions on the strength and the coherence length of the magnetic fields, and the generation of baryon isocurvature perturbations gives another constraint because it affects the big-bang nucleosynthesis. Counterintuitively, while the net baryon asymmetry is produced only from the helical part, the baryon isocurvature perturbations are generated from both helical and non-helical parts of the magnetic power spectrum. In the seminar, I focus on and explain how even non-helical magnetic fields can generate baryon isocurvature perturbations.

The second topic: In most of the scenarios of baryogenesis, sphalerons play an important role because they violates baryon number. However, when one wants to find a sphaleron solution and estimate its energy, a difficulty sometimes arises because sphalerons are not local minima but saddle-points. Also, when one studies vacuum decay, to find bounce configurations is an important problem. In this case, one has to overcome the same difficulty because bounce solutions are saddle-points. However, a method that is intuitively understandable to find these solutions is proposed recently. In the seminar, I introduce the methodology and prove its validity, mainly following [3].

The seminar will be organized as follows. The first 40 min is devoted to the first topic, based on [1,2], then 10 min break, and the next 40 min to the second, based on [3,4].

[1] M. Giovannini, and M. E. Shaposhnikov, PRD57, 2186 (1998)
[2] K. Bamba, PRD74, 123504 (2006); T. Fujita, and K. Kamada, PRD93, 083520 (2016); K. Kamada, and A. J. Long, PRD94, 063501 (2016); K. Kamada, and A. J. Long, PRD94, 123509 (2016)
[3] S. Chigusa, T. Moroi, and Y.Shoji, PLB800, 135115 (2020)
[4] R. Sato, PRD101, 016012 (2020); Y. Hamada, and K. Kikuchi, PRD101, 096014 (2020).

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