Joint Seminar on Cosmology

About the Joint Seminar

Although there are many groups in Tokyo working on gravitation, cosmology, and particle physics, there are relatively few opportunities to meet and discuss across institutions. The main aims of this seminar series are therefore to:

1. promote interaction among researchers and students,
2. provide presentation opportunities for students and early-career researchers, and
3. help participants learn about new research topics.

Registration

The next seminar will be held at RIKEN iTHEMS. If you would like to attend the seminar and/or the social gathering after the seminar, please register via the Google Form below.

If you would like to attend the social gathering (dinner) after the seminar, please register via the Google Form by Monday, July 6.

 Google Form 

Event Information

Date

Monday, July 13, 2026, starting at 14:00 (doors open at 13:30)

Venue

RIKEN Wako Campus, Okochi Hall

Program

13:30

Doors open

14:00

Presentation by Fumiya Sano (Tokyo U., 40 min)

14:40

Break & informal discussion (30 min)

15:10

Presentation by Yukihiro Kanda (ICRR, 40 min)

15:50

Break & informal discussion (30 min)

16:20

Presentation by Fumiya Okamatsu (Nihon U., 40 min)

17:00

Informal discussion (30 min)

17:30

Closing (followed by dinner from 18:30 for those who wish to join)

Talk Information

14:00–14:40

Speaker

Fumiya Sano (Tokyo U)

Title

TBA

Abstract

TBA

15:10–15:50

Speaker

Yukihiro Kanda (ICRR)

Title

Particle emission from semilocal string networks

Abstract

Topological defects can arise as remnants of symmetry-breaking phase transitions in the early universe and are usually classified according to the homotopy groups of the vacuum manifold. However, field theories can admit stable localized configurations whose stability is ensured not only by topology but also by energetic considerations. Semilocal strings provide such an example: they are string-like defects that arise even when the fundamental group of the vacuum manifold is trivial. We present results from numerical lattice simulations of semilocal string networks, focusing for the first time on their particle-emission properties. We find that semilocal string networks efficiently emit Nambu-Goldstone bosons, with emission properties similar to axion radiation from global string networks. Motivated by this result, we discuss a scenario in which the Nambu-Goldstone bosons acquire a soft-breaking mass, and their non-thermal production from the string network accounts for the observed dark matter abundance.

16:20–17:00

Speaker

Fumiya Okamatsu (Nihon U)

Title

21-cm Line Cosmology in the Dark Ages

Abstract

Neutral hydrogen has existed over a wide range of cosmic epochs, and its hyperfine transition at a wavelength of 21 cm, the so-called 21-cm line, provides a unique observational probe of the Universe. In particular, the 21-cm signal from the Dark Ages is a powerful tool for testing cosmology, since it enables us to directly probe the Universe before the formation of the first stars and galaxies. However, because this signal is redshifted into low-frequency radio bands, ground-based observations are extremely challenging due to the Earth’s ionosphere and radio-frequency interference (RFI). For this reason, observations from the lunar surface and lunar orbit are currently being discussed and explored.

In this talk, I will first review the basics of the 21-cm line, with a particular focus on its key observables. I will then discuss the cosmological implications of the Dark Ages 21-cm signal, drawing on recent developments as well as my own research.