Cosmology Study Group

Objectives

iTHEMS Cosmology Forum

The “LambdaCDM” model of cosmology, supplemented by an early period of cosmic inflation, is wonderfully successful at explaining our Universe, including the distribution of galaxies, the statistics of the cosmic microwave background radiation, and the abundances of light elements. On the other hand, it provides almost no information about what its chief components – Lambda, dark energy and CDM, dark matter – actually are. Revealing the fundamental nature of these phenomenological mysteries is now a shared goal of the cosmological, astronomical and particle physics communities. In addition, the details of the very early Universe physics remains quite unconstrained. While CMB observations are compatible with the simplest models of inflation powered by a single field, the nature of this field remains elusive, and the predicted associated primordial gravitational waves background, as well as the expected primordial black holes, have yet to be detected, leaving a lot of theoretical room for richer inflationary models. The details of the reheating period connecting (cold) inflation to the hot Big Bang model events are also poorly known.

Ongoing and scheduled experiments will improve our understanding of cosmic history by reducing the scope of possible models. Recently, a number of unexpected observations have been reported that could be tied to a richer fundamental physics of the dark sector or the very early Universe. Still, these signals may be due to astrophysical processes or uncertainties in data analysis. Careful investigation and interrogation of existing models and data, as well as inter-field discussions, are thus warranted in addition to enriched theoretical models.

This study group aims to provide a forum to discuss these emerging topics in a way that is up-to-date and accessible to a broad range of interested scientists. To do this, we intend to organize one or two-day workshops which will:

• be focused on a single theme featuring invited speakers, a panel session with audience questions, and time dedicated to discussion and collaboration. We will ask one speaker to provide an extended introduction appropriate for a physics department, with the intention that this talk is accessible to all iTHEMS physicists.
• have a target audience of local cosmologists, high-energy physicists and astronomers interested in learning about the subject, not just those who have already worked on the topic. We hope the workshop can lead to new ideas, collaborations and publications amongst the local community.
• be easy to join and schedule so that scientists, at least in Kyoto and Tokyo areas, can attend all invited talks for just a single-day event. We hope that having this low barrier to entry will lead to broad participation among potentially interested scientists.

Overall, we intend to make this study group a forum to learn, discuss and collaborate on the most exciting new areas in cosmology, and we hope this workshop series will lead to new and innovative scientific ideas.

Theoretical Cosmology Group

Describing the evolution of the Universe and its content requires modelling widely different situations from the current low energy Universe, where
matter is gathered in complex structures (galaxies, halos etc.) to the early Universe, very homogeneous but with an energy density vastly superior to that probed in colliders. Building and testing such cosmological models thus involves a large set of theoretical analysis, computational methods
(analytical and numerical) and manipulating heterogeneous observational data sets. Performing impactful cosmology research requires understanding each of these aspects, necessitating discussion and collaboration with colleagues of complementary expertise.

This theory group will bring together a small number of colleagues from iTHEMS and nearby institutions. It will allow members to:

• build collaboration on existing projects or on new ideas emerging within the group
• get feedback and input on their cosmology-related works
• learn at a technical level about state-of-the-art research in other subfields of cosmology

The group’s activities will be focused on a bi-weekly event, which will alter-
natively consist of:

• a cosmology seminar open to all iTHEMS members
• a group meeting where members briefly present their own work or a paper relevant to it

Facilitators:

Shuntaro Aoki (RIKEN iTHEMS)

Takashi Hiramatsu (Nihon University)

Nagisa Hiroshima (Yokohama National University)

Derek Inman (RIKEN iTHEMS)

Amaury Micheli (RIKEN iTHEMS)

Hironao Miyatake (Nagoya University)

Kai Murai (Tohoku University)

Ryo Namba (RIKEN iTHEMS) – Contact: ryo.namba@riken.jp

Yuichiro Tada (Rikkyo University)