iTHEMS Cosmology Forum

Objectives

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. Examples include a tension in the cosmic expansion rate measured by different probes, observations of cosmic birefringence, detection of a stochastic gravitational wave background, discovery of galaxies and black holes that formed earlier than expected. 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 working 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 three one-day workshops throughout the 2024 fiscal year. Each workshop will:

• be focused on a single theme and feature three invited speakers alongside dedicated time for discussion and collaboration. We will ask one speaker to provide an approximately thirty-minute 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. The first workshop will be held in the RIKEN Wako campus; subsequent workshops may be held at other RIKEN offices, depending on the participants. We hope that having this low barrier to entry will lead to broad participation among potentially interested scientists.

Overall, we intend to make this working 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.

Facilitators:

Ryo Namba (RIKEN iTHEMS) *Contact at ryo.namba@riken.jp

Derek Beattie Inman (RIKEN iTHEMS)

Amaury Micheli (RIKEN iTHEMS)

Nagisa Hiroshima (Univ. of Toyama)

Kohei Hayashi (National Institute of Technology, Sendai College)