AAPPS-DACG Workshop Report

Bogeun Gwak and Stephen Appleby

The Division of Astrophysics, Cosmology and Gravitation (DACG) of the Association of Asia Pacific Physical Societies (AAPPS) organized a workshop on astrophysics, cosmology and gravitation over the period 4th - 8th October, 2021. Due to the ongoing coronavirus pandemic, the workshop was the second consecutive DACG meeting to be held online. The global situation has created a unique challenge in keeping the Asia Pacific (AP) scientific communities connected, and online meetings have emerged as a key resource to allow the sharing of knowledge and new ideas.

The workshop was organized by early career scientists within the AP region, led by Professor Bogeun Gwak (Chair of the scientific organizing committee) and under the supervision of the executive committee (EXCO) and division chair Professor Sang Pyo Kim. The members of the SOC were Chang Sub Shin (Chungnam National University), Stephen Appleby (APCTP, Secretary of SOC), Eoin O Colgain (CQUeST), Qing-Guo Huang (ITP, CAS), Li Li (ITP, CAS), Tao Zhu (ZJUT), Che-Yu Chen (Academia Sinica), Yi-Zen Chu (National Central University), Kenny Ng (Chinese University of Hong Kong), Jane Dai (University of Hong Kong), Mayukh Raj Gangopadhyay (Jamia Millia Islamia), Wali Hossain (Jamia Millia Islamia), Teruaki Suyama (Tokyo Institute of Technology), Yudai Suwa (University of Tokyo and YITP), Takashi Hosokawa (Kyoto University). Furthermore, all members of the DACG expected to share scientific visions and ideas from the workshop. A total of thirty plenary speakers were invited to give talks on the latest developments in the fields of astrophysics, gravitation and cosmology. Regional experts invited to speak at the workshop were Katsuki Aoki (Yukawa Institute for Theoretical Physics), Katie Auchettl (University of Melbourne), Yi-Fu Cai (University of Science and Technology of China), Songzhan Chen (IHEP), Roland Crocker (Australian National University), Koushik Dutta (IISER Kolkata), Richard Easther (The University of Auckland), Jörg Frauendiener (The University of Otago), Dhiraj Kumar Hazra (Institute of Mathematical Sciences), Yi-Ming Hu (Sun Yat-Sen University), Kohei Inayoshi (KIAA, Peking University), Yuki Inoue (National Central University), Kazumi Kashiyama (University of Tokyo), Ranjan Laha (Indian Institute of Science), Kyung-ha Lee (Sungkyunkwan University), Yue Meng (Shanghai Jiao Tong University), Seong Chan Park (Yonsei University), Hugo Pfister (The University of Hong Kong), Lijing Shao (KIAA, Peking University), Kazuyuki Sugimura (Tohoku University), Eric Thrane (Monash University), Meng-Ru Wu (Academia Sinica), Run-Qiu Yang (Tianjin University), Hai-Qing Zhang (Beihang University), Gongbo Zhao (NAO, Chinese Academy of Science). Parallel sessions were utilized to allow young researchers to present their work to a broad audience including local experts. A small number of international speakers were invited - Mar Bastero-Gil (University of Granada), Mariam Bouhmadi-Lopez (University of the Basque Country), Soichiro Morisaki (University of Wisconsin-Milwaukee), Paolo Pani (Sapienza University of Rome), Eleonora di Valentino (University of Sheffield) - to gain a broader picture of scientific activities outside the AP region. The workshop was attended by 200 participants in total.

Scientific events such as this allow one to build a picture of the current research trends within the different physical societies. Two topics in particular that received much attention during the workshop were black holes and gravitational waves. Both are particularly relevant due to the increasing number of observations of binary black hole mergers via their gravitational wave emission, and also due to recently renewed interest in the possibility that dark matter could be described by primordial black holes.

Within the field of cosmology, ideas highlighted during the workshop included the Hubble tension, probing the reionization phase of the Universe and information extraction from upcoming galaxy surveys such as Euclid and the Dark Energy Survey. The early inflationary epoch of the Universe also remains a topic of considerable debate. This is due to the importance of this epoch in providing a window into the realm of quantum gravity and also because the exact dynamics of this process can help to determine potential dark matter candidates.

Furthermore, there were various interesting talks on astrophysics and gravitation. Since the first gravitational wave was detected, the fields of astrophysics and gravitation are closely correlated in their progress. Astrophysical experiments and the development of detectors were introduced, and it was argued that theoretical speculations and new astrophysical phenomena would be observed in the near future. Topics on black holes were introduced from various points of view. Talks on black holes provided detailed information about their number, origins, behavior as compact objects, and their role in the evolution of the universe. Neutron stars also emerged as related, important astrophysical entities. Many of these topics have a relationship with gravitational waves, which have created a common interest in the future exploration of astrophysical phenomena.

The main DACG workshop was followed by a satellite workshop 'A Discussion on the Cosmological Principle' in which international experts were invited to debate the efficacy of the standard cosmological model in describing the evolution of the Universe. The meeting was hosted by the Asia Pacific Center for Theoretical Physics (APCTP) in association with the AAPPS and the Center for Quantum Spacetime (CQUEST). The second event was organized by Stephen Appleby (APCTP), Eoin Ó Colgáin (CQUEST) and Shahin Sheikh-Jabbari (IPM, Tehran). The speakers at the event were Pravabati Chingangbam (Indian Institute of Astrophysics), Chris Clarkson (Queen Mary London), Roger Clowes and Alexia Lopez (University of Central Lancashire), Tamara Davis (University of Queensland), Ruth Durrer (Université de Genève), Asta Heinesen (Centre de Recherche Astrophysique de Lyon), Dragan Huterer (University of Michigan), Damien Hutsemékers (University of Ligè), Chethan Krishnan (Indian Institute of Science), Roy Maartens (University of the Western Cape), Konstantinos Migkas (Universität Bonn), David Parkinson (Korea Astronomy and Space Science Institute), Miguel Quartin (Univ. Federal do Rio de Janeiro), Wahidur Rahman (Imperial College London), Subir Sarkar (University of Oxford), Dominik Schwarz (Universität Bielefeld), Douglas Scott (University of British Columbia), Nathan Secrest (US Naval Observatory), Hee-Jong Seo (Ohio University), Ashok Kumar Singal (Physical Research Lab. Ahmedabad), Jiro Soda (Kobe University), Alexei Starobinsky (Landau Institute for Theoretical Physics), Christos Tsagas (Aristotle University of Thessaloniki), Shao-Jiang Wang (Chinese Academy of Sciences, Beijing), David Wiltshire (University of Canterbury), Lu Yin (Sogang University), Wen Zhao (University of Science and Technology of China). Professor George Smoot (Hong Kong University of Science and Technology) provided a historical introduction to the dipole of the Cosmic Microwave Background in a colloquium related to the event (Fig. 9).

Fig. 9. Photograph of participants

The satellite workshop was an opportunity for observational and theoretical cosmologists to discuss recent progress in the field, specifically results that indicate large scale anisotropy in the matter distribution of the Universe. These developments, if confirmed, would provide a significant challenge to the standard cosmological model. The severity of the challenge to the standard model led to a robust discussion at the workshop.

The question of the Universe being isotropic on large scales is a fundamental one, and encompasses the mathematical formulation of cosmology within General Relativity, light propagation in curved spacetime, observations of astrophysical objects of various types (supernova, quasars, galaxies), and even philosophy underlying the cosmological principle. It was argued that as data becomes increasingly precise, we can explicitly test the spacetime geometry beyond the simplest Friedmann-Robertson-Walker background that is universally adopted by the community.

The next DACG meeting will be held on-site in Gyeongju in August 2022.