Invited Speakers and Talks

  • Prof. Xian Gao (Sun Yat-sen University, China)

    • Title: Higher derivative scalar-tensor theory and its extensions
    Abstract
    Abstract: Scalar-tensor theory is one of the most important theories of modified gravity. In this talk, first I will briefly review the recent progresses in constructing scalar-tensor theories with higher derivatives but without introducing the Ostrogradsky instabilities. Then I will introduce the concept of spatially covariant gravity, which correponds to the scalar-tensor theory in the unitary gauge. I will show how to get more general scalar-tensor theory with higher derivatives from the corresponding spatially covariant gravity.

  • Prof. Qing-Guo Huang (Institute of Theoretical Physics, Chinese Academy of Sciences, China)

    • Title: Search for Gravitational-Wave Background in Pulsar Timing Array
    Abstract
    Recently, the North American Nanohertz Observatory for Gravitational Waves, the European Pulsar Timing Array, the Parkes Pulsar Timing Array, and the Chinese Pulsar Timing Array all simultaneously released their latest independent observation results, showing strong evidence for a random signal consistent with a gravitational wave background. Based on astrophysical formation models, we interpret this signal as a gravitational wave background generated by supermassive black hole binaries, and find that the orbits of these supermassive black hole binaries may have a large initial eccentricity. By introducing and extending the gravitational wave cutoff spectrum from inspiralling to merging black hole binaries in the literature, we extend the gravitational wave background generated by supermassive black hole binaries to the detection frequency band of space gravitational wave detectors. The results show that this gravitational wave background can be detected by space gravitational wave detectors such as LISA/Taiji/Tianqin in the not too distant future.

  • Prof. Antonino Marcianò (Fudan University, China)

    • Title: The Stochastic Ricci Flow: applications to cosmology and astro-particle physics and insights into the measurement problem
    Abstract
    The Stochastic Ricci Flow (SRF) leverages the Stochastic Quantization paradigm as formulated by Parisi and Wu to investigate, from a geometric perspective, the renormalization group flow of gravitational theories. According to SRF, the fields evolve along a new fictitious time variable, the thermal time, being governed by a stochastic Langevin equation. Over the flow, out-of-equilibrium propagators converge at equilibrium to the standard gauge-invariant expressions derived from the path integral. At the cosmological and astrophysical level, there exist several implications of the SRF, including: (i) the natural emergence of a cosmological constant term governed by a non-trivial out-of-equilibrium dynamics; (ii) the appearance of intermittency (i.e. non-gaussian statistics) in the near-equilibrium dynamics of spacetime fluctuations close to a blackhole horizon. The use of multiplicative noise in the Langevin equation is essential for the former point; while for the latter, it is relevant the emergence of the Kardar-Parisi-Zhang equation, which governs the dynamics of fluctuations around a Schwarzschild blackhole. The Hubble tension problem and the gravitational-waves measurements are then considered under these perspectives. Finally, recent results that may provide a natural solution to the measurement problem are reported. In the framework of SRF, the dynamical breakdown of the superposition principle emerges in a fully covariant framework, hinging toward the reconciliation among quantum mechanics and relativistic symmetries.

  • Prof. Hiranao Miyatake (Kobayashi-Maskawa Institute, Japan), online talk

    • Title: HSC Year 3 Weak Lensing Cosmology Results
    Abstract
    The accelerating expansion of the universe is one of the most mysterious phenomena. Cosmic acceleration implies the existence of dark energy or the breakdown of Einstein's general relativity. Either way, revealing the source of cosmic acceleration can result in a paradigm shift in modern physics. Weak gravitational lensing is a subtle, coherent distortion of distant galaxy images due to gravitational potential, allowing the direct measurement of the spatial distribution of dark matter. Weak lensing is one of the most powerful cosmological probes because of its capability to measure the nature of cosmic acceleration through the evolution of the large-scale structure of the universe. Hyper Suprime-Cam (HSC), a newly developed prime focus camera at Subaru Telescope, started a wide, deep galaxy imaging survey in 2014, covering 1,100 sq. degrees of the sky down to the i-band limiting magnitude of 26. The wide field of view, light-gathering power, and superb image quality of HSC make it possible to measure weak lensing distortions with unprecedented precision. In this talk, I will present cosmology results from the Subaru Hyper Suprime-Cam Survey Year 3 data, mainly about cosmological constraints from cosmic shear and the combination of galaxy-galaxy lensing and clustering.

  • Prof. Masahide Yamaguchi (Institute for Basic Science, Korea)

    • Title: Is the Coleman de Luccia action minimum?: AdS/CFT approach
    Abstract
    We use the anti-de Sitter/conformal field theory (AdS/CFT) correspondence to find the least bounce action in an AdS false vacuum state, i.e., the most probable decay process of the metastable AdS vacuum within the Euclidean formalism by Callan and Coleman. It was shown that the O(4) symmetric bounce solution leads to the action minimum in the absence of gravity, but it is non-trivial in the presence of gravity. The AdS/CFT duality is used to evade the difficulties particular to a metastable gravitational system, such as the problems of negative modes and unbounded action. To this end, we show that the Fubini bounce solution in CFT, corresponding to the Coleman de Luccia bounce in AdS, gives the least action among all finite bounce solutions in a conformal scalar field theory. Thus, we prove that the Coleman de Luccia action is the least action when (i) the background is AdS, (ii) the AdS radii, L+ and L-, in the false and true vacua, respectively, satisfy L+ / L- ～ 1, and (iii) a metastable potential gives a thin-wall bounce much larger than the AdS radii.

  • Prof. Daisuke Yamauchi (Okayama University of Science, Japan), online talk

    • Title: Testing Gravity: Connecting theoretical developments to forthcoming observations
    Abstract
    Since the discovery of the accelerated expansion of the present universe, significant theoretical developments have been made in the area of modified gravity. In the meantime, cosmological observations have been providing more high-quality data, allowing us to explore gravity on cosmological scales. In this talk, to bridge the recent theoretical developments and observations, I present an overview of a variety of modified theories of gravity and the cosmological observables in the cosmic microwave background and large-scale structure. I also discuss the cosmological scientific challenges led by the cosmologists in the Japanese research community.