New Frontiers in Observational Cosmology with Gravitational Waves and Cosmic Microwave Background (GW-CMB 2025)

6 Nov 2025, 09:00 → 7 Nov 2025, 17:00 Asia/Taipei

國立中央大學 教学研究総合大楼 羅家倫講堂 (National Central University)

Overview

We live in an era striving to uncover the origin of the Universe through observation. A century after Hubble’s discovery of cosmic expansion, our instruments have advanced dramatically. Through diverse probes—the cosmic microwave background (CMB), galaxy surveys, supernovae, and gravitational waves—the Universe’s history has become a subject of direct measurement rather than theory.

Cosmology has entered a stage where precision observations open new physics, and at its frontier lies the search for the stochastic gravitational-wave background (SGWB).

Within the next five years, two landmark discoveries may be within reach:
(1) primordial gravitational waves from inflation or cosmological phase transitions; and
(2) an astrophysical SGWB from compact binary mergers across cosmic time.

These goals unite cosmology and gravitational-wave astronomy, driving international efforts such as LIGO–Virgo–KAGRA and the Simons Observatory toward the next breakthrough.
In this mini-workshop, we explore the combined impact of GW and CMB observations on the emerging Gravitational-Wave Landscape, the cosmology and astrophysics they enable, and new ideas for cross-correlations with other probes.

The program mainly consists of invited talks, but we can accept poster presentations and a few short oral presentations.

Organizers

Chiang-Mei Chen (NCU), Yi-Kuan Chiang (ASIAA), Masashi Hazumi (NCU, KEK), Yuki Inoue (NCU, ASIAA), Albert Kong (NTHU), Chia Ming Kuo (NCU), Teppei Okumura (ASIAA), Chia-Hsien Shen (NTU, LeCosPA), Keiichi Umetsu (ASIAA)

Confirmed speakers:

• Yi-Kuan Chiang (ASIAA)
• Yuji Chinone (KEK)
• Shouvik Roy Choudhury (ASIAA)
• Giacomo Galloni (Ferrara)
• Tomo Goto (NTHU)
• Colin Hill (Columbia)
• Yuki Inoue (NCU, ASIoP)
• Teppei Okumura (ASIAA)
• Daiki Tanabe (ASIoP)
• Keiichi Umetsu (ASIAA)
• Viona Wei (NCU)
• More to be announced...

 

Sponsors

This workshop is sponsored by the Center for High Energy and High Field Physics (CHiP) and is supported by the Physical Society of Taiwan (TPS) and 中華民國重力學會(GSROC).

 

Code of Conduct

We follow the Code of Conduct of the Physical Society of Taiwan (TPS).

 

Campus map

Thursday 6 November

09:30 Registration and Welcome Coffee

1 Welcome and Introduction to CHiP

Speaker: Prof. Yung-Fu Chen (NCU)

20251106_CHiP_ShortIntro.pdf

2 Introduction to GW-CMB workshop

Speaker: Masashi Hazumi (National Central University)

20251106_Hazumi_GW-CMB2025_Intro.pdf

3 Simons Observatory and Primordial Gravitational Waves

The cosmic microwave background (CMB) remains one of the most powerful probes of the early universe, providing insights into its composition, geometry, and evolution. While early studies focused on temperature anisotropies, the next frontier lies in precise measurements of CMB polarization. In particular, detecting the curl-like B-mode pattern would provide compelling evidence for primordial gravitational waves generated during inflation, offering a unique window into physics at energy scales far beyond terrestrial experiments.
This talk will begin with an overview of CMB observations and their role in cosmology, then highlight key results from the POLARBEAR experiment, which pioneered polarization measurements and set important constraints on B-mode signals. Building on these achievements, I will introduce the Simons Observatory—a next-generation ground-based experiment—and discuss its strategy to improve sensitivity, tackle systematic challenges, and advance the search for primordial B-modes. In addition, I will touch on how polarization observations can probe new physics, including axion-like particles through cosmic birefringence, illustrating the synergy between cosmology and fundamental physics.

Speaker: Prof. Yuji Chinone (KEK)

03_NCU_mini_workshop_yuji_chinone_2025-11-06_07.web.pdf

4 GW Landscape

Speaker: Yuki Inoue (National Central University)

GW_CMB_2025.pdf

5 Photos

12:30 Lunch

6 A search for Planet 9

Speaker: Prof. Tomo Goto (NTHU)

05_20251106 Tomo Goto NCU P9 v4.pptx

7 Dark matter vs. baryons beyond equilibrium

Speaker: Prof. Keiichi Umetsu (ASIAA)

06_Keiichi_GWCMB2025.pdf

8 Dynamical Dark Energy and neutrino masses circa 2025

Speaker: Dr Shouvik Roy Choudhury (ASIAA)

07_Talk_GW_CMB_2025_Shouvik_Roy_Choudhury.pdf

15:30 Coffee Break + Posters

9 Constraints on amplitude and tilt of the tensor primordial spectrum

Speaker: Dr Giacomo Galloni (Univ. of Ferrara)

08_Taiwan_GW_CMB_2025_Galloni.pdf

19:00 Workshop Dinner

Friday 7 November

09:30 Registration + Welcome Coffee

10 Science with Simons Observatory Large Aperture Telescope

Speaker: Prof. Colin Hill (Columbia Univ.)

09a_JCH_SO_Taiwan.pdf

11 A Journey through the Full CMB–LSS Tomography: From Milky Way Dust, tSZ, and CIB to CO and [CII] Lines

CMB data are maximally projected along the line of sight, with intricate secondary anisotropies that each reveal a distinct chapter of cosmic evolution. In a series of six papers and ongoing, we embark on a deeply data-intensive, cross-correlation redshift tomography of CMB data, analyzing over two billion HEALPix pixels across more than twenty diffuse sky maps (e.g., Planck, IRAS, Herschel) together with LSS tracers from galaxy surveys with available spectroscopic redshifts pre-DESI. This unified campaign unlocks a panoramic view of cosmic baryons across space and time. Key results include: (1) a measurement of the cosmic thermal energy history, $\Omega_{\rm th}$, in hot gas and its implications for structure growth powered by gravitational energy, $\Omega_{\rm grav}$; (2) the most precise cosmic star-formation history to date, revealed by 11-band CIB tomography with fully constrained SEDs; (3) the cosmic dust evolution, $\Omega_{\rm dust}$, across all host environments; (4) the identification and removal of CIB leakage in ten Galactic dust maps, yielding a field-level component-separation algorithm as a constrained realization of 3D LSS data and producing the “CSFD” dust map; and, in the latest installment of the series, (5) the first tomographic detection of CO and [CII] line-intensity backgrounds, tracing the molecular-gas density, $\Omega_{\rm H_2}$, and the cosmic cooling budget over 12 Gyr of history.

Speaker: Dr Yi-Kuan Chiang (ASIAA)

10_CMB_secondaries_Chiang.pdf

12 Extracting parity-violating gravitational waves from the large-scale structure of the universe

Speaker: Prof. Teppei Okumura (ASIAA)

11_Okumura_202511_06-07_NCU_GW_CMB.pdf

13 Universal redshifted black hole quasinormal modes induced by environments

The ringdown phase of gravitational waves emitted by a perturbed black hole is described by a superposition of exponentially decaying sinusoidal modes, called quasinormal modes (QNMs), whose spectra depend only on the property of the black-hole geometry. The extraction of QNM frequencies of an isolated black hole thus allows for testing GR, the so-called black hole spectroscopy program. However, astrophysical black holes are not perfectly isolated. It remains unclear whether the validity of black hole spectroscopy would be affected when environmental effects surrounding the black holes are taken into account, such as accretion disk, dark matter halo, etc. In this talk, by taking a black hole surrounded by a gravitating thin disk as an example, we show that there seems to be a universal redshift relation for QNM spectrum in the presence of such environmental effects. The implications on testing GR through black hole spectroscopy will be discussed.

Speaker: Dr Che-Yu Chen (RIKEN)

12_CheYuChen2025NCUworkshop.pdf

12:30 Lunch

14 New Results from LIGO

Speaker: Dr Daiki Tanabe (ASIoP)

13_tanabe_20251107.pdf

15 Monodromy Approach to Quasi-Normal Modes of Black Holes

The quasi-normal modes (QNMs) are the oscillation modes of black holes. This kind of mode will be interesting when we study the ringdown of gravitational waves. In theoretical conception, the highly damped frequencies of QNMs would show quantum phenomena such as Bohr’s hydrogen atom model. One of the analyzing methods is called “Monodromy” which is more precise to solve the highly damped frequencies of QNMs.

Speaker: Dr Viona Wei (NCU)

14_251106 Introduction Monodromy.pdf

15:30 Coffee Break + Poster

16 Circular Polarization and Doppler-Induced Dipole Anisotropies in the Stochastic Gravitational-Wave Background

Some inflationary models predict a circularly polarized stochastic gravitational-wave background (SGWB) with potential cosmological significance. We develop a simulation and parameter-estimation framework to characterize circular polarization and Doppler-induced dipole anisotropies in the SGWB. The pipeline jointly constrains the polarization degree (Π), energy density (Ω_GW), and dipole amplitude (D_α), validated through simulated signals and applied to LIGO O1–O3 data. We derive upper limits on Π, Ω_GW, and D_α.

Speaker: Sevgi Karadağ (ASIoP)

15_GW-CMB2025_SevgiKaradag_SGWB.pdf

17 Final discussion