Workshop on Gravitational Waves, Cosmology and Fundamental Physics

30 Oct 2025, 14:00 → 3 Nov 2025, 20:00 Asia/Shanghai

The workshop on Gravitational Waves, Cosmology and Fundamental Physics will be hosted in Hangzhou Institute for Advanced Study (HIAS), University of Chinese Academy of Sciences (UCAS), on October 30-November 3, 2025. (Scientific sessions will be held on October 31- November 2.)

This workshop aims to bring together the ideas inspired by the recent progress in gravitational wave cosmology as well as other cosmological observations to explore the fundamental physics. The on-going gravitational wave observations at LIGO-Virgo-KAGRA and Pulsar Timing Arrays as well as the future observations at LISA, Taiji, TianQin, DECIGO, BBO, and so on have inspired many directions in the gravitational wave cosmology. Also recent "anomalies" reported in the Cosmic Microwave Background as well as the Baryon Acoustic Oscillation may give us hints for cosmology beyond ΛCDM. With these progress in mind, we will discuss the phenomena in the early Universe that generate stochastic gravitational wave background, which includes possible models with CP and Lorentz violation, as well as cosmology beyond ΛCDM. 

Registration fee: 1500 CNY for staffs, 1000 CNY for students. 

The deadline for registration is extended to Oct. 20, 17:00.

Invited speakers (alphabetical): 
Ligong Bian (Chongqing)
Rong-Gen Cai (Ningbo) (TBC)
Andrew Fowlie (XJTLU)
Yi-Ming Hu (SYSU Zhuhai; TianQin)
Chikako Idegawa (SYSU Zhuhai )
Eiichiro Komatsu (MPA)
Mingzhe Li (USTC)
Zi Ren Luo (IMCAS; Taiji) (TBC)
Yuichiro Nakai (TDLI)
Atsuhisa Ota (Chongqing)
Shao-Jiang Wang (ITP)
Yue-Liang Wu (ITP) (TBC)
Xunjie Xu (IHEP)
Jun'ichi Yokoyama (Kavli IPMU, Tokyo)
Yingli Zhang (Tongji)
Tao Zhu (Zhejiang U of Technology)

Organizers: 
Kohei Kamada (HIAS; chair)
Shi Pi (ITP)
Jun'ichi Yokoyama (Kavli IPMU, Tokyo)
Ye-Ling Zhou (HIAS)

Contact: 
Ms. Shutong Liu (liushutong@ucas.ac.cn)

Thursday, 30 October

Registration

Friday, 31 October

Registration

Session 1

Convener: Zong-Hong ZHU (Beijing Normal University)

1 Opening

Opening remarks

Speaker: Kohei Kamada (HIAS, UCAS)

Opening.pdf

2 Thermal production of gravitational waves in the early universe

In this work, we systematically study gravitational wave (GW) production during both the inflationary and post-inflationary epochs. While inflationary GWs can be readily derived from tensor perturbations during inflation, post-inflationary GWs arise from a variety of processes during reheating and require detailed treatment for quantitative analysis. We consider four distinct production channels: (i) pure inflaton annihilation, (ii) graviton bremsstrahlung from inflaton decay, (iii) radiation-catalyzed inflaton-graviton conversion, and (iv) scattering among fully thermalized radiation particles. For each channel, we solve the corresponding Boltzmann equation to obtain the GW spectrum and derive a simple yet accurate analytical expression for it. By employing a consistent treatment of all production channels, our analysis yields for the first time the full spectrum of GWs produced during the inflationary and post-inflationary epochs. We find that, while inflationary GWs dominate at low frequencies, post-inflationary processes generally produce high-frequency GWs with considerably high energy densities that may significantly exceed that of inflationary GWs.

Speaker: Xun-Jie Xu

Xunjie-Xu-Talk-at-HIAS.pdf

3 Gravitational Waves from the Early Universe

Gravitational waves are useful probe for the physics of the early Universe. I will discuss several sources of stochastic gravitational wave background from the early Universe and provide a historical overview of this fascinating research field.

Speaker: Jun\'ichi Yokoyama (Kavli IPMU)

20251031杭州dt.pptx

11:30 Lunch

Session 2

Convener: Jun\'ichi Yokoyama (Kavli IPMU)

4 Constraints on Lortentz and parity symmetry with gravitational waves

Gravitational wave (GW) observations offer a powerful tool for testing the fundamental Lorentz and parity symmetries of gravity. Any violation of these symmetries could manifest as deviations in GW propagation. In this talk, I will explore how current and future GW detections can constrain Lorentz- and parity-violating effects in gravity. I will introduce a systematic parameterization framework to describe potential deviations in GW propagation from general relativity in a cosmological background. Using this framework, we can construct modified GW waveforms, incorporating the effects of Lorentz and parity violations as predicted by various alternative gravity theories. Finally, I will present the latest results from our analysis of these modified waveforms using current GW data and discuss the prospects for placing even stronger constraints from future GW detections.

Speaker: 涛 朱 (Zhejiang University of Technology)

5 Space GW detection in China and progress

China's space-based gravitational wave detection projects will open a new observational window in the mid-to-low frequency band (0.1 mHz to 1 Hz), providing new approaches to understand the origin and evolution of the universe, the formation and evolution of black holes, the nature of gravity, dark energy, and dark matter. Since space-based gravitational wave detection involves a series of key technologies, the TianQin project has proposed a "0-1-2-3" development roadmap, while the Taiji program has put forward a three-step plan. Both TianQin-1 and Taiji-1 were successfully launched in 2019 and have completed their in-orbit tests with outstanding results. Supported by the National Key Program on Gravitational Wave Detection, China has achieved breakthroughs in critical technologies for space-based gravitational wave detection, with the technological levels meeting the requirements for project initiation. In this talk, I will briefly introduce recent progress in China's space-based gravitational wave detection program, especially on Taiji, and provide an outlook on future development.

Speaker: Prof. Ziren Luo

space GW in China.pptx

6 The TianQin Project and its Implication to Astronomy

The TianQin mission is a proposed space-borne gravitational wave detector, aiming to detect gravitational waves in the milli-Hertz frequency band. In this talk, I will give a brief overview of the concept of the mission, as well as its general progress and status. Furthermore, I will also briefly cover how the operation of TianQin can help deepen our understanding of the Universe.

Speaker: Prof. 一鸣 胡 (中山大学)

2025YiMingHu_Hangzhou.pptx

15:30 Coffee break

Session 3

Convener: Prof. Hongwei Yu

7 Beyond general relativity: gravitational waves in non-minimally coupled theories

Non-minimal matter-curvature couplings arise in many different contexts, and modify solutions of general relativity. If dark matter (DM) experiences non-minimal couplings, the imprint can be see in gravitational waves (GWs) emitted from binary black hole mergers, where DM densities are expected to be large. In this talk, we explore effects of various non-minimal couplings on GWs by working with a model-independent parameterization for left- and right-handed GW strains. We will write down the general solution assuming slowly-varying matter fields, and then systematically apply our results to three models: Kalb-Ramond dark matter with dimension-four operators, axion-dilaton-Chern-Simons-Gauss-Bonnet dimension-five operators, and dimension-six couplings to a (dark) vector field.

Speaker: Tucker Manton (HIAS-UCAS)

HIASworkshop.pdf

8 Scalar wave emission from particles radially infalling in a Schwarzschild black hole

I present an analytical computation of the energy emitted by a scalar particle radially infalling into a Schwarzschild black hole spacetime within the framework of the gravitational self-force. The results are expressed in Post-Newtonian expanded form, as is customary.

Speaker: Giorgio Di Russo (Hangzhou Institute for Advanced Study)

talk_hangzhou (1).pdf

9 Vanishing nonlinear black hole Love numbers in GR

Black hole Love numbers measure how a black hole deforms under external tidal fields, providing a window into the structure of spacetime, the dynamics of black holes, and the physics of gravitational waves. While linear analyses already suggest their vanishing, the role of non-linear effects has remained an open question. In this talk, I will explore the non-linear regime, presenting recent results that establish the vanishing of infinitely many non-linear Love numbers in the parity-even sector of general relativity, valid to all orders in perturbation theory. This striking result also unveals a hidden symmetry underlying black hole perturbations, offering new insights into the nature of black holes in general relativity.

Speaker: Sam Wong (CityU HK)

HIAS2025.pdf

Saturday, 1 November

09:30 Coffee break

Session 4

Convener: Yu-Feng Zhou (ITP-CAS)

10 Gravitational waves from early and late Universe

In this talk, I will introduce recent progress on gravitational waves (GWs) from cosmological first-order phase transitions associated with primordial black hole (PBH) production, the collisions between PBHs and vacuum bubbles, sound waves and GWs from gravitational collapse, induced GWs from curvature perturbations with non-Gaussianity up to all orders.

Speaker: Shao-Jiang Wang (I)

GWsEarlyUniverse(HIAS).pdf

11 Gravitational waves from symmetry breaking

In this talk, I will report our progresses on the simulation of Gravitational wave and particle productions from cosmic strings.

Speaker: 立功 边 (重庆大学)

GW from symmetry breaking_LGB.pdf

12 Nearly Monochromatic Primordial Black Holes as total Dark Matter from Bubble Collapse

We propose a two-field model where the inflaton χ is non-minimally coupled to the instanton ϕ. By choosing an appropriate coupling function, we realize the scenario where the difference of the values of potential between false vacuum (FV) and true vacuum (TV) is maximized during inflation. Most of the bubbles are created at this time. After inflation ends, the potential value of FV drops
below that of TV so that these bubbles collapse to form primordial black holes (PBHs). By tuning the parameters of our model, we analyze the Coleman-de Luccia (CDL) and Hawking-Moss (HM) process, finding that the corresponding mass function of PBHs is sharply peaked, implying that we can realize either PBHs as cold dark matter, sub-solar PBHs, or supermassive PBHs in this scenario
without enhancement of primordial curvature perturbations.

Speaker: Prof. Ying-li Zhang (Tongji University)

Hangzhou202511.pdf

11:30 Photo session

11:40 Lunch

Session 5

Convener: 颖理 章 (同济大学)

13 δn formalism and non-perturbative curvature perturbation during inflation.

Contrary to what it is typically done in the literature, we re-formulate the δN formalism by using the e-folding number n counted forward in time. Taking advantage of the complete (anti-)correlation of the initial field perturbation δϕ0 with its conjugate momenta δπ0, we find a simple formula to calculate the non-linear curvature perturbation as well as its PDF by using the δN method reformulated in terms of n, the δn formalism. We will also explore the connection of the δn formalism with the non-linear equation of motion for the curvature perturbation at superhorizon scales.

Speaker: Diego Cruces (ITP-CAS)

Presentation_HIAS (1).pdf

14 A tantalizing hint of cosmological parity violation in the polarized light of the cosmic microwave background

The polarized light of the cosmic microwave background is sensitive to new physics that violates parity symmetry. In this presentation, we present a tantalizing hint of parity violation from the polarization data of two satellite missions, WMAP and Planck, with a statistical significance of 3 sigma. This signal has also been observed in recent data from the Atacama Cosmology Telescope. Taken together, there is evidence for a cosmological parity violation with a statistical significance of 4 sigma. If confirmed in the future with higher statistical significance, this finding would have profound implications for the elusive nature of dark matter and dark energy.

Speaker: Prof. Eiichiro Komatsu (Max Planck Institute for Astrophysics)

parity_hias.pdf

15 Teleparallel gravity models with parity violations

It is not easy to have a healthy parity violating gravity models within Riemannian geometry. The famous Chern-Simons modified general relativity and its extensions are pathological due to the ghost modes appeared at high energy scales. In this talk, I will introduce some parity violating models within the framework of teleparallel gravity, especially the Nieh-Yan modified teleparallel gravity model, which is healthy and robust. It predicts the velocity birefringence phenomenon in gravitational waves and has more interesting applications in cosmology.

Speaker: Mingzhe Li (University of Science and Technology of China)

16 Gravitational waves in thermal enviornments

In this presentation, I will talk about a framework for analysing gravitational waves in thermal environments, such as the radiation-dominant early universe, from a symmetry perspective. I will focus on the effective dynamics of gravitational waves in the interaction picture of quantum field theory and discuss the symmetry constraints on the theory.

Speaker: Atsuhisa Ota (Chongqing University)

presentation.pdf

15:30 Coffee break

Session 6

Convener: Ye-Ling Zhou (Hangzhou Institute for Advanced Study)

17 High Reheating Temperature without Axion Domain Walls

We investigate a cosmological scenario in which the Peccei-Quinn (PQ) symmetry remains broken in the entire history of the Universe, thereby avoiding the formation of axion strings and domain walls. Contrary to the conventional expectation, it is demonstrated that appropriately chosen scalar
interactions are able to keep the PQ symmetry broken at arbitrarily high temperatures. We carefully examine the finite-temperature effective potential in a model with two PQ breaking scalar fields. The existence of flat directions plays a vital role in suppressing axion isocurvature perturbations
during inflation by stabilizing a PQ field at a large field value. The viable parameter space consistent with theoretical and observational constraints is identified. Our scenario provides a minimal path for PQ symmetry breaking that addresses both the axion domain wall and isocurvature problems
while permitting arbitrarily high reheating temperatures accommodating high-scale baryogenesis scenarios such as thermal leptogenesis.

Speaker: 耀铎(Yaoduo) 王(Wang) (李政道研究所(TDLI))

25_10_HIAS_SNR.pdf

18 Probing Light Dark Matter with Cosmic Gravitational Focusing

We investigate the possibility of using the cosmic gravitational focusing (CGF) to probe the minor light dark matter (DM) component whose mass is in the range of $(0.1 \sim 100)$\,eV. Being a purely gravitational effect, the CGF offers a mode-independent probe that is complementary to the existing ways such as Lyman-$\alpha$ and $\Delta N_{\rm eff}$. Such effect finally leads to a dipole density distribution that would affect the galaxy formation and hence can be reconstructed with galaxy surveys such as DESI. Both the free-streaming and clustering limits have been studied with analytical formulas while the region in between is bridged with interpolation. We show the projected sensitivity at DESI with the typical phase space distribution of a freeze-in DM scenario as illustration.

Speaker: Liang (亮) Tan (谭) (Tsung-Dao Lee Institute)

light DM with CGF.pptx

19 Constraining the Primoridal Black Hole Abundance with Space-Based Detectors

Primordial black holes, hypothesized to form from the collapse of large density fluctuations in the early universe, remain a compelling candidate for dark matter and a powerful probe of primordial cosmological physics. In this work, we explore the capabilities of future space-based gravitational wave detectors, including LISA, Taiji, and TianQin, to constrain the PBH abundance. We focus on the stochastic background of scalar-induced gravitational waves associated with PBH formation, and derive upper bounds on the PBH abundance across a wide mass range. We systematically account for the width of the primordial curvature power spectrum and determine the mass windows in which PBHs can be excluded as the dominant component of dark matter under null detection scenarios. Notably, we find that the exclusion window is narrowest for a power spectrum width of $\Delta \sim 0.28$, yet PBHs with masses in the range $10^{−17}–10^{−9} M_\odot$ can still be robustly ruled out. This implies that if PBHs constitute the majority of dark matter, the associated SIGW signal must be detectable by upcoming space-based interferometers.

Speaker: Wencong Hong (ITP-CAS)

HIAS2025-HWC.pdf

20 Constraining reheating temperature by ACT DR6

The latest Atacama Cosmology Telescope (ACT) Data Release 6 suggests the spectral index of the curvature perturbation n_s = 0.974 ± 0.003, about 0.01 larger than the Planck 2018 result, which revives chaotic inflation with a power-law potential V(ϕ) ∼ |ϕ|p. We revisit such a model in the light of ACT DR6, and carefully takes into account the intermediate oscillatory inflation stage after slow-roll inflation when p < 1. We find that such models are consistent with ACT DR6, which also implies p < 2/3 and T_reh ≥ 10^11 GeV.

Speaker: Zi-Han Li

21.Zi-Han Li constraining?? reheating.pdf

17:00 Workshop dinner

Sunday, 2 November

09:30 Coffee break

Session 7

Convener: Yi Yang

21 Darkogenesis via Supercooled Phase Transition

We discuss the intriguing possibility that the recently reported nano-Hz gravitational wave signal by Pulsar Timing Array (PTA) experiments is sourced by a strong first-order phase transition in a dark sector. The phase transition has to be strongly supercooled to explain the signal amplitude. However, such strong supercooling exponentially dilutes away any pre-existing baryon asymmetry and dark matter, calling for a new paradigm of their productions. We then develop a mechanism of cold darkogenesis that generates a dark asymmetry during the phase transition from the textured dark SU(2) Higgs field. This dark asymmetry is transferred to the visible sector via neutron portal interactions, resulting in the observed baryon asymmetry. Furthermore, the mechanism naturally leads to the correct abundance of asymmetric dark matter. Collider searches for mono-jets and dark matter direct detection experiments can dictate the viability of the model. We also discuss another scenario of darkogenesis where the number asymmetry is generated from the decay of a mother particle produced via parametric resonance during the phase transition induced due to its coupling to the order parameter scalar. It is shown that the correct baryon asymmetry and dark matter abundance can be realized for a dark phase transition at O(1) GeV. The scenario will be tested further in neutron-antineutron oscillation experiments.

Speaker: Yuichiro Nakai (Shanghai Jiao Tong University)

HIAS2025.pdf

22 Gravitational waves and Primordial black holes formation in a complex singlet extension of the Standard Model

A complex singlet extension of the Standard Model not only incorporates scalar dark matter candidates but also enables the realization of a strong electroweak first-order phase transition essential for electroweak baryogenesis. This phase transition may generate gravitational waves and primordial black holes. We discuss the correlation between the strength of the phase transition and these signals, as well as their future observability.

Speaker: Chikako Idegawa (Sun Yat-sen University)

CI_251102.pdf

11:00 Lunch

Session 8

Convener: Anna Tokareva (HIAS)

23 Dynamics of coherent vacuum domains from axion fragmentation

In this talk we will present the formation of multiple vacuum states in axion models with non-degenerate vacua, which originate from vacuum fluctuations amplified via parametric resonance. The domains are classified as sub-horizon, cross-horizon, or super-horizon on their scale upon formation, leading to the different cosmic evolution as well as gravitational wave spectrum

Speaker: Dr 贇 蒋 (Sun Yat-Sen (Zhongshan) University)

24 Probing spin-2 ultralight dark matter with PTA and space-based GW Detectors

Spin-2 ultralight dark matter (ULDM) is a viable dark matter candidate, and it can be constrained using gravitational wave (GW) observations. The pulsar timings are sensitive to both the nanohertz gravitational-wave background and the oscillation of ultralight dark matter. We study the angular correlation of the timing residuals induced by spin-2 ultralight dark matter, which differs from the usual Hellings-Downs correlation for stochastic gravitational-wave backgrounds. We also investigate the detectability of spin-2 ULDM by space-based GW interferometers. By considering a direct coupling between spin-2 ULDM and ordinary matter, we derive the corresponding response functions and sensitivity curves for various time-delay interferometry channels and calculate the optimal sensitivity curves for future millihertz GW detectors. Thus, the GW detectors can serve as powerful tools not only for detecting GWs but also for probing fundamental properties of ultralight dark matter.

Speaker: Dr Yun-Long Zhang (NAOC (National Astronomical Observatories, CAS))

251102ZhangYLspin2.pdf

25 Closing

Closing talk

Speaker: Kohei Kamada (HIAS, UCAS)