Gravitational Waves and the Early Universe: Accelerated Expansion, Dynamical Inhomogeneity, and Beyond

Contribution List

1. Opening remark

12/03/2026, 09:30

2. John Ellis

12/03/2026, 09:40

3. Mikage Kobayashi, "NANOGrav 15-year gravitational-wave signals from binary supermassive black-holes seeded by primordial black holes"

12/03/2026, 10:50

In 2023, NANOGrav and other pulsar timing arrays (PTAs), reported evidence for a stochastic gravitational-wave background (GWB) in the nanohertz (nHz) frequency band. One of the most promising candidates for the origin of this signal is the merger of supermassive black hole binaries (SMBHBs). Supermassive black holes (SMBHs), with masses exceeding $10^6 M_\odot$, are believed to reside at the...

4. Luis Gil, "Hot news on the phase structure of the SMEFT"

12/03/2026, 11:10

We compute the high-temperature limit of the Standard Model Effective Field Theory (SMEFT) up to full order $g^4$ in coupling constants $g$. This analysis includes one-loop contributions to kinetic terms and quartic couplings; as well as two-loop contributions to squared mass terms. Using lattice data, we provide evidence that, in contrast with previous statements in the literature, the SMEFT...

5. Ashu Kushwaha, "An upper limit on cosmological chiral gravitational wave background"

12/03/2026, 11:30

Within the standard framework in which electroweak sphaleron processes relate lepton and baryon number, we derive an upper limit on the amplitude of a chiral gravitational wave background produced prior to the electroweak epoch. This bound is independent of the production time of chiral GWs for superhorizon modes, while it becomes sensitive to the production time for subhorizon modes. For...

6. Junseok Lee, "Induced domain walls of QCD axion, and gravitational waves"

12/03/2026, 11:50

We show that heavy axion domain walls induce domain walls of the QCD axion through a mixing between the heavy axion and the QCD axion, even when the pre-inflationary initial condition is assumed for the QCD axion. The induced domain walls arise because the effective $\theta$ parameter changes across the heavy axion domain walls, shifting the potential minimum of the QCD axion. When the heavy...

18. Jun-Peng Li, "The Imprints of Primordial Non-Gaussianity on the Scalar-Induced Gravitational-Wave Background"

12/03/2026, 13:30

Scalar-induced gravitational waves (SIGWs), produced nonlinearly by enhanced curvature perturbations, offer a promising probe of primordial non-Gaussianity (PNG) in the early Universe. In this talk, I will present our recent work on the imprints of local-type PNG on the SIGW background. I will introduce a “renormalized” diagrammatic approach we developed, which simplifies calculations of the...

19. Pankaj Saha, "A Cosmic Chorus of coherent Scalars: Gravitational Waves probe of Baryogenesis and Dark Sectors"

12/03/2026, 14:40

A scalar field undergoing coherent oscillations can exhibit parametric resonance, leading to resonant amplification of field fluctuations that can source gravitational waves with characteristic spectra. We illustrate two key implications of this framework. First, such oscillating spectator scalar fields can naturally accommodate a candidate for the (ultralight) dark matter or dark radiation....

20. Danilo Artigas, "δN formalism: non-linear evolution of tensor perturbations during inflation"

12/03/2026, 15:00

Linear-perturbation theory has proven to be an extremely powerful tool to compare inflationary models with observational data. Recently, the newcoming high-precision observations call for predictions beyond linear perturbations. Such effects are known to be relevant for example in the production of primordial black holes or scalar-induced gravitational waves. The δN formalism proposes to...

21. Sam Wong, "Probing the Squeezed Tensor Bispectrum and Soft Theorem via PTA-Interferometer Correlations"

12/03/2026, 15:20

While the power spectrum of the stochastic gravitational wave background (SGWB) characterizes the energy density of the background, the three-point function (bispectrum) encodes the dynamics of interactions. In this talk, we focus on the squeezed limit of the tensor bispectrum, where one mode is significantly longer than the other two. First, we review the theoretical connection between...

22. Andres Gonzalez Morales, "Complexity-Bounded State Selection in Discrete Cosmology for Primordial Gravitational Waves"

12/03/2026, 15:40

We consider discrete quantum gravity models in which the early universe is described by a finite discrete history rather than an infinite past. Such a history can be viewed as a finite depth local quantum circuit acting on a simple seed state, which immediately links the microscopic circuit complexity of the resulting cosmological boundary state to the complexity of the underlying quantum...

23. Kazunori Kohri, "Merger signals of gravitational waves from binary PBHs or SMBHs seeded by PBHs"

12/03/2026, 16:20

In my invited talk, I will explain recent topics concerning three types of merger signals of gravitational waves related to primordial black holes: 1) dark matter primordial black holes, 2) nano-Hz gravitational waves produced by binary supermassive black holes seeded by primordial black holes, and 3) ultra-light binary primordial black holes with the memory burden effect.

13. Kazunori Nakayama, "High frequency gravitational waves from reheating"

13/03/2026, 09:30

I will review some recent developments on the production of high frequency gravitational waves during the reheating stage.

14. Kamil Mudrunka, "High Frequency Spectrum of Primordial Gravitational Waves"

13/03/2026, 10:40

During inflation gravitational waves are produced in the superhorizon regime, which form stochastic background in the present universe with very wide range of frequencies. Higher frequency gravitational waves never experience superhorizon regime, but they are also amplified after inflation due to the inflaton oscillation. Taking account of the inflaton dynamics after inflation, we calculate...

15. Sourav, "Stochastic gravitational wave from graviton bremsstrahlung in inflaton decay into massive spin 3/2 particles"

13/03/2026, 11:00

The detection of primordial gravitational waves would offer direct evidence of inflation and valuable insights into the dynamics of the early universe. During the post-inflation reheating period, when the inflaton coherently oscillates at the bottom of its potential, primordial stochastic gravitational waves may be sourced by its perturbative decay into particles of different spins. Assuming...

16. Takahiro Terada, "Quintessence with Sharp Transitional Feature and Late-Time Cosmological Signals"

13/03/2026, 11:20

"Combinations of recent cosmological observations, including Dark Energy Spectroscopic Instrument, show hints of a dynamical nature for dark energy. While the data suggest the possibility of the phantom crossing, it is worth thoroughly exploring quintessence models. Given that phenomenological parametrisations of the equation-of-state parameter $w(a)$ with a sharp transitional feature fit the...

17. Yannis Georis, "Upper Bound on Thermal Gravitational Wave Backgrounds from Hidden Sectors"

13/03/2026, 11:40

Hot viscous plasmas unavoidably emit a gravitational wave background, similar to electromagnetic black body radiation. We study the contribution from hidden particles to the diffuse background emitted by the primordial plasma in the early universe. While this contribution can easily dominate over that from Standard Model particles, we find that both are capped by a generic upper bound that...

7. Alexander Vikman, "Challenges in Modelling Domain Walls and Their Gravitational Wave Emission"

13/03/2026, 13:30

I will discuss biased and melting domain walls (DW) and their resulting gravitational waves (GW). In particular, I will discuss recently identified subtle issues with initial and boundary conditions in numerical simulations of biased DW and how these issues influence resulting GW. Moreover, I will compare GW signatures of biased and recently introduced melting DW. This talk is based on...

8. Kazuya Furusawa, "Probing Helical Primordial Magnetic Fields via Circular Polarization of Gravitational Waves in the LISA-TAIJI Network"

13/03/2026, 14:40

Primordial magnetic fields (PMFs) are anticipated to be a cosmological candidate that generates a stochastic gravitational wave background (SGWB) by their anisotropic stress. In particular, if parity violation is present in the production of PMFs, it can induce their helical component, leading to circular polarization in the isotropic SGWB. In this study, we consider parity-violating...

9. Van Que Tran, "Refining Gravitational Wave and Collider Physics Dialogue via Singlet Scalar Extension"

13/03/2026, 15:00

Employing effective field theory techniques, we advance computations of thermal parameters that enter predictions for the gravitational wave spectra from first-order electroweak phase transitions. Working with the real-singlet-extended Standard Model, we utilize recent lattice simulations to confirm the existence of first-order phase transitions across the free parameter space. For the first...

10. Kohei Kamada, "Gravitational wave background from metastable cosmic strings in the delayed scaling scenario"

13/03/2026, 15:20

Recent observations by pulsar timing arrays (PTAs) such as NANOGrav, EPTA, PPTA, and CPTA suggest the presence of nanohertz stochastic gravitational wave background (GWB), which may be a hint for new physics. Among several possible sources, those from metastable cosmic string would be attractive since the spectral tilt of the GWB can be easily consistent with those suggested in PTAs. However,...

11. Javier Rubio, "Gravity-Induced Phase Transitions in the Standard Model and Beyond"

13/03/2026, 15:40

Scalar fields coupled to gravity can display spectacular dynamics in the aftermath of inflation. In cosmological scenarios featuring a brief period of kination, the rapid evolution of spacetime curvature can destabilize otherwise symmetric vacua, triggering curvature-driven phase transitions through tachyonic instabilities. These purely gravitational effects can explosively amplify field...

12. Philipp Schicho, "Cosmological phase transitions as sources of gravitational waves"

13/03/2026, 16:20

The possibility that future gravitational-wave detectors could observe the relic background of a cosmological phase transition has triggered intense progress in the theoretical description of these events. A detection of any such background can therefore probe energies far above those accessible to particle colliders, shedding light on fundamental physics questions, such as the state of the...

36. Maya Perez "Phase transitions of a noncommutative Reissner-Nordstrom Black Hole"

13/03/2026, 17:20

The thermodynamic properties of a noncommutative Reissner–Nordström black hole (NCRN BH) are presented. The analysis is carried out using a Lorentzian source and the framework of Geometrothermodynamics (GTD), which provides a Legendre-invariant geometric description of thermodynamics and allows for a quasihomogeneous analysis. Within this formalism, phase transition curves are obtained from...

37. Manas Agrawal "Exploring Non-Hermitian & PT-Symmetric Quantum Systems: Energy Spectra, Integrability, and Stability"

13/03/2026, 17:40

We investigate the spectral and dynamical properties of a class of non-Hermitian quantum systems endowed with Parity–Time (PT) symmetry, focusing on both continuum and
lattice models. First, we consider the one-parameter family H = p2 + x2 (ix)ϵ,
and compute its energy spectrum across the unbroken and broken PT phases using
complementarytechniques:...

24. Workshop dinner

13/03/2026, 18:30

25. Naoya Kitajima

14/03/2026, 09:30

26. Takumi Fujimori, "Distinction of an ultra-light dark matter signal from narrow-band spectral artifacts using signal correlation"

14/03/2026, 10:40

Recently, ultra-light dark matter (ULDM) has attracted attention as a new candidate for dark matter. In addition, there are experiments to detect a ULDM signal with ground-based gravitational wave (GW) detectors. While currently operating GW detectors can interact with and be sensitive to ULDM, the data from detectors contains narrow-band spectral artifacts, which have similar spectral...

27. Albert Escrivà, "Ripples from the Dark Ages: Gravitational Waves in an Early Matter-Dominated Era"

14/03/2026, 11:00

We investigate gravitational-wave emission from the nonlinear collapse of matter overdensities during an early matter-dominated era using cosmological N-body simulations. In this talk, I will present our simulation framework and summarize current results on the waveform and energy spectrum.

28. Marco Antonio Merchand Medina, "Gravitational waves from phase transitions during reheating"

14/03/2026, 11:20

In this talk I wall show recent results on the gravitational wave spectra that are generated during perturbative reheating.Within a concrete inflationary framework, we derive how the thermodynamic parameters governing the phase transition depend on the effective equation-of-state parameter, which is in turn determined by the inlaton potential. We show that phase transitions occurring during...

29. Rinku Maji, "Cosmological Implications of Unification: Topological Structures and Gravitational Waves"

14/03/2026, 11:40

Grand unified theory (GUT) provides a rationale for the arbitrariness of the Standard Model (SM) and explains many enigmas of nature at the outset of a single gauge group. We will discuss the realizations of 'metastable' strings (MSS), 'quasistable' strings (QSS), and 'walls bounded by strings' (WBS) in SO(10) GUTs. We will explore the stochastic gravitational wave background emitted from such...

30. Takashi Hiramatsu, "Gravitational waves from cosmic textures"

14/03/2026, 13:30

Cosmic textures are non-trivial field configurations arising from spontaneous symmetry-breaking, G->H, for which \pi_3(G/H) is nontrivial. Global textures are known to be sources of gravitational waves in the literature, whereas gauged textures have not been well studied, since some of them are pure gauge. In the present study, we focus on gauged SU(2) and O(4) textures, following the...

31. Akifumi Chitose, "Do Cosmic String Segments Emit Gravitational Waves?"

14/03/2026, 13:50

Cosmic strings are predicted in various extensions of the Standard Model, including grand unified theories. Depending on the symmetry-breaking pattern, they can be either topologically stable or metastable. Intriguingly, metastable strings have been proposed as a possible origin of the gravitational wave (GW) background observed by recent pulsar timing array experiments. When metastable...

32. Ivan Rybak, "Gravitational-Wave Signatures of Superconducting Cosmic Strings"

14/03/2026, 14:10

The early Universe offers a powerful probe of physics at the highest energy scales, including the formation of cosmic strings in many extensions of the Standard Model. Most predictions for the stochastic gravitational-wave background (SGWB) from cosmic strings rely on the Nambu–Goto approximation and neglect internal string structure. In realistic particle-physics models, however, cosmic...

33. Qaisar Shafi, "Superconducting strings and composite topological structures"

14/03/2026, 14:30

We review the appearance of superconducting strings and composite topological structures in realistic extensions of the Standard Model. Some of these structures also appear in condensed matter systems.

34. Kai Schmitz, "Cosmic Strings at the PTA Frontier and Beyond"

14/03/2026, 15:10

I will review the gravitational-wave (GW) phenomenology of various types of cosmic strings, including stable, metastable, and current-carrying strings, and discuss the capacity of cosmic strings to explain the evidence for a GW background in the latest pulsar timing array (PTA) data sets. In doing so, I will emphasize current uncertainties in the modeling of the GW signal from cosmic strings...

35. Closing remark

14/03/2026, 16:00