Opening remark Room: ES635

John Ellis, "Chasing the Biggest Bangs since the Big Bang with Gravitational Waves" Room: ES635

Mikage Kobayashi, "NANOGrav 15-year gravitational-wave signals from binary supermassive black-holes seeded by primordial black holes" Room: ES635 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 centers of nearly all massive galaxies. However, the formation and growth mechanisms of these black holes remain poorly understood. Previous simulation studies have suggested that the amplitude of the stochastic GWB generated by SMBHB mergers is smaller than the value reported by the NANOGrav 15-year data (NG15). In this study, we propose that the nHz-band GWB recently reported by the NANOGrav 15-year dataset can be explained by mergers of SMBHBs with masses of $10^9 M_\odot$ that are formed through the growth of primordial black holes (PBHs). If PBHs accrete matter at a high rate, they emit a large number of high-energy photons, which heat the surrounding plasma and induce cosmological 21 cm radiation at high redshifts. Since such radiation has not been observed, stringent upper limits are imposed on the accretion rate. We show that, for a PBH abundance in the range $10^{-14} \lesssim f_{\rm PBH} \lesssim 10^{-12}$ and PBH masses of $1 M_\odot \lesssim m_{\rm PBH} \lesssim 10^3 M_\odot$, it is possible to explain the nHz-band GWB observed by the NANOGrav 15-year data while avoiding constraints from 21 cm radiation. We propose that future gravitational-wave observations and cosmological 21 cm measurements will provide critical tests of this scenario.

Luis Gil, "Hot news on the phase structure of the SMEFT" Room: ES635 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 may undergo a first-order phase transition even in the absence of direct modifications of the Higgs potential, opening the door to entirely unexplored directions in model building. In such case, however, our analysis implies that the phase transition is not sufficiently strong, narrowing down the new physics that allows for electroweak baryogenesis and the production of observable gravitational waves.

Ashu Kushwaha, "An upper limit on cosmological chiral gravitational wave background" Room: ES635 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 sufficiently high reheating temperatures, the bound becomes significantly more stringent than the conventional big bang nucleosynthesis constraints at frequencies above the MHz scale, thereby providing a powerful and model-independent probe of parity-violating physics in the early Universe. Based on the work [2601.13532]

Junseok Lee, "Induced domain walls of QCD axion, and gravitational waves" Room: ES635 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 axion domain walls collapse, the induced QCD axion domain walls collapse as well. This novel mechanism for producing the QCD axions can explain dark matter even with the axion decay constant as small as ${\mathcal O}(10)\,{\rm GeV}$. In particular, this scenario requires domain wall collapse near the QCD crossover, potentially accounting for the stochastic gravitational wave background suggested by recent pulsar timing array observations, including NANOGrav. Using this mechanism, it is also possible to easily create induced domain walls for string axions or axions with a large decay constant, which would otherwise be challenging.

Jun-Peng Li, "The Imprints of Primordial Non-Gaussianity on the Scalar-Induced Gravitational-Wave Background" Room: ES635 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 SIGW energy-density spectrum in the presence of high-order PNG and enables systematic analysis up to arbitrary order. This work includes the energy-density fraction spectrum of the isotropic background, the angular power spectrum describing anisotropies, and the angular bispectrum and trispectrum characterizing non-Gaussianity. Specifically, using this framework, we perform numerical calculations for PNG up to quartic order. Our results show that PNG can significantly enhance the SIGW energy density and generate distinctive anisotropic and non-Gaussian signatures. These findings highlight the potential of the SIGW background as a sensitive probe of primordial non-Gaussianity.

Pankaj Saha, "A Cosmic Chorus of coherent Scalars: Gravitational Waves probe of Baryogenesis and Dark Sectors" Room: ES635 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. Second, in a low-energy realization of Affleck–Dine baryogenesis, a light scalar field with mass of order a few GeV generates the baryon asymmetry while sourcing gravitational waves peaking in the CE/DECIGO frequency band. We highlight a compelling complementarity between gravitational-wave observations, cosmology, and laboratory searches for physics beyond the Standard Model.

Danilo Artigas, "δN formalism: non-linear evolution of tensor perturbations during inflation" Room: ES635 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 capture some of these non-linearities by describing the universe as a set of homogeneous patches evolving independently. Originally formulated to describe scalar perturbations only, I will show how it can be generalised to also describe gravitational waves and their correlation functions. As an application, I will discuss a model where a U(1) gauge field sources the gravitational-wave signal.

Sam Wong, "Probing the Squeezed Tensor Bispectrum and Soft Theorem via PTA-Interferometer Correlations" Room: ES635 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 gravitational soft theorems and the squeezed bispectrum in flat spacetime, drawing a parallel to their well-known cosmological counterpart: the consistency relation in the tensor sector. Second, we propose a multi-frequency observational strategy to search for this signal. By leveraging the vast scale separation between Pulsar Timing Arrays ( ~nano Hz) and space-based interferometers like LISA (~0.001 Hz ), we construct a cross-correlation statistic that treats the PTA residuals as a background field modulating the power of the short modes observed by LISA.

Kazunori Kohri, "Merger signals of gravitational waves from binary PBHs or SMBHs seeded by PBHs" Room: ES635 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.

Kazunori Nakayama, "High frequency gravitational waves from reheating" Room: ES635 I will review some recent developments on the production of high frequency gravitational waves during the reheating stage.

Kamil Mudrunka, "High Frequency Spectrum of Primordial Gravitational Waves" Room: ES635 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 the spectrum of high frequency gravitational waves produced during and after inflation in detail, in particular focusing on the connection between the low and high frequency regime, and show that the detailed spectrum can distinguish inflation models.

Sourav, "Stochastic gravitational wave from graviton bremsstrahlung in inflaton decay into massive spin 3/2 particles" Room: ES635 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 the behaviour of the potential near the minimum as a polynomial $V(\phi)= \phi^k$, where $k>=2$, and treating the inflaton as a coherently oscillating classical field, we calculate the decay of the inflaton into a pair of spin particles accompanied by graviton emission. We numerically study the reheating dynamics and calculate the stochastic gravitational wave spectra. Our analysis shows that the gravitational wave spectra can offer insights into the microscopic physics during inflation.

Takahiro Terada, "Quintessence with Sharp Transitional Feature and Late-Time Cosmological Signals" Room: ES635 "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 data well, we study the realisation of such models in quintessence. In the late Universe, the quintessence field begins to oscillate abruptly, changing the behaviour of $w$. Naturally, such a model entails tachyonic instability, and particle production modifies $w$. We perform numerical lattice simulations to study the time dependence of $w$. In addition, the violent particle production produces significant density perturbations and the stochastic gravitational-wave background, whose characteristic scale depends on the mass scale of the quintessence around the minimum of the potential. We discuss the observability of these late-time cosmological signals through cosmic microwave background, quasar astrometry, pulsar timing arrays, and other observational probes."

Yannis Georis, "Upper Bound on Thermal Gravitational Wave Backgrounds from Hidden Sectors" Room: ES635 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 makes them difficult to detect with interferometers in the foreseeable future. We illustrate our results for axion-like particles and heavy neutral leptons. Finally, our results suggest that previous works overestimated the gravitational wave background from particle decays out of thermal equilibrium. This talk will be based on: ArXiv:2312.13855

Alexander Vikman, "Challenges in Modelling Domain Walls and Their Gravitational Wave Emission" Room: ES635 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 e-Print: 2504.07902, e-Print: 2410.21971, e-Print: 2406.17053, e-Print: 2307.04582, e-Print: 2112.12608 and e-Print: 2104.13722.

Kazuya Furusawa, "Probing Helical Primordial Magnetic Fields via Circular Polarization of Gravitational Waves in the LISA-TAIJI Network" Room: ES635 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 inflationary magnetogenesis phenomenologically and compute the intensity and circular polarization of SGWB for various assumed power spectra of helical PMFs. By using the planned sensitivity of the LISA-TAIJI network, we calculate the signal-to-noise ratio (SNR) and also perform a Fisher forecast to discuss the possibility of constraining the parameters of helical PMFs. We conclude that it is possible to estimate the fractional helicity~$r_\mathrm{H}$ with $\mathrm{SNR}^{V}>2$ if the PMF strength is $\mathcal{B}\gtrsim 10~\mathrm{nG}$, particularly in the case of the delta function type Pspectrum. Our results offer a useful criterion that indicates the future observational limit on helical PMFs on the small scale $k_\mathrm{LISA} \approx 10^{12}~\mathrm{Mpc}^{-1}$.

Van Que Tran, "Refining Gravitational Wave and Collider Physics Dialogue via Singlet Scalar Extension" Room: ES635 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 time, we account for several important two-loop corrections in the high-temperature expansion for determining thermal parameters, including the bubble wall velocity in the local thermal equilibrium approximation. We find that the requirement of completing bubble nucleation imposes stringent bounds on the new scalar boson mass. Moreover, the prospects for detection by LISA require first-order phase transitions in a two-step phase transition, which display strong sensitivity to the portal coupling between the Higgs and the singlet. Interestingly, signals from di-Higgs boson production at the HL-LHC probe parameter regions that significantly overlap with the LISA-sensitive region, indicating the possibility of accounting for both signals if detected. Conversely, depending on the mixing angle, a null result for di-Higgs production at the HL-LHC could potentially rule out the model as an explanation for gravitational wave observations.

Kohei Kamada, "Gravitational wave background from metastable cosmic strings in the delayed scaling scenario" Room: ES635 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, there are two issues in this scenario; i) it is inconsistent with the non-observations of the stochastic GWB at LVK, ii) it needs fine-tuning in the highest temperature of the Universe to have cosmic string formation without monopole formation. In this talk, I will discuss if cosmic strings are formed during inflation after sufficient dilution of monopoles, they start emitting gravitational waves at relatively later time to avoid the LVK bound. I also show the numerical and analytic evaluation of the shape of the whole GWB spectrum and its parameter dependence.

Javier Rubio, "Gravity-Induced Phase Transitions in the Standard Model and Beyond" Room: ES635 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 fluctuations, seed short-lived networks of topological defects, and efficiently convert vacuum energy into radiation—opening a new and economical route to reheating and to the production of stochastic gravitational-wave backgrounds. In this talk, I will present the basic mechanism and explore its broader cosmological implications, with the Standard Model Higgs field as a particularly compelling realization. The resulting interplay between spacetime curvature, electroweak vacuum stability, and non-perturbative field dynamics offers a minimal yet predictive framework that links fundamental particle physics to potentially observable gravitational-wave signals in upcoming experiments.

Philipp Schicho, "Cosmological phase transitions as sources of gravitational waves" Room: ES635 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 early Universe, the baryon asymmetry of the Universe, the nature of the dark matter, or whether exotic objects like primordial black holes or cosmic strings exist. First-order phase transitions proceed through the nucleation and expansion of bubbles, whose microscopic properties, such as the bubble nucleation rate, fluctuation determinants, and wall velocity, directly determine the resulting gravitational-wave spectrum. Reliable predictions therefore require a precise treatment of thermal field theory, out-of-equilibrium dynamics, and the interaction of the bubble wall with the primordial plasma. To enable systematic studies of models beyond the standard model, these developments must be automated and implemented in robust computational tools. Recent and forthcoming programs for determining bubble determinants and bubble-wall velocities represent important steps in this direction. I will review key advances in modelling phase-transition dynamics, discuss the challenges that remain, and outline how improved automation will sharpen gravitational-wave predictions for upcoming experiments.

Maya Perez "Phase transitions of a noncommutative Reissner-Nordstrom Black Hole" Room: ES635 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 singularities of the Ricci scalar.

Workshop dinner Room: ES635

Naoya Kitajima, "Primordial black hole formation and stochastic gravitational waves with numerical relativity" Room: ES635 In the first part of my talk, I focus on the primordial black hole formation from collapsing closed domain walls. To follow the dynamical process of the black hole formation, I employed numerical simulations with fully general relativistic framework (numerical relativity). I present the results of our simulations, showing that the initial closed domain wall shell collapses into a black hole, even in the case with non-spherical initial conditions. In the second part, I focus on the stochastic gravitational waves induced by initial scalar perturbations. The simulation based on the numerical relativity can follow the gravitational wave emission in fully nonlinear regime. In this talk, I present our tentative results of the simulation for the scalar-induced gravitational waves in matter dominated universe.

Takumi Fujimori, "Distinction of an ultra-light dark matter signal from narrow-band spectral artifacts using signal correlation" Room: ES635 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 morphologies to a possible ULDM signal. To detect a possible ULDM signal, such artifacts must be distinguished from the true signal. We introduce a method to characterize them and discuss the possibility of distinction by utilizing the signal correlation.

Albert Escrivà, "Ripples from the Dark Ages: Gravitational Waves in an Early Matter-Dominated Era" Room: ES635 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.

Marco Antonio Merchand Medina, "Gravitational waves from phase transitions during reheating" Room: ES635 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 reheating generically produce gravitational wave signals that are suppressed compared to the conventional radiation dominated scenario. The degree of suppression and spectral features depend qualitatively on whether reheating proceeds through fermionic or bosonic channels, with bosonic reheating exhibiting stronger suppression.

Rinku Maji, "Cosmological Implications of Unification: Topological Structures and Gravitational Waves" Room: ES635 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 networks of composite structures. We will discuss the gravitational waves emitted from MSS, QSS, and WBS with superheavy (~GUT scale) strings, which can explain the recent exciting evidence of stochastic gravitational waves in NANOGrav and other pulsar timing array data.

Takashi Hiramatsu, "Gravitational waves from cosmic textures" Room: ES635 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 pioneering work of Davis (1987), in which it was shown that the gauged SU(2) texture is pure gauge, while the gauged O(4) texture behaves like radiation. We estimate the gravitational wave spectra using field-theoretic simulations and discuss the differences between these cases.

Akifumi Chitose, "Do Cosmic String Segments Emit Gravitational Waves?" Room: ES635 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 strings decay, they fragment into segments with monopoles and antimonopoles attached at their endpoints. The monopole and antimonopole are strongly pulled by the string tension. Violent oscillations of these segments have been considered as a potential GW source, in addition to contributions from string loops. We show that, in realistic situations, the monopoles frequently collide with thermal fluctuations on the string segments, which act as a resistance and prevent the oscillation. As a result, we find that the contribution from string segments to the GW background is negligible.

Ivan Rybak, "Gravitational-Wave Signatures of Superconducting Cosmic Strings" Room: ES635 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 strings can carry bosonic or fermionic currents and become superconducting. In this work, we extend SGWB predictions to networks of cosmic strings carrying chiral currents. We analyze the coupled evolution of the string network and the current and its impact on gravitational-wave emission. We show that superconducting cosmic strings generate gravitational-wave spectra that differ from the standard case: strong coupling between the current and a massless vector field can substantially suppress the SGWB, while moderate coupling may still yield detectable signals. These effects open new observational avenues for probing fundamental field interactions in the early Universe through superconducting cosmic strings.

Kai Schmitz, "Cosmic Strings at the PTA Frontier and Beyond" Room: ES635 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 and how they impact the interpretation of the PTA data. Along the way, I will also mention recent results on low-scale strings (see 2405.10937 and 2505.04537) and present a fully analytical treatment of the GW spectrum from stable strings in the velocity-dependent one-scale (VOS) model (see 2412.20907).

Closing remark Room: ES635