Gravitational Wave Probes of Physics Beyond Standard Model 4

23 Jun 2025, 09:00 → 27 Jun 2025, 18:30 Europe/Warsaw

Faculty of Physics, University of Warsaw

Bogumiła Świeżewska (University of Warsaw), John Ellis, Marek Lewicki (University of Warsaw), Nobuchika Okada, Qaisar Shafi, Anish Ghoshal (University of Wasrsaw, Poland)

The purpose of this workshop is to discuss how gravitational waves may serve as tools to explore possible fundamental physics beyond the Standard Model, such as first-order phase transitions in the early Universe, cosmic strings, primordial black holes and scenarios for cosmological inflation. In addition, it will explore how gravitational wave signals may reveal possible modifications of general relativity. With these aims in mind, the workshop will bring together experts in particle physics, cosmology and the gravitational wave community.

Confirmed speakers:

• Lara Sousa (Porto U) [cosmic defects]
  
• Fabrizio Rompineve (UA Barcelona) [cosmic defects]
  
• Alberto Roper Pol (U Geneva) [phase transition]
  
• Oliver Gould (Nottingham U) [phase transition]
  
• David Mateos (ICREA and U Barcelona) [phase transition]
  
• Simone Blasi (DESY) [phase transition]
  
• Nicola Bartolo (U Padua) [inflation & PBH]
  
• Marek Szczepańczyk (U Warsaw) [astrophysics]
  
• Alexander Jenkins (Cambridge U) [phase transition]
  
• Christoph Ringeval (CP3, Louvain) [cosmic defects]
• Gianmassimo Tasinato (U Swansea) [inflation & PBH] 
• Achamveedu Gopakumar (TIFR Mumbai) [PTA]
• Kazunori Kohri (NAOJ Tokyo) [Inflation and PBH]

 

Please beware of fake e-mails attempting to offer you to book accommodation. The only reliable communications are those that come directly from the organisers. 

Organisers:

Monday 23 June

Inflation1

Gravitational Waves from Inflationary Sources

1 Infl 1

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Speaker: Gianmassimo Tasinato (U Swansea)

2 Gravitational waves from preheating in α-attractors

In multi-field inflationary models, couplings between fields are not limited to the potential of the model, but can also be present in kinetic terms. In such a case, they can be interpreted as a non-trivial structure of the space of fields. Non-vanishing curvature of this space can lead, if negative, to a new phenomenon called geometrical destabilization.

α-attractors are a very promising class of inflationary models. Their
predictions for small values of α parameter are in remarkably good agreement with the CMB data obtained by the Planck satellite. Since α-attractor models are by construction multi-field models, dynamics of the additional spectator field can play a role in the (p)reheating.

When investigating the two-field α-attractor T-model, we have found that geometrical destabilization during preheating leads to efficient fragmentation of the spectator field. As a result, the transition from the inflationary era to the radiation domination era is practically instantaneous and much faster than previously found in the effective theory including only the inflaton field. Recently, due to development of numerical software, we were able to overcome deficiencies of our primary analysis and got deeper insight into parametric dependence of the problem. Moreover, we estimated the spectrum of gravitational waves emitted during preheating in this model.

Speaker: Tomasz Krajewski (Institute of Fundamental Technological Research PAS)

3 Gravitational Waves from a First-Order Phase Transition of the Inflaton

We explore the production of gravitational waves resulting from a first-order phase transition (FOPT) in a non-minimally coupled "Dark Higgs Inflation" model. Utilizing a single scalar field both as the inflaton and as the Dark Higgs breaking the gauge symmetry in the dark sector, we demonstrate the feasibility of a unified framework for inflation and observable gravitational waves from a FOPT.

Speaker: Jörn Kersten (Yonsei University)

Coffee

Inflation2

4 Inflation 2

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Speaker: Nicola Bartolo (U Padua)

5 Probing leptogenesis with scalar induced gravitational waves

We demonstrate in this work that scalar-induced gravitational waves (SIGWs) can serve as a natural and powerful probe of thermal leptogenesis occurring at extremely high scales-beyond the reach of conventional particle physics experiments. As a proof of concept, we present a simple leptogenesis model in which an early matter-dominated phase, tracking the leptogenesis scale, is responsible for the production of GWs sourced by the early structure (halos of the scalar field giving mass to the right-handed neutrinos seeding leptogenesis) formation taking place before the Big Bang Nucleosynthesis (BBN). Leveraging recent N-body and lattice simulation results for GW computations in the nonlinear regime, we show that it is possible to establish a direct link between the frequency and the amplitude of these SIGWs and the thermal leptogenesis scale.

Speaker: Theodoros Papanikolaou (Scuola Superiore Meridionale)

6 Gravitational Waves from a Higgs-like spectator field

We study stochastic gravitational waves (GW) sourced by a Higgs-like spectator field via the modulated reheating mechanism. The curvature perturbations generated by the spectator in de Sitter vacuum are blue-tilted and strongly non-Gaussian. They must be suppressed on scales probed by the CMB anisotropies but can grow large on sub-Mpc scales, generating observationally testable stochastic GWs. We study the GW signals from a Higgs-like spectator scalar with a non-minimal coupling to spacetime curvature in a modulated reheating setup with shift-symmetric dimension-five couplings for the inflaton field. We find that for Standard Model values of the gauge and Yukawa couplings, the GWs are unobservably small but for larger couplings the setup can produce GWs detectable with the BBO survey.

Speaker: Michele Benaco (University of Jyväskylä)

Lunch

Inflation3

7 Inflation 3

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Speaker: Kazunori Kohri (NAOJ / KEK)

8 Pre-thermalized Gravitational Waves

We investigate a novel gravitational wave (GW) production mechanism from gravitons generated during the pre-thermal phase of cosmic reheating, where the energy density is dominated by non-thermalized inflaton decay products, dubbed reheatons. We consider multiple production channels, including: $i)$ pure inflaton-inflaton annihilation, $ii)$ graviton Bremsstrahlung from inflaton decay, $iii)$ scatterings between an inflaton and a reheaton, and $iv)$ scatterings among reheatons. To determine the resulting GW spectrum, we solve the Boltzmann equation to obtain the graviton phase-space distribution for each channel. We find that the third channel, $iii)$, dominates due to the large occupation number of reheatons at highly-energetic states during the pre-thermalization phase. Notably, in scenarios with a low inflaton mass, the GW spectrum could fall within the sensitivity range of future experiments such as the Einstein Telescope, the Cosmic Explorer, the Big Bang Observer, and ultimate DECIGO.

Speaker: Nicolás Bernal (New York University Abu Dhabi)

9 Searches for Inflationary Gravitational Waves in the NANOGrav 15 yr data-set

In this talk, I will summarize some of our efforts to interpret the GWB-like signal observed by the NANOGrav collaboration as inflationary gravitational waves, by sampling reheating temperature, tensor spectral index, and the running of the tensor spectral index. I will show that there is available parameter space to describe the signal, even if we impose constraints from CMB, LVK, and do not overabound the effective number of neutrino species. Two scenarios are possible: i) one in which a very late reheating period takes place, falling into the NANOGrav band, or ii) one with a more conventional large reheating temperature with a non-vanishing running of the tensor spectral index.

Speaker: Rafael Robson Lino dos Santos (NCBJ, Warsaw)

Coffee

Discussion

Tuesday 24 June

Defects1

10 Defects 1

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Speaker: Fabrizio Rompineve (UA Barcelona)

11 Supercooled Audible Axions

We present the audible axion mechanism extended by a period of supercooling that delays the onset of axion oscillations. While the original setup relies on a large axion decay constant and coupling to a dark Abelian gauge field to produce sizable gravitational wave signals, in this talk we discuss how supercooling opens up the testable parameter space and reduces the required coupling to $\alpha \gtrsim 1$.
Added to that, we showcase that the emission of gravitational waves via the axion coupling to the Standard Model photon in the presence of Schwinger pair production becomes possible, generating a strong signal in the $\mu$Hz or ultra-high frequency range.

Speaker: Christopher Gerlach (Johannes Gutenberg-Universität Mainz)

12 Probing SUSY at Gravitational Wave Observatories

Under the assumption that the recent pulsar timing array evidence for a stochastic gravitational wave (GW) background at nanohertz frequencies is generated by metastable cosmic strings, we analyze the potential of present and future GW observatories for probing the change of particle degrees of freedom caused, e.g., by a supersymmetric (SUSY) extension of the Standard Model (SM). We find that signs of the characteristic doubling of degrees of freedom predicted by SUSY could be detected at Einstein Telescope and Cosmic Explorer even if the masses of the SUSY partner particles are as high as about 10^4 TeV, far above the reach of any currently envisioned particle collider. We also discuss the detection prospects for the case that some entropy production, e.g. from a late decaying modulus field inducing a temporary matter domination phase in the evolution of the universe, somewhat dilutes the GW spectrum, delaying discovery of the stochastic GW background at LIGO-Virgo-KAGRA. In our analysis we focus on SUSY, but any theory beyond the SM predicting a significant increase of particle degrees of freedom could be probed this way.

Speaker: Kevin Hinze

Coffee

Defects2

13 Defects 2

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Speaker: Christoph Ringeval (CP3, Louvain)

14 Defects 3

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Speaker: Lara Sousa (Porto U)

Lunch

Discussion

Coffee

Wednesday 25 June

Phase Transitions 1

15 PT1

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Speaker: Oliver Gould (University of Nottingham)

16 PT2

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Speaker: Yann Gouttenoire (MITP)

Coffee

Phase Transitions 2

17 PT2

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Speaker: Alexander Jenkins (Cambridge U)

18 Supercooled Dark Scalar Phase Transitions explanation of NANOGrav data

The evidence of a Stochastic Gravitational Wave Background (SGWB) in the nHz frequency range is posed to open a new window on the Universe. A preferred explanation relies on a supercooled first order phase transition at the 100 MeV–GeV scale. We address address its feasibility going from the particle physics model to the production of the gravitational waves. We take a minimal approach for the dark sector model introducing the fewest ingredient required, namely a new U(1) gauge group and a dark scalar that dynamically breaks the symmetry. Supercooling poses challenges in the analysis that put under question the feasibility of this explanation: we address them, going beyond previous studies by carefully considering the effects of a vacuum domination phase and explicitly tracking the phase transition from its onset to its completion. We find that the proposed model can successfully give origin to the observed PTA SGWB signal. The strong supercooling imposes a correlation between the new gauge coupling and the scalar quartic one, leading to a significant hierarchy between the (heavier) gauge boson and the dark scalar. ultimately, information on phase transitions from SGWB observations could provide a direct probe of the microphysics of the Early Universe and be used to guide future searches of dark sectors in laboratories.

Speaker: Jaime Hoefken Zink (Narodowe Centrum Badań Jądrowych (NCBJ))

Data 1

19 Data 1

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Speaker: Mauro Pieroni (Instituto de Estructura de la Materia (IEM - CSIC))

Lunch

Thursday 26 June

Phase Transitions 3

20 PT3

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Speaker: Alberto Roper Pol (U Geneva)

21 Bubble wall velocity from hydrodynamics

Terminal velocity reached by bubble walls in cosmological first-order phase transitions is an important parameter determining both primordial gravitational wave spectrum and the production of baryon asymmetry in models of electroweak baryogenesis. In this talk I discuss the recent results for local thermal equilibrium approximation for which, using hydrodynamic simulations, we have confirmed that pure hydrodynamic backreaction can lead to steady-state expansion. However, this is not the generic outcome. Instead, it is much more common to observe runaways, as the early-stage dynamics right after the nucleation allow the bubble walls to achieve supersonic velocities before the heated fluid shell in front of the bubble is formed. In order to capture this effect, we generalized the analytical methods beyond the local thermal equilibrium and find a qualitative way to predict whether the runaway is physical, which has a crucial impact on cosmological observables.

Speaker: Mateusz Zych (University of Warsaw)

22 Paths Through the Dark: Comparing Approaches to Cosmological FOPTs

We explore the dynamics of cosmological phase transitions in a dark sector model featuring a dark photon associated with a U(1)D gauge symmetry. Our analysis focuses on two complementary approaches to construct the effective potential: an  scale invariant model with the Coleman-Weinberg potential, both using the high-temperature approximation and a full numerical evaluation of the thermal integrals, and a dimensionally reduced 3D effective theory built with DRalgo, at both leading and next-to-leading order.

We investigate how these methods impact the characterization of the phase transition, particularly in the supercooled regime.

Our results show the importance of method choice when predicting observable signatures, and establish a benchmark for future studies of first-order phase transitions in models with weakly coupled dark sectors.

Speaker: Cristina Puchades Ibáñez (JGU Mainz)

Coffee

Phase Transitions 4

23 PT4

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Speaker: Simone Blasi (DESY)

24 Bubble wall dynamics from non equilibrium quantum field theory

Gravitational waves (GWs) produced during a first-order phase transition are a favourite candidate as a probe of beyond the standard model physics at future space-based experiments. The spectrum of produced GWs depends strongly on the terminal velocity at which bubbles of the broken phase expand in the plasma, and in particular on whether this velocity can become ultrarelativistic.
In this talk, I show how the language of non-equilibrium quantum field theory, together with the 2PI effective action, is the natural framework to study the dynamics of the bubble wall. I derive the full equation of motion for the bubble wall and the two-point functions, and show that it can be brought into a convenient form by expanding in gradients. This yields an equation that generalises the one usually employed.
In the ultrarelativistic regime, I compute the pressure induced by pair production of heavy particles, mixing with a heavy species and transition radiation of gauge bosons, thus providing a first-principle derivation of the known sources of friction.
This talk is based on 2504.13725

Speaker: Matthias Carosi (Technical University of Munich)

25 Finite-temperature (supercooled) bubble-nucleation with shifting scale hierarchies

Focusing on supercooled phase transitions in models with classical scale symmetry, we investigate the limitations of derivative expansions in constructing a thermal EFT description for bubble nucleation. We demonstrate that derivative expansions for gauge field fluctuations diverge after the two leading orders because the gauge field mass varies strongly between the high- and low-temperature phases. By computing such contributions directly using the fluctuation determinant, we show that these effects can be captured while at the same time accounting for large explicit logarithms at two loops utilising the exact renormalisation group structure of the EFT. We show how this construction can be used to improve nucleation rate calculations, providing a more robust framework for describing gravitational waves from supercooled phase transitions in models like the SU(2)cSM.

Speaker: Maciej Kierkla (University of Warsaw)

Lunch

Phase Transitions 5

26 PT5

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Speaker: David Mateos (ICREA and U Barcelona)

27 Black holes and gravitational waves from phase transitions in realistic models

Among all the possible candidates for Dark Matter, one appealing example is a population of Primordial Black Holes, which could have been borne by various processes in the early stages of the Universe. In this talk, I will investigate the formation of such objects as the result of the collapse of energy density fluctuations originating from supercooled first-order phase transitions. I will stress the importance of including the second-order corrections in the expansion of the bubble nucleation rate and show its implications for the production of Primordial Black Holes as well as the emission of Gravitational Wave signals. Finally, I will illustrate the application of this formalism to a realistic particle model, showing that in certain regions of parameter space both abundant production of Primordial Black Holes and emission of strong Gravitational Wave signals is realized.

Speaker: Piotr Toczek (University of Warsaw)

28 Primordial Gravitational Waves from Phase Transitions during Reheating

We study primordial gravitational waves (GWs) generated from first-order phase transitions (PTs) during cosmic reheating. Using a minimal particle physics model, with a general parametrization of the inflation energy density and the evolution of the Standard Model temperature, we explore the conditions under which PTs occur and determine the corresponding PT parameters. We find that, in certain cosmological scenarios, PTs can be delayed and prolonged compared to the standard post-inflationary evolution. Finally, incorporating these PT parameters, we compute the resulting GW spectrum while accounting for the uncertainties related to cosmic reheating.

Speaker: Amitayus Banik (Chungbuk National University)

Coffee

Discussion

Friday 27 June

Astrophysics 1

29 Astro 1

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Speaker: Marek Szczepanczyk (University of Warsaw)

30 Microlensing effect in the long-duration gravitational wave signals originating from Galactic sources

Detection of quasi-monochromatic, long-duration (continuous) gravitational wave radiation emitted by, e.g., asymmetric rotating neutron stars, planetary or asteroid mass - primordial BH (PBH) binaries during their in-spiral phase in our Galaxy requires a long observation time to distinguish it from the detector’s noise. If this signal is additionally microlensed by a lensing object located in the Galaxy, its magnitude would be temporarily magnified, which may lead to its discovery and allow probing of the physical nature of the lensing object and the source. We study the observational feasibility of Galactic microlensing of continuous gravitational wave signals in the point mass lens approximation by discussing the parameter space of the problem as well as by applying a gravitational wave detection method, the Time Domain F-statistic search, to ground-based detectors in the simulated data.

Speaker: Sudhagar Suyamprakasam (Nicolaus Copernicus Astronomical Center)

31 Using graviational waves to look for dark matter (in)directly

Gravitational wave detectors offer promising probes to search for dark matter, both directly and indirectly. If dark matter originates from a background of ultralight gauge bosons, these bosons could exert forces on the test masses of gravitational wave detectors, inducing displacements with characteristic frequencies determined by the boson mass. On the indirect side, the Fermi satellite has observed an excess of GeV gamma rays from the Galactic Center. The origin of this excess remains under debate, with leading hypotheses including dark matter annihilation or emission from a population of millisecond pulsars. The continuous gravitational wave searches conducted by the LVK collaboration have strong potential to help resolve this long-standing question.

Speaker: Yue Zhao (University of Utah)

Coffee

Astrophysics 2

32 Astro 2

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Speaker: Achamveedu Gopakumar (TIFR Mumbai)

33 Gravitational waves from supermassive black holes at pulsar timing arrays

Inspiralling SMBH binaries constitute a natural explanation of the gravitational wave (GW) background discovered in pulsar timing array (PTA) data. In this talk, I will present a fast semianalytical computation of the expected GW background from SMBH binaries and discuss the SMBH fit to the PTA data that shows evidence of environmental effects or binary eccentricities. I will identify signatures that can be used to distinguish between these effects and to confirm whether the signal comes from SMBH binaries, and show how the PTA GW observations can be linked to the JWST observations of dual AGNs and little red dots. Finally, I will discuss potential ways to test dark matter properties through SMBH observations.

Speaker: Ville Vaskonen (University of Padova and KBFI)

34 Pulsar Timing Array Evidence for Self–Interacting Dark Matter in Super-Massive Black-Hole Mergers

Pulsar timing arrays (PTAs) have now revealed a nano-hertz stochastic gravitational-wave background whose amplitude and spectral shape are consistent with a cosmic population of merging super-massive black-hole (SMBH) binaries. Explaining how such binaries bridge the ``final parsec’’ separation before gravitational radiation dominates remains a key challenge. Following the mechanism proposed by Alonso-Álvarez et al. (2024), we investigate whether dynamical friction from a dense spike of self-interacting dark matter (SIDM) surrounding each SMBH can simultaneously solve the final-parsec problem and imprint the mild low-frequency turnover hinted at in current PTA data.

We perform the first full-likelihood Bayesian analysis of this scenario using the NANOGrav 15-year data set. Employing a custom-corrected version of the holodeck pipeline, we sample
the joint posterior of SMBH-population, host-galaxy and SIDM parameters with an MCMC and marginalise over astrophysical uncertainties. The velocity-weighted cross section per unit mass is
constrained to

$$ \bigl\langle \sigma v / m \bigr\rangle \;=\; 10^{\,2.9 \pm 0.5}\, \mathrm{cm^{2}}\!/\!(\mathrm{g\,km\,s^{-1}}), $$ This range is fully compatible with independent inferences from dwarf-galaxy cores and galaxy-cluster offsets, favouring a Yukawa-like velocity dependence mediated by an $O(10\text{–}100)$ MeV dark photon. In our model, the SIDM spike supplies enough dynamical friction to merge typical $10^{8\text{–}9}\,M_\odot$ binaries within a Hubble time, whereas collisionless cold-DM spikes are disrupted too early.

Speaker: Chris Gordon (University of Canterbury (NZ))

Lunch

Discussion

Coffee