# Gravitational Wave Probes of Fundamental Physics

Europe/Amsterdam
Riet (Volkshotel, Amsterdam, The Netherlands)

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
Description

The Gravitational Wave Probes of Fundamental Physics (GW4FP) workshop will take place from 11-13 November at the Volkshotel in Amsterdam, The Netherlands.

As the first workshop by the European Consortium for Astroparticle Theory (EuCAPT), GW4FP aims to bring together the Gravitational Wave and Fundamental Physics communities to discuss topics in Dark Matter, exotic objects, tests of GR and early Universe physics, as well as tests of Standard Model physics in unexplored regimes.

The workshop will involve invited plenary talks covering these broad areas as well as shorter submitted talks for researchers to present their work. Since a key goal of the workshop is to foster collaboration, there will also be discussions, lightning talks and a "four corners" discussion session.

Slides: Slides from the conference are being collected and archived on Zenodo (https://zenodo.org/communities/gw4fp-2019/).

Confirmed speakers: Tessa Baker, Andrea Bertoldi, Vitor Cardoso, Siyuan Chen, Djuna Croon, Thomas Edwards, Stephen Feeney, Alexander Jenkins, Peter Johansson, Bradley Kavanagh, Eugene Lim, Masha Okounkova, Paolo Pani, Pedro Schwaller, Ulrich Sperhake, Yevgeny Stadnik, Bert Vercnocke, Marta Volonteri, Anna Watts, Helvi Witek

Organizing committee: Daniel Baumann, Gianfranco Bertone, Vitor Cardoso, Sarah Caudill, Adam Coogan, Thomas Edwards, Tanja Hinderer, Bradley Kavanagh, Philipp Moesta, Suvodip Mukherjee, Samaya Nissanke, Christoph Weniger

Scientific Advisory Board (EuCAPT Steering Committee): Gianfranco Bertone, Philippe Brax, Vitor Cardoso, Gian Giudice, David Langlois, David Marsh, Silvia Pascoli, Antonio Riotto, Subir Sarkar, Andrew Taylor, Piero Ullio, Licia Verde

Participants
• A. Kalaja
• Aleksandr Chatrchyan
• Alexander Jenkins
• Alice Garoffolo
• Amelia Drew
• Andi Hektor
• Andrea Bertoldi
• Andreas Guerra
• Andrew Tolley
• Anish Ghoshal
• Anna Puecher
• Anna Watts
• Anwesh Majumder
• Archisman Ghosh
• Banafshe Shiralilou
• Benjamin Stefanek
• Bert Vercnocke
• Christoffel Doorman
• Christoph Weniger
• Dan Stinebring
• Daniel Baumann
• Deyan Mihaylov
• Djuna Croon
• Eduard Masso Soler
• Eugene Lim
• Florian Kühnel
• Gastón Creci Keinbaum
• Geert Raaijmakers
• Giacomo Bruno
• Gianfranco Bertone
• Giovanni Tambalo
• Helvi Witek
• Joachim Kopp
• John Stout
• Jonathon Baird
• Jose Agustin Lozano Torres
• Kamile Lukosiute
• Kenny Chun Yu Ng
• Khun Sang Phukon
• Kristjan Kannike
• Kwinten Fransen
• Laura Sagunski
• Leïla Haegel
• Lorenzo Küchler
• Lukas Mittnacht
• M.K. Haris
• Maarten van de Meent
• Marco Chianese
• Maria Ramos
• Marieke Postma
• Marta Volonteri
• Masha Okounkova
• Massimo Mannarelli
• Matteo Biagetti
• Moritz Breitbach
• Neal Dalal
• Omar Contigiani
• Otto Hannuksela
• Paolo Pani
• Pedro Klaus Schwaller
• Peter Johansson
• Peter Tsun Ho Pang
• Qin Tan
• Rafael Porto
• Ranjan Laha
• Rob Tielemans
• Ryan Magee
• Samaya Nissanke
• Sarah Caudill
• Serguei Ossokine
• Shin'ichiro Ando
• Siyuan Chen
• Stephen Feeney
• Subodh Patil
• Sumedha Biswas
• Suvodip Mukherjee
• Sébastien Clesse
• Tanja Hinderer
• Tessa Baker
• Thomas Edwards
• Tim Dietrich
• Ulrich Sperhake
• Vitor Cardoso
• Wolfram Ratzinger
• Zeyd Sam
• Zheng-Quan Cui
Contact
• Monday, 11 November
• 12:30 13:00
Check-in 30m Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
• 13:00 13:10
Welcome 10m Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
Speaker: Gianfranco Bertone
• 13:10 14:10
Overview talks Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
• 13:10
Dense matter equation of state constraints from NICER 30m

NICER, the Neutron Star Interior Composition Explorer, is an X-ray telescope that was installed on the International Space Station in 2017. Its mission is to study the nature of the densest matter in the Universe, in the cores of neutron stars. NICER does this by exploiting the effects of General and Special Relativity on radiation emitted by hotspots at the magnetic polar caps of X-ray pulsars. I will explain some of the challenges we have encountered along the way and present preliminary results from the mission. NICER is also paving the way for the Next Generation of larger area X-ray telescopes to be launched in the mid in the next decade, which will enable even bigger strides in our understanding of dense matter.

Speaker: Anna Watts
• 13:40
Gravitational-wave echoes 30m

Gravitational wave (GW) astronomy allows us for unprecedented tests of the nature of dark compact objects and to probe into outstanding foundational issues, such as the fate of spacetime singularities and the loss of unitarity in Hawking evaporation. In this context, I will discuss a striking signature of new physics at the horizon scale: GW “echoes” in the postmerger ringdown phase of a binary coalescence. The ringdown waveform of exotic ultracompact objects is initially identical to that of a black hole, and putative corrections at the horizon scale appear only at later times as a modulated and distorted train of echoes of the modes of vibration associated with the photon sphere. These corrections display a universal logarithmic dependence on the location of the surface in the black-hole limit, allowing to probe even Planckian corrections. I will discuss challenges in modelling this signal and the ability of present and future GW detectors to measure this effect.

Speaker: Paolo Pani
• 14:10 14:25
Short talks Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
• 14:10
Gravitational Collider Physics 15m

Gravitational wave astronomy will play a transformative role in astrophysics; can it do the same for particle physics? An ultralight bosonic field will extract mass and angular momentum from a rapidly spinning black hole, forming a gravitationally bound condensate reminiscent of the hydrogen atom. This "gravitational atom" will have nontrivial dynamics if it is part of a binary inspiral. I will argue that these dynamics can be described as a series of "scattering" events, quantified by an S-matrix, during which there can be large corrections to the inspiral trajectory and thus the resulting gravitational wave signal. These corrections can then be used to infer the mass and spin of the boson, turning binary inspirals into ultralight particle detectors.

Speaker: John Stout (University of Amsterdam)
• 14:25 15:25
Discussion Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
• 14:25
Discussion session: particle physics around black holes 1h
• 15:25 15:55
Coffee break 30m Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
• 15:55 16:40
Short talks Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
• 15:55
Fundamental and Gravitational Wave Science with Pulsar Timing 15m

Since the discovery of pulsar 50 years ago, they have proven to be very useful astronomical objects. Spinning neutron stars with a very stable rotation period as low as a few milliseconds, emitting radio pulses, similar to a lighthouse. These periodic radio signals can be detected and timed on Earth with an accurate timing model describing the entire process from emission at the pulsar through its travel through the interstellar medium and the solar system to Earth. A very large number of different effects can be measured. I will give a brief overview on the science of Pulsar Timing with a focus on fundamental and gravitational wave science.

With the emission process we can study the properties of the neutron star itself. The timing model is very sensitive to the masses if the pulsar is in a binary with another object. This allows us to put very tight constraints on the mass of the neutron star itself and by extension put some meaningfull limits to the equation of state of neutron stars. Some binary system can be very extreme, like a double pulsar system, and thus are a very great system to test General Relativity and alternative theories.

The path that the photons travel between pulsar and Earth can be up to about a thousand of lightyears, this is similar to having a galactic scale detector for gravitational waves (GW) and other interstellar medium effects. The most likely source of GW are supermassive black hole binaries, which create both a stochastic background as well potential single sources affecting all pulsars in a characteristic fashion. Other interesting sources could be GWs from burst of memory events, cosmic string loops and primordial black hole binaries. To disentangle noise from one pulsar, we need to look at many pulsars and look for common processes. This is what Pulsar Timing Arrays do to look for Gws.

Speaker: Siyuan Chen (NRT Nancay, CNRS Orleans, France)
• 16:10
Dark Energy Instabilities induced by Gravitational Waves 15m

In this talk I will discuss the decay of gravitational waves (GW) into dark energy fluctuations π in the context of the EFT of Dark Energy.
In such theories, the time-dependence of the Dark Energy (DE) field spontaneously breaks Lorentz invariance. Therefore as for light in a material, GW travelling in the cosmic medium are affected by dispersion phenomena and can decay into DE fluctuations. For cubic Horndeski and beyond Horndeski theories, the gravitational wave acts as a classical background for π and thus modifies its dynamics. In particular, for a sufficiently large amplitude of the wave, the kinetic term of π becomes pathological, featuring gradient and ghost instabilities. For smaller gravitational wave amplitude, π fluctuations are described by a Mathieu equation and feature instability bands that grow exponentially. The gravitational wave signal is affected by the π back-reaction and this provides very stringent bounds on cubic and quartic GLPV theories.

Speaker: Giovanni Tambalo (SISSA)
• 16:25
Constraining the speed of gravity using astrometric measurements 15m

Stochastic gravitational wave backgrounds induce correlated patterns in the redshift and astrometric shifts of objects on the sky. The astrometric equivalent of the Hellings-Downs curve depends on the polarization content as the group velocity of the GWs making up the stochastic background. I will explain how these results relate to the measurements taken by the Gaia mission and how they can be leveraged to produce new constraints on the mass of the graviton and the speed of gravity from an ultra-low frequency point of view. I will also draw parallels between these new results and the existing PTA literature.

Speaker: Deyan Mihaylov (University of Cambridge)
• 16:40 17:40
Four corners discussion Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
• 19:00 21:00
Welcome reception 2h The Eye Film Museum

#### The Eye Film Museum

Drinks and snacks in a futuristic-looking building on the river in the center of Amsterdam

• Tuesday, 12 November
• 09:00 10:00
Overview talks Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
• 09:00
Cosmological Tests of Gravity with Gravitational Waves 30m

The first direct detections of gravitational waves have had a strong impact on attempts to extend GR in the cosmological regime. In particular, GW170817 effectively ruled out some significant chunks of the modified gravity model space. I’ll summarise what the theoretical options for extensions of GR are, and what we’ve learnt about them from gravitational waves so far. I’ll also talk briefly about what LISA data can contribute to the picture.

Speaker: Tessa Baker (Queen Mary University of London)
• 09:30
New prospects in numerical relativity 30m

Both observations and deeply theoretical considerations indicate that general relativity, our elegant standard model of gravity, requires modifications at high curvatures scales. Candidate theories of quantum gravity, in their low-energy limit, typically predict couplings to additional fields or extensions that involve higher curvature terms.

At the same time, the breakthrough discovery of gravitational waves has opened a new channel to probe gravity in its most extreme, nonlinear regime. Modelling the expected gravitational radiation in these extensions of GR enables us to search for -- or place novel observational bounds on -- deviations from our standard model. In this talk I will give an overview of the recent progress on simulating binary collisions in these situations.

Speaker: helvi witek (KCL)
• 10:00 11:00
Discussion Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
• 10:00
Discussion session: numerics 1h
Speakers: Ms Amelia Drew (Department of Applied Mathematics and Theoretical Physics, University of Cambridge), Masha Okounkova (Flatiron Institute), Serguei Ossokine, Tim Dietrich (Nikhef), helvi witek (KCL)
• 11:00 11:30
Coffee break 30m Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
• 11:30 12:15
Short talks Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
• 11:30
Binary black holes beyond general relativity 15m

At some length scale, Einstein's theory of general relativity (GR) must break down and be reconciled with quantum mechanics in a quantum theory of gravity. Binary black hole mergers probe the strong field, non-linear, highly dynamical regime of gravity, and thus gravitational waves from these systems could contain beyond-GR signatures. While LIGO presently performs model-independent and parametrized tests of GR, in order to perform model-dependent tests, we must have access to numerical relativity binary black hole waveform predictions in beyond-GR theories through full inspiral, merger, and ringdown. In this talk, I will discuss our results in producing full numerical relativity waveforms in beyond-GR theories.

Speaker: Masha Okounkova (Flatiron Institute)
• 11:45

The groundbreaking detection of gravitational waves by LIGO has opened up a brand new window into observational cosmology, catalysing research into gravitational wave signatures from a wide range of astrophysical and cosmological sources. In this work, we calculate accurate radiative signatures from topological cosmic’ strings, implementing adaptive mesh refinement (AMR) simulations of global strings using the numerical relativity code, GRChombo. We investigate the resulting massless (Goldstone boson or axion) radiation and massive (Higgs) radiation signals, using quantitative diagnostic tools and geometries to determine the eigenmode decomposition of these radiation components. Given analytic radiation predictions, we compare the oscillating string trajectory with a backreaction model accounting for radiation energy losses, finding excellent agreement: we establish that backreaction decay is accurately characterised by the inverse square of the amplitude being proportional to the inverse tension μ for 3≲λ≲100. We conclude that analytic radiation modelling in the thin-string (Nambu-Goto) limit provides the appropriate cosmological limit for global strings. We also make a preliminary study of massive radiation modes, including the large λ regime in which they become strongly suppressed relative to the preferred massless channel. We comment on the implications of this study for predictions of axions and gravitational waves produced by cosmic string networks.

Speaker: Ms Amelia Drew (Department of Applied Mathematics and Theoretical Physics, University of Cambridge)
• 12:00
Cross-correlation of the astrophysical gravitational-wave background with galaxy clustering 15m

We investigate the correlation between the distribution of galaxies and the predicted gravitational-wave background of astrophysical origin. We show that the large angular scale anisotropies of this background are dominated by nearby non-linear structure, which depends on the notoriously hard to model galaxy power spectrum at small scales. In contrast, we report that the cross-correlation of this signal with galaxy catalogues depends only on linear scales and can be used to constrain the average contribution to the gravitational-wave background as a function of time. Using mock data based on a simplified model, we explore the effects of galaxy bias and the matter abundance on these constraints. Our results suggest that the gravitational-wave background when combined with near-future galaxy surveys, is a powerful probe for both gravitational-wave merger physics and cosmology.

Reference: https://arxiv.org/abs/1910.08353

Speaker: Omar Contigiani (Leiden University)
• 12:15 12:30
Lightning talks Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam

Very short talks before lunch intended to spur discussion.

• 12:15
Extreme Dark Matter Tests with Extreme Mass Ratio Inspirals 5m

Future space-based laser interferometry experiments such as LISA are expected to detect $\mathcal{O}(100-1000)$ stellar-mass compact objects (e.g., black holes, neutron stars) falling into massive black holes in the centers of galaxies, the so-called extreme-mass-ratio inspirals (EMRIs). If dark matter forms a "spike" due to the growth of the massive black hole, it will induce a gravitational drag on the inspiraling object, changing its orbit and gravitational-wave signal. We show that detection of even a single dark matter spike from the EMRIs will severely constrain several popular dark matter candidates, such as ultralight bosons, keV fermions, MeV--TeV self-annihilating dark matter, and sub-solar mass primordial black holes, as these candidates would flatten the spikes through various mechanisms. Future space gravitational wave experiments could thus have a significant impact on the particle identification of dark matter.

Speaker: Dr Otto Hannuksela (Nikhef)
• 12:20
Gravity waves from ALP dark matter fragmentation 5m

Axion-like particles (ALP) are appealing candidates for dark matter if produced non-thermally via the vacuum misalignment mechanism. In certain cases, such as in the presence of a monodromy, the self-interactions of ALPs can be sufficiently strong and lead to the fragmentation of the homogeneous field soon after the onset of oscillations. We investigate numerically the dynamics of fragmentation, as well as of the subsequent turbulent regime, and calculate the stochastic gravitational wave (GW) background that is produced from this process. We find that a particularly strong background can be produced when ALPs exhibit an extended intermediate phase of ultra-relativistic dynamics, which can be induced by a small mass at the bottom of the potential. Such background can partially be explored with future GW detectors, offering an important probe of the properties of dark matter.

Speaker: Aleksandr Chatrchyan (Institute for Theoretical Physics, Heidelberg University)
• 12:25
Detecting Gauged $L_\mu-L_\tau$ using Neutron Star Binaries 5m

We show that gravitational wave emission from neutron star binaries can be used to discover ultra-light U(1)$_{L_\mu-L_\tau}$ vectors by making use of the large inevitable abundance of muons inside neutron stars. In pulsar binaries the U(1)$_{L_\mu-L_\tau}$ vectors induce an anomalously fast decay of the orbital period through the emission of dipole radiation. We study a range of different pulsar binaries, finding the most powerful constraints for vector masses below ${\mathcal O}(10^{-18} \, {\rm eV})$. For merging binaries the presence of muons in neutron stars can result in dipole radiation as well as a modification of the chirp mass during the inspiral phase. We make projections for a prospective search using the GW170817 event and find that current data can discover light vectors with masses below ${\mathcal O}(10^{-18} \,{\rm eV})$. In both cases, the limits attainable with neutron stars reach gauge coupling $g' < \sim 10^{-20}$, which are many orders of magnitude stronger than previous constraints. We also show projections for next generation experiments, such as Einstein Telescope.

Speaker: Dr Ranjan Laha (CERN)
• 12:30 14:00
Walking lunch 1h 30m Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
• 14:00 15:00
Overview talks Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
• 14:00
Hearing the sirens of the early Universe: Primordial Black Holes and Gravitational Waves 30m

Since the first detections of gravitational waves (GW) from merging binary black holes (BH), there has been a renewed interest in the possibility that at least some of these BHs could be primordial in origin. I will briefly discuss motivations for such primordial black holes (PBHs), formed from the collapse of large over-densities in the early Universe. I will then examine the on-going debate over whether LIGO and Virgo have indeed detected merging PBHs, and what we might learn if they did. Of course, the power of GWs in probing PBHs extends far beyond these 'direct' detections. I will also discuss more indirect probes, such as the stochastic GW background which may be produced along with PBH formation. The observation of GWs has thus opened up a new way to detect and study PBHs and to learn about the physics of the early Universe from which they formed.

Speaker: Bradley Kavanagh (GRAPPA, University of Amsterdam)
• 14:30
Simulating black hole dynamics and gravitational wave emission in galactic-scale simulations 30m

We will briefly review how supermassive black holes (SMBH) are modelled in galactic-scale simulations. Recently, large-scale cosmological simulations have been used to predict the gravitational wave background. These simulations typically rely on semi-analytic models to describe the small-scale black hole binary dynamics and gravitational wave emission, as these processes cannot be directly resolved in simulations employing gravitational softening. An alternative is to use a hybrid approach, such as the KETJU code, recently developed in our group. The KETJU code includes algorithmically regularized regions around every SMBH. This allows for simultaneously following global galactic-scale dynamical and astrophysical processes, while solving accurately the dynamics of SMBHs at sub-parsec scales. The KETJU code includes also post-Newtonian terms in the equations of motions of the SMBHs, which allows us to directly calculate the expected gravitational wave signal from the motion of the resolved SMBH binary in mergers of massive gas-poor galaxies.

Speaker: Peter Johansson (University of Helsinki)
• 15:00 16:00
Discussion Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
• 15:00
Discussion session: primordial black holes 1h
Speakers: A. Kalaja, Florian Kühnel (Stockholm University), Ryan Magee (Pennsylvania State University), Dr Subodh Patil (Niels Bohr Institute (DK)), Sébastien Clesse (Catholic University of Louvain)
• 16:00 16:30
Coffee break 30m Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
• 16:30 17:30
Short talks Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
• 16:30
A Unique Multi-Messenger Probe of QCD Axion Dark Matter 15m

We propose a multi-messenger probe of QCD axion dark matter (DM) based on observations of black hole-neutron star binary inspirals. It is suggested that a dense DM spike may grow around intermediate mass black holes. The presence of such a spike produces two unique effects: a distinct phase shift in the gravitational wave strain during the inspiral period and an enhancement of the radio emission from the resonant axion-photon conversion occurring in the neutron star magnetosphere. Remarkably, the observation of the gravitational wave signal can be used to infer the DM density and, consequently, to predict the radio emission. Given a sufficiently nearby detection with the LISA interferometer and next-generation radio telescope Square Kilometre Array, I will show that such observations can explore the QCD axion in the mass range 10−7 eV to 10−5 eV, potentially providing a striking multi-messenger signature of QCD axion DM.

Speaker: Thomas Edwards (University of Amsterdam)
• 16:45
Novel signatures of dark matter in laser-interferometric gravitational-wave detectors 15m

Dark matter may induce apparent temporal variations in the physical `constants'', including the electromagnetic fine-structure constant and fermion masses. In particular, a coherently oscillating classical dark-matter field may induce apparent oscillations of physical constants in time, while the passage of macroscopic dark-matter objects (such as topological defects) may induce apparent transient variations in the physical constants. We point out several new signatures of the aforementioned types of dark matter that can arise due to the geometric asymmetry created by the beam-splitter in a two-arm laser interferometer. These new signatures include dark-matter-induced time-varying size changes of a freely-suspended beam-splitter and associated time-varying shifts of the main reflecting surface of the beam-splitter that splits and recombines the laser beam, as well as time-varying refractive-index changes in the freely-suspended beam-splitter and time-varying size changes of freely-suspended arm mirrors. We demonstrate that existing ground-based experiments already have sufficient sensitivity to probe extensive regions of unconstrained parameter space in models involving oscillating scalar dark-matter fields and domain walls composed of scalar fields. In the case of oscillating dark-matter fields, Michelson interferometers --- in particular, the GEO\,600 detector --- are especially sensitive. The sensitivity of Fabry-Perot-Michelson interferometers, including LIGO, VIRGO and KAGRA, to oscillating dark-matter fields can be significantly increased by making the thicknesses of the freely-suspended Fabry-Perot arm mirrors different in the two arms. Not-too-distantly-separated laser interferometers can benefit from cross-correlation measurements in searches for effects of spatially coherent dark-matter fields. In addition to broadband searches for oscillating dark-matter fields, we also discuss how small-scale Michelson interferometers, such as the Fermilab holometer, could be used to perform resonant narrowband searches for oscillating dark-matter fields with enhanced sensitivity to dark matter. Finally, we discuss the possibility of using future space-based detectors, such as LISA, to search for dark matter via time-varying size changes of and time-varying forces exerted on freely-floating test masses.

Reference: H. Grote and Y. V. Stadnik, arXiv:1906.06193

Speaker: Dr Yevgeny Stadnik (Kavli IPMU, University of Tokyo)
• 17:00
Gravitational Wave Detection at low frequency with Atom Interferometry 15m

Atom interferometry promises to extend the detection bandwidth of GW detectors in the mid-frequency band (10 mHz - 10 Hz), where Earth based optical detectors are limited by low frequency gravity noise. Adopting as probes arrays of atomic ensembles in free fall, and tracking their motion on geodesics with atom interferometry allows the suppression of Newtonian Noise, enables low frequency sensitivity, and opens the way toward the realization of low frequency GW detectors on Earth. I will report on the MIGA project, an atom interferometry based demonstrator for GW detection being developed in the underground environment of LSBB (Rustrel, France), and on the potential role of atom interferometry in GW astronomy.

Speaker: Andrea Bertoldi (Institut d'Optique)
• 17:15
Black holes in string theory and observations 15m

In this talk I will address a theoretical underpinning of potential quantum modifications to classical GR black holes from the perspective of string theory. I will discuss theoretical motivations, new insights over the last two decades, and give a view on observational consequences.

Speaker: Bert Vercnocke (KU Leuven)
• 18:30 20:30
Workshop dinner (Restaurant De Tropen) 2h Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
• Wednesday, 13 November
• 09:00 10:00
Overview talks Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
• 09:00
Clarifying the Hubble Constant Tension 30m

Our best estimate of the Universe's current expansion rate (the Hubble constant) from the local Universe (via the Cepheid distance ladder) is in four-sigma tension with the value extrapolated from cosmic microwave background data assuming the standard cosmology. Whether this discrepancy represents physics beyond the Standard Model or deficiencies in our understanding of the data is the subject of intense debate. In this talk, I will review the community's attempts to explain and interpret the Hubble constant tension, clarifying the current picture using Bayesian probability theory, and consider the potential for independent gravitational wave observations to arbitrate the dispute.

Speaker: Stephen Feeney (University College London)
• 09:30
Phase Transitions, Dark Sectors and Gravitational Waves 30m

I will give an overview on gravitational waves from phase transitions, and then focus on specific hidden sector scenarios such as dark photons or axions, and discuss how they could be probed by future gravitational wave observations in pulsar timing arrays, space and ground based detectors.

Speaker: Pedro Klaus Schwaller (Mainz University)
• 10:00 11:00
Discussion Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
• 10:00
Discussion session: cosmology 1h
Speakers: Andrew Tolley (Imperial College London), Archisman Ghosh (Nikhef), Maria Sakellariadou (University of London (GB)), Neal Dalal, Suvodip Mukherjee
• 11:00 11:30
Coffee Break 30m Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
• 11:30 12:15
Short talks Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
• 11:30
Gravitational Waves signatures from collisions of exotic compact objects 15m

I will talk about GW signatures from the collisions of exotic compact objects (ECOs). I will argue that getting good high quality signatures require good control over the initial conditions which is currently one of the key challenges. I will also discuss whether ECOs can act as black hole mimics.

Speaker: Eugene Lim
• 11:45
Probing the Early and Late Universe with the Gravitational-Wave Background 15m

One of the most exciting targets for current and future gravitational-wave (GW) observatories is the stochastic GW background (SGWB)---a persistent all-sky signal, sourced by the incoherent emission of GWs from many independent sources throughout the history of the Universe. In particular, the SGWB is a sensitive probe of cosmic strings: line-like topological defects formed through spontaneous symmetry breaking at extreme energy scales in the early Universe. Searches for cosmic strings therefore allow us to probe particle physics at scales far beyond the reach of collider experiments. I will discuss the GW signals associated with cosmic strings, and their detection prospects with LIGO/Virgo and LISA.
Another important SGWB signal at much lower redshift is the astrophysical GW background (AGWB), generated by the superposition of many compact binary coalescences. These act as tracers of the galaxy distribution, and therefore offer a novel probe of large-scale structure. I will describe the calculation of the AGWB angular power spectrum using large N-body simulations, and discuss the implications of these observables for late-Universe cosmology.

Speaker: Mr Alexander Jenkins (King's College London)
• 12:00
Massive black hole binaries in the cosmos 15m

I will outline our current understanding on how massive black holes, routinely found in galaxy centers, form in galaxies in the first billion years of the Universe and how they form binaries, eventually coalescing emitting gravitational waves in the frequency range observable by LISA and PTAs.

Speaker: Marta Volonteri
• 12:15 12:30
Lightning talks Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam

Very short talks before lunch intended to spur discussion.

• 12:15
Audible Axions 5m

Conventional approaches to probing axions and axion-like particles (ALPs) typically rely on a coupling to photons. However, if this coupling is extremely weak, ALPs become invisible and are effectively decoupled from the Standard Model. We show that such invisible axions, which are viable candidates for dark matter, can produce a stochastic gravitational wave background in the early universe. This signal is generated in models where the invisible axion couples to a dark gauge boson that experiences a tachyonic instability when the axion begins to oscillate. Incidentally,the same mechanism also widens the viable parameter space for axion dark matter. Quantum fluctuations amplified by the exponentially growing gauge boson modes source chiral gravitational waves. We discuss the parameter space where this signal can possibly be detected by pulsar timing arrays or space/ground-based gravitational wave detectors, taking into account obstructions to the tachyonic growth like kinetic mixing of the gauge boson resulting in a thermal mass.

Speaker: Wolfram Ratzinger (Johannes Gutenberg Universität Mainz)
• 12:20
Gravitational waves and collider probes of extended Higgs sectors 5m

Extended scalar sectors often emerge in models motivated by the electroweak hierarchy problem. In particular, the scalar triplet extension of the SM is interesting because the triplet decay is very constrained at the renormalizable level. Therefore, effective operators with a low cutoff make the triplet components decay promptly, leading to a drastically different collider phenomenology. In this talk, I will discuss the reach of ongoing searches at the LHC, as well as projected bounds for the HL-LHC. The non-minimal scalar sector also modifies the electroweak phase transition, which can be first-order and produce sizable gravitational waves. Therefore, I will also discuss the possibility of electroweak baryogenesis in this model and constraints from gravitational waves observatories.

Speaker: Maria Ramos (LIP)
• 12:25
Dark, Cold, and Noisy: Constraining Secluded Hidden Sectors with Gravitational Waves 5m

We explore gravitational wave signals arising from first-order phase transitions occurring in a secluded hidden sector, allowing for the possibility that the hidden sector may have a different temperature than the Standard Model sector. Secluded hidden sectors are of particular interest for dark matter models at the MeV scale or below, which falls into the sensitivity range of pulsar timing arrays. Cosmological constraints on light degrees of freedom restrict the number of sub-MeV particles in a hidden sector, as well as the hidden sector temperature. Nevertheless, we find that observable first-order phase transitions can occur in two minimal benchmark models.

Speaker: Moritz Breitbach (University Mainz)
• 12:30 14:00
Walking Lunch 1h 30m Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
• 14:00 14:10
Lightning talks Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam

Very short talks before lunch intended to spur discussion.

• 14:00
NANOGrav: progress toward detecting a SMBHB stochastic background 5m

The North American Nanohertz Observatory for
Gravitational Waves (NANOGrav) has recently released its
12.5 year data set. I will summarize the GW results and their
implications for limits on Supermassive Black Hole Binaries
through hierarchical galaxy merging. These limits are starting
to substantially constrain models for SMBHB. We anticipate
detection of the stochastic background of GWs from these
sources within the next 3 - 7 years.

Speaker: Dan Stinebring (Oberlin College)
• 14:05
Primordial Gravitational Waves from Modified Gravity and Cosmology 5m

We will review gravitational wave propagation in standard and non-standard cosmological history. Particularly, we will discuss the spectrum of primordial gravitational (PWG) waves spectrum induced due to inflation in such scenarios. Then we will show the predictions in scenarios as predicted by various modified gravity theories, motivated by beyond ΛCDM model of cosmology and dark energy scenarios. As an example, we will discuss scalar-tensor theory as modified cosmology candidate. Next we will comment upon the sensitivity reaches of such predictions within the future and current GW detectors.

Speaker: Anish Ghoshal (L)
• 14:10 14:40
Overview talks Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
• 14:10
Gravitational probes of exotic compact objects 30m

In this talk I will discuss experimental probes of dark compact objects in the new era of gravitational wave astrophysics. Such proposed objects include scalar (boson) stars, Q-balls, and dark matter clumps inside neutron stars. I will review the properties that will help us distinguish them from astrophysical objects, and the resulting gravitational wave phenomenology. I will also discuss connections with other astrophysical probes, such as gravitational (micro)lensing.

Speaker: Dr Djuna Croon (TRIUMF)
• 14:40 15:00
Four corners discussion: Recap Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
• 15:00 15:25
Wrap-up talk 25m Riet

### Riet

#### Volkshotel, Amsterdam, The Netherlands

Wibautstraat 150 1091 GR Amsterdam
Speaker: Vítor Cardoso (IST)