10th LISA Cosmology Working Group Workshop

5 Jun 2023, 08:00 → 9 Jun 2023, 19:00 Europe/Oslo

University of Stavanger

The 10th LISA Cosmology Working Group Workshop will take place at the University of Stavanger on June 5th-9th, 2023.

The workshop aims at gathering the LISA CosWG community after the break due to the Covid pandemic. The goal is to review the recent progress in cosmology relevant to LISA and initiate collaborative projects to investigate new directions and tackle open problems in LISA cosmology.

The discussed topics include:

• Predictions for SGWBs from the early universe
• Detection of stochastic backgrounds and foregrounds
• Characterization of isotropic and anisotropic SGWB components
• Standard sirens and cosmological tests of the late universe
• Cosmological probes of general relativity and non-standard paradigms
• Primordial black holes and dark matter
• Gravitational-wave lensing
• Structure formation

Around half of the time will be dedicated to the CosWG collaborative projects.

The registration remains open until May 3rd. A 600 NOK (~60 euros) contribution for lunches and coffee breaks is requested for in-person participants.

The workshop is restricted to LISA CosWG members. Non-LISA members interested in the workshop should apply for membership, but due to the time delays in the application process, they should apply as soon as possible and definitely some months before the registration deadline. For instructions on the LISA application, see here https://signup.lisamission.org/.

 

The workshop is supported by the ROMFORSK grant Project. No. 302640 “Gravitational Wave Signals From Early Universe Phase Transitions”.

Monday, 5 June

08:00 → 12:30 Optional self-organized outdoor activity. For suggestions, see the "Leisure and touristic activities" tab on the left. The workshop starts at 13.10.

See "outdoor options" tab on the left

13:10 → 13:40 Registration

13:40 → 14:00 Welcome and introduction

14:00 → 15:30 Talks

14:00 Status of the LISA mission

Speaker: Antoine Petiteau (APC - Université de Paris)

14:30 The LISA Red Book

Speaker: Chiara Caprini (CNRS France)

15:00 Presentation of new CosWG collaborative rules

Speaker: CosWG Chairs

15:30 → 16:00 Coffee break

16:00 → 18:00 Discussion: CosWG collaborative rules

18:30 → 20:00 Welcome reception (different building)

Tuesday, 6 June

09:00 → 10:30 Talks

09:00 Gravitational Wave Propagation Beyond GR

Departures from General Relativity impact the propagation of gravitational waves (GW) in multiple ways. I’ll explain the theoretical underpinnings of these phenomena, and how they connect to potentially observable features of waveforms. I’ll review the current status of constraints on modified GW propagation, and outline how LISA and other detectors are expected to enhance these bounds.

Speaker: Tessa Baker (Queen Mary University of London)

LISA_Cos_Stavanger_June23.pdf

09:30 Gravitational wave signals from cosmological phase transitions and cosmic strings

I will review the stochastic gravitational wave backgrounds produced by cosmological phase transitions and cosmic strings.

Speaker: Marek Lewicki (University of Warsaw)

2023_06 MLewicki cosWGmeeting.pdf

10:00 Cosmic string parameter estimation with SGWBinner

We present recent results on predictions of LISA's ability to determine cosmic string model parameters. These predictions are made using SGWBinner to fit an injected signal for a variety of cosmic string templates (to be discussed in a complementary talk). We find that LISA should be able to accurately and precisely recover the parameters of simple field-theoretic string models in standard cosmology, such as the string tension, over a wide range of values; we discuss regions of parameter-space where this recovery breaks down. We comment briefly on more complicated cosmological models, such as new degrees of freedom, and their impact on LISA's ability to reconstruct string backgrounds.

Speaker: Jeremy Wachter (Wentworth Institute of Technology)

jmwachter_coswg23.pdf

10:15 Update on the cosmic strings parameter estimation project

In this talk, I will report on the progress of the cosmic string parameter estimation project and on its contribution to building up the LISA catalogue of templates for cosmic strings.

Speaker: Lara Sousa (IA/U. Porto)

10:30 → 11:00 Coffee break

11:00 → 12:30 Talks

11:00 The SGWB produced by MHD turbulence in the early universe

The stochastic gravitational wave background (SGWB) produced at the electroweak phase transition is expected to be peaking within LISA's sensitivity frequency range, being a promising test of high energy physics and beyond Standard Model extensions. The contribution of magnetohydrodynamic (MHD) turbulence to the cosmological SGWB is one of the least understood sources due to the necessity, in general, to perform large-scale numerical simulations solving MHD equations. In this talk, I will review recent numerical simulations that have addressed this issue and studied the potential detectability of the resulting SGWB and its polarization by space-based GW detectors like LISA. I will focus on magnetically dominated MHD turbulence and compare to astrophysical constraints that can provide a multi-messenger study of primordial magnetic fields. In particular, I will present the SGWB produced by decaying MHD turbulence, which has been validated by numerical simulations for a particular range of parameters. This model has been recently used to constrain the characteristics of a primordial magnetic field produced at the QCD phase transition with the common-spectrum process reported by the different pulsar timing array collaborations in the last few years.

Speaker: Alberto Roper Pol (University of Geneva)

lisa_stavanger_jun23.pdf

11:15 First-order phase transition SGWB in LISA: template databank, reconstruction pipeline and science interpretation

In this project, we explore the reconstruction of stochastic gravitational wave backgrounds generated by cosmological phase transitions using LISA, taking into account signals from bubble collisions, sound waves and turbulence. We implement the corresponding signal templates in terms of the both the geometric parameters related to the spectral shape, as well as in terms of the thermodynamical phase transition parameters such as the transition temperature $T_*$, the transition strength $\alpha$ and the time scale $\beta/H_*$. We estimate the prospective parameter reconstruction reach of LISA, employing nested sampling techniques and Fisher analysis and compare the two approaches. To assess the impact of astrophysical foregrounds, we further evaluate the reach with and without their inclusion. Moreover, we interpret our results in terms of fundamental benchmark models and derive constraints on the corresponding model parameters. Our study provides a promising framework to search for gravitational wave signals from cosmological phase transitions and to investigate their properties with LISA.

Speaker: Eric Madge (Weizmann Institute of Science)

EMadge-CosWG23.pdf

11:30 The PBH Review

We give a detailed review of the theory and observations of Primordial Black Holes and specialize on the prospects for detection with the LISA mission.

Speaker: Prof. Juan Garcia-Bellido (Universidad Autonoma Madrid)

LISA_CosWG_6June2023_JGB.pdf

12:00 PrimBHoles: a public code for the computation of Primordial Black Hole abundances and GW signatures

I will present the status of the development of the PrimBHoles code. The code implements various models of PBH formation based on the most recent theoretical prescriptions, as well as their merger rates, stochastic backgrounds (2nd order, from binaries, etc), and other gravitational-wave signatures.

Speaker: Sebastien Clesse (UCLouvain & UNamur)

CLESSE_PrimBHoles_Stavanger.pdf

12:30 → 14:00 Lunch

14:00 → 15:30 Talks

14:00 On the inference of cosmological parameters with gravitational waves

Speaker: Walter Del Pozzo

14:30 Cosmological constraints from the 3rd observing run of Advanced LIGO, Virgo and KAGRA

The 3rd observing run of Advanced LIGO, Virgo and KAGRA (LVK) increased the number of confident gravitational-wave detections from compact binary coalescences to 90. This includes the second ever binary neutron star merger to be detected, and a small number of neutron star-black hole mergers - the first of their kind to be seen by the LVK. However, none of these events were observed with a confirmed electromagnetic counterpart, meaning that cosmological analyses applied to the latest detections must use so-called "dark siren" methods. This talk will look at the latest cosmological results produced by the LVK, from two different approaches: the use of galaxy catalogues to provide redshifts for potential host galaxies, and the use of features in the mass distribution of compact binaries to break the mass-redshift degeneracy.

Speaker: Rachel Gray (University of Glasgow)

LISA_CosWG_2023.pdf

15:00 LISA data analysis: from measurements to discoveries

The future space-based gravitational-wave detector LISA will deliver rich, information-dense data by listening to the milliHertz universe. The measured time series will contain the imprint of tens of thousands of detectable astrophysical sources, emitting continuous, transient and stochastic signals. Finding and characterising all the resolvable sources is a vast and unsolved task. Detecting a cosmological background hidden behind all these signals would be a breakthrough requiring the finest analysis. Various noises, data artefacts and interruptions will further complicate the search. I will give an overview of the state of the art in LISA data analysis research and discuss the framework offered by the LISA Data Challenges (LDCs) to prepare the community for analysing LISA data. I will insist on the challenges posed by detecting a stochastic gravitational-wave background and try to sketch out the issues we have to clear before the launch.

Speaker: Quentin Baghi (CEA Paris-Saclay)

qbaghi-cosmowg.pdf

15:30 → 16:00 Coffee break

16:00 → 18:00 Talks

16:00 Lensing of Gravitational Waves

Gravitational lensing is an unavoidable phenomenon in which the propagation path of gravitational waves changes due to the presence of structures along the line of sight. In this talk, I will give an overview of the lensing signatures relevant for LISA.

Speaker: Macarena Lagos (Columbia University)

Lensing LISA Talk_PDF.pdf

16:30 Parameter estimation for inflationary gravitational wave stochastic backgrounds

Early Universe dynamics beyond the standard single-field slow-roll paradigm may produce a Stochastic Gravitational Wave Background (SGWB) in the LISA band. We adopt a recently developed data analysis pipeline to forecast the constraints on the model parameters and discuss the implications of our forecast for inflationary model builders.

Speaker: Matteo Braglia (New York University)

Lisa coswg.pdf

16:45 Inflationary Stochastic Gravitational wave Background in LISA

I will discuss the current Cosmology Working Group effort to initiate the first LISA template databank for SGWB signals sourced by​ ​inflationary models.​ ​

Speaker: Jacopo Fumagalli (ICCUB - University of Barcelona)

JFumagalli_LISA_Workshop.pdf

17:00 A review on Astrophysical Stochastic signals measured by LISA

LISA is going to be a signal-dominated observatory, thus, multiple Gravitational Wave signals from different types of sources are going to populate our future data streams. From those, the ones with high-enough SNR will be identified and subtracted from the data. However, we expect populations of sources that will generate a stochastic type of signal across the LISA band.

In this talk I will summarize the different possible contributions to the overall stochastic signal that we expect to measure with LISA, and also discuss the possible implications to data analysis.

Speaker: Nikolaos Karnesis (AUTh)

20230605_coswg_stavanger.pdf

17:30 GWSB anisotropies: Challenges and Opportunities

Speaker: Carlo Contaldi

Stavanger_LISA_COSMOWG_Talk-4.pdf

Wednesday, 7 June

09:00 → 09:30 Talks

09:00 Cosmology with Einstein Telescope

Speaker: Angelo Ricciardone

09:30 → 10:30 Discussion: Collaborative projects parallel sessions

10:30 → 11:00 Coffee break

11:00 → 12:30 Discussion: Collaborative projects parallel sessions

12:30 → 14:00 Lunch

14:00 → 15:30 Talks

14:00 Constraining the expansion of the Universe with massive black hole binaries

In $\sim$2034 the Laser Interferometer Space Antenna (LISA) will detect the coalescence of massive black hole binaries (MBHBs) from $10^4$ to $10^7 \, \rm M_{\odot}$ up to $z\sim20$. The gravitational wave (GWs) signal is expected to be accompanied by a powerful electromagnetic (EM) counterpart, from radio to X-ray, generated by the gas accreting on the binary.
GWs are standard sirens (StSis) since they carry the direct information of the luminosity distance of the source. If an EM counterpart is present, the galaxy hosting the merger can be identified and the redshift can be determined with follow-up observations, opening the possibility to test the expansion rate of the Universe.
In this talk, I will present the recent LISA forecasts to constrain cosmological parameters with multi-messenger observations of MBHBs.

Speaker: Alberto Mangiagli (APC)

cosmo_mangiagli.pdf

14:23 Gravitational wave cosmology with LISA Standard sirens

In this talk I will discuss some of the LISA sources that can be used as standard sirens, focussing on Extreme Mass-Ratio Inspirals used as dark sirens. I will review current forecasts of the precision that can be achieved for measurements of the cosmological parameters in both a LCDM model and scenario with a redshift-dependent dark energy equation of state.

Speaker: Danny Laghi (CNES-L2IT)

DL_Stavanger.pdf

14:45 Modified gravitational-wave propagation with extreme mass-ratio inspirals

Extreme mass-ratio inspirals (EMRIs) are systems consisting of a massive black hole and a stellar-mass compact object. These systems are one of the gravitational-wave (GW) sources that will be detected by the Laser Interferometer Space Antenna (LISA). By using dark sirens with galaxy catalogs, EMRIs detected by LISA have the potential to place stringent constraints on cosmological parameters within the framework of general relativity (GR). However, some modified gravity theories predict deviations from GR in the propagation of GWs at cosmological distances, which have a more dominant effect than the modification of the background evolution. Such modified GW propagation effects could be characterized by the phenomenological parameter (Ξ0,𝑛). In this work, we explore the detectability of possible deviation from GR using different EMRI populations detected by LISA. We study the constraints on modified GW propagation effect and provide limits on cosmological parameters (𝐻0,Ω𝑚,Ξ0,𝑛).

Speaker: Chang Liu

backup.pdf

ChangLiu_Stavanger.key

15:00 On the effectiveness of null TDI channels as instrument noise monitors in LISA

In this talk, we will explore some of the challenges and possibilities of detecting stochastic gravitational wave backgrounds (SGWBs) with LISA. The main scientific observables produced by LISA will be three semi-independent TDI data streams, one of which is a so-called null channel, which has reduced sensitivity to gravitational waves (GW). LISA will certainly have the ability to directly constrain the level of a possible SGWB present in the data via the observed noise level in the sensitive channels, but actually distinguishing it from instrumental effects will require some method to characterize the latter.

I will present the results of a study on the ability to directly constrain the level of instrumental noise in a sensitive channel through the level observed in the null channel. We consider two limiting noise sources, test-mass acceleration noise and interferometric phase measurement noise, and discuss how these noises appear differently in the considered TDI channels, which limits the ability of the null channel to quantify the instrumental noise in the sensitive channel. However, we demonstrate that in-flight noise estimations from the null channel can still be used to claim the detection of a strong SGWB in this scenario. While less stringent than other results that have appeared in the literature, our method has the advantage of being quite general, making few assumptions on the noise or signal shapes and properties.

Speaker: Olaf Hartwig (SYRTE - Observatoire de Paris)

20230606-sgwb.pdf

15:15 Stochastic gravitational wave background reconstruction for a non-equilateral and unequal-noise LISA constellation

We explore the impact of choosing different sets of Time-Delay Interferometry
(TDI) variables for detecting and reconstructing Stochastic Gravitational Wave
Background (SGWB) signals and estimating the instrumental noise in LISA. Most
works in the literature build their data analysis pipelines relying on a
particular set of TDI channels, the so-called AET variables, which are
orthogonal under idealized conditions. By relaxing the assumption of a
perfectly equilateral LISA configuration, we investigate to which degree these
channels remain orthogonal and compare them to other TDI channels. We show that
different sets of TDI variables are more robust under perturbations of the
perfect equilateral configuration, better preserving their orthogonality and,
thus, leading to a more accurate estimate of the instrumental noise. Moreover,
we investigate the impact of considering the noise levels associated with each
instrumental noise source to be independent of one another, generalizing the
analysis from two to twelve noise parameters. We find that, in this scenario,
the assumption of orthogonality is broken for all the TDI variables, leading to
a misestimation of measurement error for some of the noise parameters.
Remarkably, we find that for a flat power-law signal, the reconstruction of the
signal parameters is nearly unaffected in these various configurations.

Speaker: Marc Lilley (SYRTE, Observatoire de Paris)

talk_lilley.pdf

15:30 → 16:00 Coffee break

16:00 → 17:30 Talks

16:00 Impact of the noise knowledge uncertainty for the science exploitation of cosmological and astrophysical stochastic gravitational wave background with LISA

LISA, the first space-based gravitational wave observatory designed for detecting gravitational waves in the mHz frequency range, is expected to operate in a regime where gravitational wave signals continuously dominate the instrumental noise. This poses a technical challenge of accurately quantifying and understanding the instrumental noise in the presence of gravitational waves, which is crucial for maximizing the scientific potential of the observatory and identifying possible stochastic gravitational wave backgrounds (SGWB). Despite current ongoing efforts to study this problem, existing approaches for SGWB detection with LISA have limitations in terms of noise or signal model assumptions.
In this study, we contribute to these efforts by investigating the impact of lacking accurate noise knowledge on parameter estimation for different models of gravitational wave cosmological and astrophysical stochastic backgrounds. We consider various signal models, including a power law for stellar-origin binary black holes, a Gaussian bump for primordial black hole generation, a power law with running for non-standard inflation, and a first-order phase transition for the production of sound waves in the cosmic fluid from colliding phase transition bubbles. We conduct analyses that incorporate prior uncertainties in the instrumental noise and we confirm our results by trying to reconstruct noise and signal in case of a power law signal.
Our findings provide insights for developing more robust and agnostic pipelines for SGWB detection with LISA. Overall, we aim to provide a comprehensive analysis that enhances our understanding of the challenges and opportunities in detecting SGWB with LISA and we give a critical analysis of the possibility to detect these types of sources with LISA

Speaker: Martina Muratore (Potsdam-Hannover (AEI))

noise_knowledge_study_Muratore.pdf

16:15 Recovering Primordial Stochastic Gravitational Wave Backgrounds in the LISA Global Fit

Primordial stochastic gravitational wave backgrounds in the LISA band could have been generated from cosmic inflation, an early phase transition, or a myriad of other dynamic processes. Any detection would almost certainly provide the earliest constraints on cosmology. But the rich science potential of these signals does not come for free: LISA's band will be signal-dominated at all times. The galactic binary foreground, as well as extra-galactic binary coalescences will be so dense in time and frequency that a simultaneous, global fit of all signals in the LISA band is necessary for unbiased parameter estimation. In this talk I will describe the prototype global fit pipeline in development at NASA, focusing on our stochastic search. We parametrize known backgrounds with frequency-domain templates, including a large library of potential sources. We are able inject and recover many of these primordial backgrounds on top of the Sangria LISA Data Challenge, setting limits on cosmology even in the presence of the galactic foreground.

Speaker: Robert Rosati (NASA - Marshall)

Rosati_CosmoWG2023_stavanger.pdf

16:30 Doppler-boosted anisotropies of SGWB and LISA: a lever of separation from instrumental and confusion noise.

Crucial aspects of LISA data analysis regard our ability to distinguish the various signals and signatures the LISA data will contain. In particular, signals of stochastic nature will be particularly challenging to sort out, as they could be of instrumental, galactic, astrophysical or cosmological origins, lacking of unique signature to identify them. To tackle this issue, efforts have been put to develop a noise model agnostic strategy in order to extract astrophysical Stochastic Gravitational Waves Background (SGWB) with very flexible assumptions on the noise characteristic [Baghi et al. 2023], utilizing as a lever of separation the distinctive instrument response between single-arm instrumental noise and the signature of a stochastic gravitational strain. On the same lines, now on the signal side, one can use specificities of an astrophysical signal that the instrumental noise has no reason to observe. Kinematic anisotropies (dipolar and quadripolar) are a good example, and could add interesting, complementary information to the Bayesian separation between signal and noise.
Towards such ultimate goal, and following up recent published works from the LISA CosWG, we have put together a full simulation of the response of the instrument to an anisotropic GW sky using the most up-to-date simulation tools from the consortium (full time-domain simulation, LISAGWResponse, pyTDI). From > 3 years simulated datasets, we are studying the capability of LISA to resolve dipolar / quadrupolar kinematic anisotropies, for a scale-free generation as a first test case, then applied to SGWB signals with richer spectral profile (and enhanced anisotropy). We aim at reproducing results from [Bartolo et al. 2022] and [Cusin et al. 2022]. We apply a bayesian analysis method to measure the dipole and quadrupole component which is adapted from [Contaldi et al. 2020] using spectrum averaging and MCMC sampling. More generally, this works put together a first framework for a full time-domain simulation of LISA response to an anisotropic GW sky.

Speaker: Henri Inchauspé (Institute for Theoretical Physics, Universität Heidelberg)

LISA CosWG Meeting - June 2023.pdf

16:45 Reconstructing phase transitions from future LISA data

While a first-order phase transition at the electroweak scale is not expected within the Standard Model, these occur in many BSM scenarios. Therefore, detecting a stochastic gravitational wave background consistent with a first-order phase transition could point to new physics, while a null detection could constrain or even exclude many BSM models. However, recovering the physical parameters of an underlying phase transition from a possible signal at LISA is not straightforward, partly due to other possible sources of a stochastic gravitational wave background. In this talk I will present our recent advances in reconstructing the phase transition parameters from mock LISA data using parameterised templates as an approximation to a more complete physical model, which greatly speeds up the reconstruction process. I will also discuss how we hope to include more realistic noise sources in our mock LISA data in future analysis.

Speaker: Deanna Hooper (University of Helsinki)

Hooper_coswg23.pdf

17:00 Computation of stochastic background from extreme mass ratio inspiral populations for LISA

Extreme mass ratio inspirals (EMRIs) are among the primary targets for the Laser Interferometer Space Antenna (LISA). The extreme mass ratios of these systems result in relatively weak GW signals, that can be individually resolved only for cosmologically nearby sources (up to $z\approx2$). The incoherent piling up of the signal emitted by unresolved EMRIs generate a confusion noise, that can be formally treated as a stochastic GW background (GWB). In this paper, we estimate the level of this background considering a collection of astrophysically motivated EMRI models, spanning the range of uncertainties affecting EMRI formation. To this end, we employed the innovative \textit{Augmented Analytic Kludge} waveforms and used the full LISA response function. For each model, we compute the GWB SNR and the number of resolvable sources. Compared to simplified computations of the EMRI signals from the literature, we find that for a given model the GWB SNR is lower by a factor of $\approx 2$ whereas the number of resolvable sources drops by a factor 3-to-5. Nonetheless, the vast majority of the models result in potentially detectable GWB which can also significantly contribute to the overall LISA noise budget in the 1-10 mHz frequency range.

Speaker: Federico Pozzoli (Università dell'Insubria, Como)

Computation of Stochastic Background from Extreme Mass Ratio Inspirals populations for LISA.pdf

17:15 Parity violation from anisotropies in gravitational waves V modes

According to the standard paradigm, parity is a fundamental symmetry of the Universe. A measure of parity violation in the Universe will inevitably be a smoking-gun for new physics. About one year ago a parity breaking signature from the scalar sector of inflation was claimed by the analysis of the BOSS-galaxy parity-odd trispectrum. More recently, a CMB analysis of the temperature anisotropies parity-odd trispectrum revealed that no parity violation should occur in the inflationary scalar sector, opening up a new (large) tension between LSS and CMB measurements of inflation. In this presentation I will outline the important role of the tensor sector of inflation and scalar-tensor primordial non-Gaussianities in constraining parity violation from the early-Universe. In particular, I will talk about the direct detection of the V-mode gravitational waves anisotropies as a new observational channel to measure parity-violation from inflation.

Speaker: Giorgio Orlando

Presentation_LISA_workshop.pdf

Thursday, 8 June

09:00 → 09:50 Talks

09:00 Primordial Gravitational Waves from Axion-Gauge Fields Dynamics

I will first quickly review the mechanism for primordial gravitational wave (GW) generation in axion-inflation models coupled with a gauge sector via a Chern-Simons term. I will then specialise to the case of an SU(2) sector in a setup where the inflaton is non-minimally coupled to gravity and detail on the gravitational wave spectrum at small scales, those of interest for GW interferometers.

Speaker: Matteo Fasiello (IFT Madrid)

10th_LISA_CosWG_Stavanger_Fasiello.pdf

09:20 Probing Primordial Black Holes and Inflation with LISA

If Primordial Black Holes (PBHs) are formed due to an enhanced scalar-perturbation amplitude at small scales, their formation is inevitably accompanied by the generation of gravitational waves (GWs). Thanks to a coincidence of scales, LISA will be able to observe the GWs associated with the formation of asteroidal mass PBHs, which may constitute the entirety of the dark matter. We will review some recent progress on the characterization of such a signal, alongside the potential constraints LISA will set on the inflationary dynamics underlying these formation models.

Speaker: Gabriele Franciolini (University of Rome la Sapienza)

Stavanger_v2.pdf

09:50 → 10:30 Discussion: Collaborative projects parallel sessions

10:30 → 11:00 Coffee break

11:00 → 12:40 Discussion: Collaborative projects parallel sessions

12:40 → 14:10 Lunch

14:10 → 15:30 Talks

14:10 Status of hydrodynamical simulations of early universe phase transitions

Excellent progress has been made in developing high-level modelling of the gravitational wave power spectrum from early universe phase transitions. This progress has been made possible in part by large-scale simulations of bubble collisions. However, challenges remain, particularly in understanding and modelling the development of nonlinearities, including shocks and turbulence, which become increasingly important for strong phase transitions. Phenomena such as hot droplets can also affect the outcome of the phase transition. I will present the results of recent relativistic hydrodynamical studies of nonlinear effects encountered during and after first-order thermal phase transitions, and discuss how the results can inform theoretical understanding and future analytical models of the resulting gravitational wave power spectra.

Speaker: David Weir (University of Helsinki)

Link to slides

14:30 Gravitational waves from primordial black holes

We will discuss how gravitational wave astronomy can probe various primordial black hole (PBH) scenarios. This includes gravitational waves originating from coalescing PBH binaries in the late universe or from PBH formation in the early universe. These channels can be accessed by LISA if PBHs occupy the (sub)solar or the asteroid mass range, respectively.

Speaker: Hardi Veermae (NICPB, Tallinn)

Stavanger_PBH_GW.pdf

14:45 Primordial Gravitational Waves in non-Minimally Coupled Chromo-Natural Inflation

In this talk we discuss, as recently proposed in arXiv:2303.10718 [astro-ph.CO], how chromo-natural inflation (CNI) can be made compatible with observations at CMB scales through the introduction of a non-minimal coupling of the axion-inflaton kinetic term with the Einstein tensor. The resulting gravitationally-enhanced friction allows the gauge friction to be reduced just enough to avoid the overproduction of chiral GWs at CMB scales, while this compelling feature (characteristic of the CNI model) is delayed at much higher frequencies, where the signal may even be detectable by next-generation interferometers.

Speaker: Mr Martino Michelotti (Van Swinderen Institute for Particle Physics and Gravity, University of Groningen)

LISA presentation Martino Michelotti.pdf

15:00 SGWB anisotropies from inflation with non-Bunch-Davies initial states

It has been found that squeezed tensor non-Gaussianities can induce anisotropies in the Stochastic Gravitational Wave Background (SGWB). One of the models predicting enhanced non-Gaussianities at the squeezed limit is inflation with non-Bunch-Davies initial states. In this talk, we will show that the non-Bunch-Davies effects can potentially enhance the SGWB anisotropies.

Speaker: Shingo Akama

LISA file (A. Shingo).pdf

15:15 Maximal temperature of strongly-coupled dark sectors

A thermal plasma necessarily generates a spectrum of gravitational waves that is, however, too weak to be measurable unless the maximum temperature of the system is large. We present a mechanism ensuring a high dark-sector temperature as a result of reheating dynamics. As an example, we consider axion inflation coupled to a non-Abelian dark sector and we calculate the maximum temperature as a function of the confinement scale. We notice that additional gravitational wave signal can be obtained due to the confinement phase transition in the dark sector.

Speaker: Helena Kolesova (University of Stavanger)

CosWG_Kolesova.pdf

15:30 → 16:00 Coffee break

16:00 → 17:15 Talks

16:00 Improving predictions for thermal bubble nucleation

The gravitational wave signal produced by a cosmological phase transition depends strongly on the rate of bubble nucleation. This determines the size of bubbles at collision, and consequently affects both the peak frequency and amplitude of the gravitational wave signal. Making reliable predictions of the nucleation rate is made difficult by its spacetime dependence, and by non-equilibrium aspects. In this talk, I will give an overview of recent developments in the theory of thermal bubble nucleation. I will also present nonperturbative lattice simulations for a simple real scalar theory with a tree-level barrier. The results of these simulations provide an unambiguous benchmark for perturbative calculations of the nucleation rate, and highlight the importance of higher-order corrections. For perturbative predictions, the inclusion of functional determinants is crucial for agreement with lattice results.

Speaker: Oliver Gould (University of Nottingham)

Nucleation_OG_Stavanger.pdf

16:15 General relativistic bubble growth in cosmological phase transitions

We use a full general relativistic framework to study the self-similar expansion of bubbles of the stable phase in a first order phase transition in the early universe. Self-similarity requires a near-conformal equation of state in both phases, where the ratio between the energy density and pressure of the fluid is constant but different in the two phases. The Israel junction conditions at the interface in the limit of thin shell and large bubble have a structure similar to the energy-momentum conservation in Minkowski spacetime. We find that self-similar detonation and deflagration solutions exist under these assumptions, with peculiar features that discriminate them from the analogous solutions in Minkowski. Both deflagration and detonation solutions describe bubbles with interior spatial negative curvature expanding in a flat Friedmann-Lamaître-Robertson-Walker Universe. The amount of spatial curvature in the interior of the bubble is significantly larger than the naive expectation based on considerations on the energy density perturbation in bubbles expanding on a Minkowski background. We infer that general relativistic effects might have a significant impact in the generation of scalar induced gravitational waves when the bubble size becomes comparable to the cosmological Hubble radius.

Speaker: Lorenzo Giombi (University of Helsinki)

LG_Lisa_workshop.pdf

16:30 Audible Gravitational Echoes of New Physics

Based on the recent article [2023.02399], we discuss the LISA potential for finding evidence of New Physics from measurements of the Stochastic GW Background (SGWB). As a benchmark scenario, we study a version of the low-scale Majoron model equipped with lepton number symmetry and an inverse seesaw mechanism for neutrino mass generation. In particular, we discuss under which circumstances the model can be probed at LISA and which implications result for collider physics observables, such as the Higgs trilinear coupling, the scalar mixing angle and the mass of a new CP-even Higgs boson. If time allows, we will also report on a scenario of symmetry restoration at zero temperature based on a model with two scalar leptoquarks.

Speaker: António Morais (Aveiro University)

Talk_LISA.pdf

16:45 Correlating new physics searches at colliders with a possible gravitational-wave detection

In this talk, I will present a few examples on how collider searches for New Physics may be correlated with a future observation of primordial gravitational waves. One such example concerns the recently observed anomaly in W-boson mass by CDF, and another one elaborates on trilinear Higgs coupling constraints and their impact on the strength of the electroweak phase transition.

Speaker: Roman Pasechnik (Lund university)

LISA_CosWG23-Pasechnik.pdf

17:00 Cosmological gravitational wave anisotropies from adiabatic and isocurvature perturbations

The anisotropies of the stochastic gravitational wave background, as produced in the early phases of cosmological evolution, can act as a key probe of the primordial universe particle content. We point out a universal property of gravitational wave anisotropies of cosmological origin: for adiabatic initial conditions, their angular power spectrum is insensitive to the equation of state of the cosmic fluid driving the expansion before BBN. Any deviation from this universal behaviour points to the presence of non-adiabatic sources of primordial fluctuations. In this work we prove this general result, and we illustrate its consequences for a representative realisation of initial conditions based on the curvaton scenario. In the case of the simplest curvaton setup, we also find a four-fold enhancement in the cross-correlation between gravitational wave anisotropies and the CMB temperature fluctuations, vis-à-vis the purely adiabatic scenario.

Speaker: Ema Dimastrogiovanni

Friday, 9 June

09:00 → 10:30 Discussion: Writing down proposals for projects (parallel sessions)

10:30 → 11:00 Coffee break

11:00 → 12:00 Discussion: Presentation of the project proposals

12:00 → 12:45 Final discussion, future plans and farewell

13:00 → 19:00 Optional self-organized outdoor activity. For suggestions, see the "leisure and touristic activities" tab on the left.