# 5th LISA Cosmology Working Group Workshop

Europe/Helsinki
Physicum, University of Helsinki

#### Physicum, University of Helsinki

Gustaf Hällströmin katu 2 00560 Helsinki Finland
Description

The 5th LISA Cosmology Working Group Workshop will take place from the 11th to the 15th of June on the Kumpula Campus of the University of Helsinki. It is hosted by the Computational Field Theory group.

The main aim of the workshop will be to finalise the Working Group's currently ongoing projects and start follow-up work.

More generally, topics that will be covered include:

• First order phase transitions
• Detection of stochastic backgrounds
• Topological defects
• Standard sirens
• Testing General Relativity
• Inflation and beyond
• Primordial black holes and dark matter
• Structure formation

The workshop is supported by the Magnus Ehrnrooth Foundation.

Participants
• Anders Tranberg
• Angelo Ricciardone
• Anna Kormu
• Antoine Petiteau
• Antonio Riotto
• Asier Lopez-Eiguren
• Bithika Jain
• Catarina Cosme
• Daniel Cutting
• Daniel G. Figueroa
• David Chernoff
• David Weir
• Djuna Croon
• Elena Maria Rossi
• Emanuela Dimastrogiovanni
• Germano Nardini
• Gianluca Calcagni
• Gianmassimo Tasinato
• Giorgio Orlando
• Jani Dahl
• Jeremy Wachter
• Joanes Lizarraga
• Jose Miguel No
• Juan Garcia-Bellido
• Kari Enqvist
• Kari Rummukainen
• Keijo Kajantie
• Kimmo Kainulainen
• Kimmo Tuominen
• Lara Sousa
• Lauri Niemi
• Lorenzo Sorbo
• Lucio Mayer
• Marco Peloso
• Marek Lewicki
• Mark Hindmarsh
• Martin Hewitson
• Matteo Fasiello
• Matts Roos
• Mauro Pieroni
• Michele Maggiore
• Mikael Chala
• Mikko Laine
• Nicola Bartolo
• Nicola Tamanini
• Niko Jokela
• Nikolaos Karnesis
• Oliver Gould
• Paul McNamara
• Purnendu Karmakar
• Raphael Flauger
• Riccardo Buscicchio
• Robert Caldwell
• Sabino Matarrese
• Sachiko Kuroyanagi
• Sara Tähtinen
• Stephan Huber
• Syksy Räsänen
• Tania Regimbau
• Tommi Tenkanen
• Tuomas Tenkanen
• Valerie Domcke
• Venus Keus
Contact
• lundi, 11 juin
• 10:30 11:00
Registration and coffee 30m Physicum lobby

#### Physicum lobby

• 11:00 12:30
Parallel discussions: Discussions (amongst those already arrived)
• 12:30 13:30
Lunch 1h Unicafe Chemicum

#### Unicafe Chemicum

• 13:30 15:00
Talks and group discussions: Consortium information D101 (Physicum)

### D101

#### Physicum

• 13:30
Consortium status 30m
Orateur: Martin Hewitson (AEI Hannover)
• 14:00
ESA Perspectives 30m
Orateur: Paul McNamara (ESA)
• 14:30
Consortium discussion 30m

Consortium-related discussion on:

• Publications & presentations policy

• Template for project proposals

• "SGWB characterization" WP

• 15:00 15:30
Coffee 30m
• 15:30 16:00
Talks and group discussions: Consortium information D101 (Physicum)

### D101

#### Physicum

• 15:30
LISA work packages 30m
Orateur: Martin Hewitson (AEI Hannover)
• 16:00 17:00
Talks and group discussions: Late universe D101 (Physicum)

### D101

#### Physicum

• 16:00
Multimessenger WP details 30m
Orateur: Elena Maria Rossi (Leiden Observatory)
• 16:30
Standard sirens 30m
Orateur: Nicola Tamanini (AEI Potsdam)
• 17:00 18:00
Welcome drinks 1h Physicum lobby

#### Physicum lobby

• mardi, 12 juin
• 09:30 10:30
Talks and group discussions: Late universe D101 (Physicum)

### D101

#### Physicum

• 09:30
Astrophysics interplay and work packages 30m
Orateur: Lucio Mayer (Zurich)
• 10:00
PBH: summary of the CERN workshop 30m
• 10:30 11:00
Coffee 30m Physicum lobby

#### Physicum lobby

• 11:00 12:30
Talks and group discussions: Late universe D101 (Physicum)

### D101

#### Physicum

• 11:00
Primordial Black Holes as Dark Matter 15m

I will revisit the most important cosmological and astrophysical constraints on the fraction of dark matter (DM) in primordial black holes (PBHs). I consider a variety of production mechanisms and mass functions for PBHs and discuss whether they can constitute the observed DM abundance or not. I also discuss how one can constrain the physics of the early Universe with the constraints on PBHs.

Orateur: Tommi Tenkanen (Queen Mary University of London)
• 11:15
Local galactic measurements 15m
Orateur: Elena Maria Rossi (Leiden Observatory)
• 11:30
• 12:30 13:30
Lunch 1h Unicafe Chemicum

#### Unicafe Chemicum

• 13:30 15:00
Talks and group discussions: Data analysis D101 (Physicum)

### D101

#### Physicum

• 13:30
LISA Data Challenge 30m
• Intro
• Plans/status for data analysis for standard sirens
• Production of SGWB
Orateur: Antoine Petiteau (APC - Université Paris-Diderot)
• 14:00
SGWB data analysis for Radler 30m
Orateur: Riccardo Buscicchio (University of Birmingham)
• 14:30
Bin reconstruction 30m
Orateur: Mauro Pieroni (Instituto de Física Teórica (IFT))
• 15:00 15:30
Coffee 30m Physicum lobby

#### Physicum lobby

• 15:30 17:45
Talks and group discussions: Data analysis D101 (Physicum)

### D101

#### Physicum

• 15:30
SGWB parameter estimation 15m
Orateur: Nikolaos Karnesis (APC Paris)
• 15:45
Probing the bispectrum of the SGWB 30m
Orateur: Valerie Domcke (DESY)
• 16:15
Detector Geometries for a Chiral SGWB 15m
Orateur: Robert Caldwell (Dartmouth)
• 16:30
Discussion on sky realisations, figures of merit etc. 1h 15m
• mercredi, 13 juin
• 09:30 10:30
Talks and group discussions: Data analysis D101 (Physicum)

### D101

#### Physicum

• 09:30
SGWB data analysis in LIGO/Virgo 1h
Orateur: Tania Regimbau (CNRS)
• 10:30 11:00
Coffee 30m Physicum lobby

#### Physicum lobby

• 11:00 11:30
Talks and group discussions: Data analysis D101 (Physicum)

### D101

#### Physicum

• 11:00
Data analysis discussion 30m
• 11:30 12:30
Talks and group discussions: Strings D101 (Physicum)

### D101

#### Physicum

• 11:30
Update on strings project 30m
Orateur: Lara Sousa (IA-Porto)
• 12:00
Stochastic GW background from Cosmic Strings 15m

I will give a brief introduction to the theory of cosmic strings in the early universe and discuss recent progress in our understanding of the evolution of a cosmic string network using Nambu-Goto simulations. I will then discuss the production of gravitational waves from this network and how this leads to a prediction of an stochastic background of gravitational waves covering a wide range of frequencies. Finally, I will also discuss the implications of these predictions for the gravitational wave observations at the PTA range as well as the LIGO and LISA observatories.

Orateur: Jose Juan Blanco-Pillado (IKERBASQUE and UPV/EHU)
• 12:15
Analytics of gravitational self-interactions of cosmic strings 15m

We discuss an analytic approach to solving the problem of the gravitational self-interaction of cosmic strings. We demonstrate this approach on a simple toy model of string loops, and discuss how it might be applied to a string loop network.

Orateur: Jeremy Wachter (UPV/EHU)
• 12:30 13:30
Lunch 1h Unicafe Chemicum

#### Unicafe Chemicum

• 13:30 14:00
Talks and group discussions: Strings D101 (Physicum)

### D101

#### Physicum

• 13:30
Prospects for detecting gravitational waves from cosmic strings with LISA 15m

Cosmic strings are one-dimensional topological defects which may have been generated in the early Universe. LISA will be an excellent probe of cosmic strings as they emit strong gravitational wave bursts. In my talk, I will review the possibility of probing cosmic strings through detection of rare bursts as well as of a stochastic background, which consists of many overlapped bursts. I will also mention efforts for developing a data analysis code in KAGRA collaboration to search for cosmic string bursts with matched-filtering, which might have important applications also for LISA.

Orateur: Sachiko Kuroyanagi (Nagoya University)
• 13:45
Effect of the local population of superstrings on detection rates 15m

Cosmic superstrings of string theory differ from conventional cosmic strings of field theory, influencing the expectations for searches for relics from inflation. The universe's average density of cosmic superstrings can easily exceed that of conventional cosmic strings having the same tension by two or more orders of magnitude. The cosmological behavior of the remnant superstring loops is qualitatively distinct because the string tension is exponentially smaller than the string scale in flux compactifications in string theory. Low tension superstring loops live longer, experience less recoil (rocket effect from the emission of gravitational radiation) and tend to cluster like dark matter in galaxies. Clustering enhances the string loop density with respect to the cosmological average in collapsed structures in the universe. The enhancement at the Sun's position is $\sim 10^5$. I present a model encapsulating the leading order string theory effects, the current understanding of the string network loop production and the influence of cosmological structure formation suitable for forecasting the detection of superstring loops by LISA. Clustering dominates rates for $G \mu<10^{-11.2}$ and $G \mu < 10^{-10.6}$ for cusps and kinks, respectively. I forecast experimentally accessible gravitational wave bursts for $G \mu>10^{-15}$ and $G \mu>10^{-14.1}$ for cusps and kinks, respectively.

Orateur: Prof. David Chernoff (Cornell University)
• 14:00 15:00
Talks and group discussions: Phase transitions D101 (Physicum)

### D101

#### Physicum

• 14:00
Phase transitions: signal update 15m
Orateur: Mark Hindmarsh (University of Sussex)
• 14:15
Phase transitions project: BSM update 15m
Orateur: Jose Miguel No (Kings College London, IFT Madrid)
• 14:30
More on the signal 15m
Orateur: Mikko Laine (AEC, ITP, U. Bern)
• 15:00 15:30
Coffee 30m Physicum lobby

#### Physicum lobby

• 15:30 16:15
Talks and group discussions: Phase transitions D101 (Physicum)

### D101

#### Physicum

• 15:30
Collider and gravitational wave signatures of the electroweak phase transition in the standard model effective field theory 15m

We explore new-physics setups that at low energy exhibit a Higgs potential with sizeable higher-dimensional operators. We focus on the parameter regions promoting a first-order electroweak phase transition (FOEWPT). For weakly-interacting setups, we find that such regions are always tuned when the operators receive contributions from integrating out SU(2)_L singlets or triplets. The reason is the emergence of other operators that are naturally unavoidable and strongly experimentally constrained. The custodial quadruplet extension we propose, instead, does not suffer of this problem. We also analyse strongly interacting setups for which dimension- eight operators need to be considered. In this context we compute for the first time the parameters leading to a FOEWPT. We estimate the reach of LISA for probing all these low-energy scenarios and compare it with the capabilities of the Higgs measurements at current and future colliders. For some parameter regions, LISA can prove the existence of new physics before colliders.

Orateur: Mikael Chala (IPPP Durham)
• 15:45
Model discrimination in Gravitational Waves from Phase Transitions 15m

Besides the EWPT, various dark phase transitions could give an observable broken power-law spectrum at LISA. In this talk I will consider the opportunities for model discrimination, by examining the thermal parameters predicted in classes of models. These thermal parameters are functions of the parameters in the scalar potential, the quantum numbers of the scalar, and the couplings to fermions. I will also comment on exotic spectra from multi-step phase transitions, and finish with an outlook to the Effective Field Theory of Gravitational Waves from Phase Transitions.

Orateur: Djuna Croon (Dartmouth)
• 16:00
Catalogues of sources (and webpage) 15m
Orateur: David Weir (University of Helsinki)
• 16:15 17:30
Parallel discussions: Discussions E204 (Physicum)

### E204

#### Physicum

• jeudi, 14 juin
• 09:30 10:30
Talks and group discussions: Beyond the standard paradigms D101 (Physicum)

### D101

#### Physicum

• 09:30
Beyond the standard paradigms - status 15m
Orateur: Gianmassimo Tasinato (Swansea University)
• 09:45
Cosmic structures and gravitational waves in ghost-free scalar-tensor theories of gravity 15m
Orateur: Purnendu Karmakar (University of Padova)
• 10:00
The gravitational-wave luminosity distance in modified gravity 15m

In modified gravity the propagation of gravitational waves is in general different from that in general relativity. As a result, the luminosity distance for GWs can differ from that for electromagnetic signals, and is affected both by the dark energy equation of state and by a function describing modified propagation. The effect of modified propagation in general dominates over the effect of the dark energy equation of state, making it easier to distinguish a modified gravity model from $\Lambda$CDM using standard sirens. (based on arXiv:1712.08108).

Orateur: Michele Maggiore (University of Geneva)
• 10:15
Testing quantum gravity with LISA 15m

We briefly review whether and how some theories of quantum gravity could be tested by LISA, either with stand-alone data or in combination with other observations. In particular, we focus our attention on string theory, loop quantum gravity, asymptotic safety, nonlocal quantum gravity, and multifractional spacetimes. We stress the importance and meaning of following top-down approaches (from a well grounded theory to experiments) rather than a bottom-up strategy using ad hoc phenomenological models.

Orateur: Gianluca Calcagni (IEM-CSIC)
• 10:30 11:00
Coffee 30m Physicum lobby

#### Physicum lobby

• 11:00 12:30
Talks and group discussions: Inflation D101 (Physicum)

### D101

#### Physicum

• 11:00
Inflation project: short summary 15m
Orateur: Angelo Ricciardone (University of Stavanger)
• 11:15
Primordial Gravitational Waves from Axion-Gauge Fields Dynamics 15m

I will first review an inflationary model that includes an axion coupled with SU(2) gauge fields. This setup can generate a chiral gravitational waves signal. I will then detail on how the parameter space of the theory supports a blue tensor spectrum and large tensor as well as scalar-tensor-tensor non-Gaussianities.

Orateur: Matteo Fasiello
• 11:30
Chern-Simons gravitational term coupled to a scalar field during inflation 15m

Chern-Simons gravity is a modified gravity theory where a term like $f(\phi) C \tilde C$ is added to the Einstein-Hilbert Lagrangian. Here $C$ symbolically labels the Weyl tensor, the trace-less part of the Riemann tensor. In this context $\phi$ can be both a scalar or a pseudo-scalar field and $f(\phi)$ is a generic scalar coupling function. Phenomenologically, it is interesting to study the effect of Chern-Simons gravity during inflation where we can identify the scalar field $\phi$ as the inflaton field. The result is that we have the production of primordial chiral gravitational waves, whose chirality is extremely small in the regime in which we avoid ghost field formation. Analysing also the implications to the bispectrum statistics of primordial perturbations, it is possible to point out that only the mixed $\langle \phi \gamma \gamma \rangle$ bispectrum takes a non negligible contribution. In particular, it is possible to show that, admitting a particular fine-tuning to the second order derivative of the coupling function $f(\phi)$, we could get a signature of this kind of modified gravity model through measurements of CMB primordial non-Gaussianities. However, a new interesting scenario can be identifying $\phi$ with an external scalar field $\chi$ different from the inflaton field. In this case we get a squeezed modulation of primordial gravitational waves chirality. This may enhance the 2-point function chirality, making it interesting for a possible detection through both CMB experiments and interferometers sensitive to the circular polarization of gravitational waves. In the talk I discuss Chern-Simons gravity and give the basic idea of this amplification mechanism.

Orateur: Giorgio Orlando (Dipartimento di Fisica e Astronomia, Padova)
• 11:45
Scalar field dark matter with spontaneous symmetry breaking and the 3.5 keV line 15m

We show that the present dark matter abundance can be accounted for by an oscillating scalar field that acquires both mass and a non-zero expectation value from interactions with the Higgs field. The dark matter scalar field can be sufficiently heavy during inflation, due to a non-minimal coupling to gravity, so as to avoid the generation of large isocurvature modes in the CMB anisotropies spectrum. The field begins oscillating after reheating, behaving as radiation until the electroweak phase transition and afterwards as non-relativistic matter. The scalar field becomes unstable, although sufficiently long-lived to account for dark matter, due to mass mixing with the Higgs boson, decaying mainly into photon pairs for masses below the MeV scale. In particular, for a mass of $\sim 7$ keV, which is effectively the only free parameter, the model predicts a dark matter lifetime compatible with the recent galactic and extragalactic observations of a 3.5 keV X-ray line.

Orateur: Catarina Cosme (University of Porto)
• 12:00
Production of gravitational waves during inflation 15m

I will review mechanisms of production of matter during inflation that will in its turn lead to the generation of gravitational waves. These gravitational waves can be of relevance both for the phenomenology of the CMB and for gravitational interferometers.

Orateur: Lorenzo Sorbo (University of Massachusetts, Amherst)
• 12:15
Reminder: Source catalogue, parametrisation, figures of merit, project templates 10m
• 12:25
Orateur: Dr Germano Nardini
• 12:30 13:30
Lunch 1h Unicafe Chemicum

#### Unicafe Chemicum

• 13:30 15:00
Parallel discussions: Project development E204 (Physicum)

### E204

#### Physicum

• 15:00 15:30
Coffee 30m Physicum lobby

#### Physicum lobby

• 15:30 17:00
Parallel discussions: Project development E204 (Physicum)

### E204

#### Physicum

• 17:00 17:30
Parallel discussions: Filling the project templates E204 (Physicum)

### E204

#### Physicum

• 19:00 21:00
Conference dinner 2h
• vendredi, 15 juin
• 09:30 10:30
Talks and group discussions: Summary of the parallel discussions E204 (Physicum)

### E204

#### Physicum

• 10:30 11:00
Coffee 30m Physicum lobby

#### Physicum lobby

• 11:00 12:15
Talks and group discussions: Project template discussion E204 (Physicum)

### E204

#### Physicum

• 12:15 12:30
Talks and group discussions: Closing E204 (Physicum)

### E204

#### Physicum

• 12:30 13:30
Lunch 1h Unicafe Chemicum