Welcome

• Sébastien Renaux-Petel (IAP-CNRS)

Cosmology with LISA

• Chiara Caprini

Measuring the net circular polarization of the stochastic gravitational wave background with interferometers

• Lorenzo Sorbo

Stochastic Gravitational Wave Backgrounds (SGWB) characterization with LISA

• Mauro Pieroni

In this talk I will discuss Stochastic Gravitational Wave Backgrounds (SGWB) characterization with LISA. After a general introduction on SGWB detection, I will explain the peculiar features of LISA with a focus on the response function and on the noise spectrum. The core of my talk will be the presentation of two different methods for model independent SGWB frequency reconstruction: - The so-called ``binning method'' (1906.09244, 2009.11845) based on the idea of approximating the signal with a piecewise-defined function where each sub-function is a power law. - A more recent method (2004.01135) based on principal components analysis. The problem of component separation to disentangle a cosmological SGWB from astrophysical foregrounds is addressed in both cases.

Open discussion

Gauge Fields in Inflation, Origin of Matter, and Gravitational Waves

• Azadeh Maleknejad

Modern cosmology inevitably involves particle physics beyond the Standard Model to explain long-standing puzzles: the origin of matter asymmetry, nature of dark matter and cosmic inflation. In this talk, I will explain that gauge fields in inflation can possibly solve and relate these seemingly unrelated mysteries of modern cosmology. As a smoking gun, this setup has robust observable signatures for the gravitational background to be probed by future CMB missions and laser interferometer detectors.

Probing the Early Universe Particle Content with Primordial Messengers

• Matteo Fasiello

Some of our best ideas on early universe physics are about to be put to the test by an unprecedented array of cosmological probes. The data these will collect span a vast range of scales, from the CMB to large scale structure, from pulsar timing arrays all the way to laser interferometers. This combined wealth of new information holds the potential to transform not just our understanding of cosmology, but also particle physics. Probing the earliest accessible epoch, the accelerated expansion known as inflation, is absolutely crucial: inflation can provide a cosmological portal to otherwise inaccessible energy scales. This is the “cosmological collider” idea. The spectacular success of the inflationary paradigm in explaining the origin of cosmic structure demands that we tackle a number of compelling questions still in need of an answer: - what is the energy scale of inflation? - what fields were active during inflation? In this talk I will review recent progress on the inflationary field content focussing on an effective theory approach. I will also highlight key observables, starting with primordial gravitational waves, and discuss the prospect for detection.

Testing inflation with small-scale anisotropies

• Ema Dimastrogiovanni

Inflation predicts a stochastic background of gravitational waves. In this talk I will discuss how anisotropies in the gravitational wave energy density can be a powerful tool in characterizing the inflationary GW background and potentially distinguishing it from backgrounds due to other sources.

Open discussion

Probing gravitational waves using spectral distortions

• Subodh Patil

CMB anisotropies probe primordial gravitational wave backgrounds of frequencies between 10^{-18} and 10^{-15} Hz. Pulsar timing arrays begin to be sensitive to backgrounds starting in the nano-hertz regime, seemingly leaving an approximately six decade gap in frequency coverage. This talk will discuss how spectral distortions of the CMB covers this gap in a meaningful way, and discuss various phenomenological consequences for a variety of beyond the standard model scenarios, including low scale (dark sector) phase transitions, cosmic strings and ultra-light axions.

Features in the stochastic gravitational wave background from two-field inflationary models.

• Matteo Braglia

Oscillations in the stochastic gravitational wave background from small-scale features

• Lukas Witkowski

Observing primordial GWs from excited states

• Spyros Sypsas

Open discussion

Probing the Physics of Inflation with Gravitational Wave Experiments

• Gianmassimo Tasinato

Imprints of Primordial Non-Gaussianity on Gravitational Wave Spectrum

• Caner Ünal

I will talk about the imprints of small, mild and large primordial local NG in density perturbations on the induced GW spectrum.

Are induced GWs gauge dependent?

• Guillem Domènech

Open discussion

NANOGrav 12.5-yr result and induced gravitational waves

• Shi Pi

I will briefly review the properties of SGWB induced by the scalar perturbation, as well as its connection to the primordial black holes (PBHs). NANOGrav reported the discovery of a common-spectrum process of the time residuals in their 12.5-yr data set of pulsar timing array, which might be the first detection of nano-hertz SGWB. Based on the recent developments on the spectral shape of SGWB, I will show that this signal could be connected to the planet-mass PBH candidates found by OGLE recently, if there is a dust-like stage prior to the radiation dominated era.

Primordial Nano Black Holes

• Antonio Riotto

Primordial Black Holes: gravitational-wave signatures and possible contribution to the dark matter

• Sebastien Clesse

The detection of gravitational-waves from binary black hole mergers has rekindled the idea that primordial black holes may have formed in the early Universe. I will review some recent developments in the field of primordial black holes: formation mechanisms, mass function, astrophysical limits, possible contribution to the Dark Matter and links to the Baryogenesis. I will particularly focus on their gravitational-wave signatures: merging rates, explanation of recent LIGO/Virgo events (GW190521, GW190815, GW190425), subsolar black hole searches, stochastic gravitational-wave backgrounds and continuous waves or high-frequency GW detectors as a probe of planetary-mass primordial black holes.

Open discussion

Deformation of the gravitational wave spectrum by density perturbations

• Jinno Ryusuke

In recent years, the possibility of having large density perturbations at high wavenumbers has been actively studied, especially in the study of primordial black holes (PBHs). In this presentation, we point out that such density perturbations affect any pre-existing gravitational wave (GW) spectrum through CMB-like effects. Since GWs produced in the early universe propagate through the density perturbations before reaching the observer, GWs coming from each direction experience different evolution depending on the density perturbations. As a result, the observed (directionally averaged) GWs generally have a deformed spectrum compared to the original one. Therefore, by carefully comparing the observed spectrum with the theoretical GW spectrum at the time of production, we may be able to indirectly observe density perturbations at high wevenumbers. In this presentation, we explain how this effect shows up and estimate the maximum expected impact on the GW spectrum, taking into account the latest bound on density perturbations from PBH searches.

Probing the 'primordial dark ages’ with gravitational waves. Implications for reheating.

• Daniel Figueroa

Guided discussion