Extreme mass ratio inspirals: a review

• Stanislav Babak

Room: 4/3-006 - TH Conference Room

Limits and limitations of perturbative approaches to cosmological phase transitions

• Philipp Schicho (University of Geneva)

Room: 4/3-006 - TH Conference Room Scale hierarchies are required to reliably describe the thermodynamics of cosmological first-order phase transitions using perturbation theory. At finite temperature, such a hierarchy is provided naturally. One can then use this hierarchy to construct a three-dimensional effective field theory (EFT) that systematically includes thermal resummations to all orders. Using this EFT setup, I discuss completing the perturbative program for equilibrium thermodynamics of cosmological first-order phase transitions by determining the finite-temperature effective potential of gauge-Higgs theories at their perturbative limit, reaching next-to-next-to-next-to-next-to-leading order (N4LO) [1]. I then explore the limitations of thermal EFTs, discussing their regime of validity and the conditions under which thermally induced higher-dimensional operators become relevant for gravitational-wave predictions. [1] A. Ekstedt, P. Schicho, and T. V. I. Tenkanen, Cosmological phase transitions at three loops: The final verdict on perturbation theory, Phys. Rev. D 110, 096006 (2024), [2405.18349].

Backreaction and Cosmic Butterflies: what simulations can tell us about inflation (Cosmo Coffee, https://indico.cern.ch/event/1519218/)

• Angelo Caravano

Room: 4/3-006 - TH Conference Room The small-scale physics of inflation can leave unique observational signatures in the gravitational wave background and may also generate primordial black holes as a dark matter candidate. These phenomena often involve a significant enhancement of inflationary fluctuations, potentially leading to the breakdown of standard perturbation theory. In this talk, I will discuss how lattice simulations provide a crucial tool for addressing these challenges. Focusing on single-field inflationary models with deviations from slow-roll, I will demonstrate how nonlinear physics can significantly alter the inflationary dynamics and their predictions, even in regimes where perturbation theory is typically assumed valid. This highlights the indispensable role of lattice simulations in probing inflation with current and future gravitational wave experiments.

The first massive black holes: recent discoveries and open questions (Theory Colloquium https://indico.cern.ch/category/100/)

• Marta Volonteri

Room: 4/3-006 - TH Conference Room The presence of massive black holes in the first galaxies has been predicted theoretically for many years. Until the launch of the space telescope JWST the bulk of observational data hinged on very bright sources — luminous quasars — detected about one billion years after the Big Bang. Now a large number of fainter sources, candidate active massive black holes, have been identified, some at much earlier cosmic times. The extent of the population of potential massive black holes as well as their peculiar properties in terms of spectral distribution and compactness of the host galaxies challenge standard approaches. I will first discuss the challenges of interpreting these observations and comparing them to theoretical models. I will then connect massive black holes observed via their electromagnetic emission to those that can be detected via emission of gravitational waves: merging massive black holes.

High-Frequency Gravitational Waves from Axion Experiments

• Sung Mook Lee (CERN)

Room: 4/3-006 - TH Conference Room In recent years, there has been growing interest and significant progress in the study of high-frequency gravitational waves (HFGWs) within both theoretical and experimental realms. Of particular significance is the potential for electromagnetic signals induced by gravitational waves in the presence of background electromagnetic fields, highlighting promising synergies between axion experiments and gravitational wave detection in the high-frequency regime, beyond the reach of LIGO. In this talk, I will provide an overview of the opportunities and challenges in advancing the HFGW program and present illustrative examples of potential HFGW searches, including the use of axion haloscopes and various astrophysical sources, such as pulsars.