Gravitational Wave Signals From Early Universe Phase Transitions

In the very early Universe, a fraction of a second after the Big Bang, a number of phase transitions may have taken place. Just as for the boiling of water (which is also a phase transition) bubbles form. These grow and collide which each other, and for the energies present in the early Universe, these collisions create gravitational waves. The LIGO experiment on Earth is much too small to detect these gravitational waves from the Big Bang. Hence, the LISA experiment has been designed to be a million times larger, and will go into orbit around the Sun in 2034. This research project is part of the larger LISA project, and is about calculating the expected shape, magnitude and frequency spectrum of gravitational waves from phase transitions in the early Universe, and forecasting at which accuracy LISA will reconstruct this class of signals.

The Laser Interferometer Space Antenna (LISA), mainly funded by the European Space Agency, will be the first Gravitational Wave (GW) observatory in space, a flagship project in fundamental science over the next 20 years. LISA is designed to detect GW sources spanning the whole history of the Universe. While the loudest sources will be resolved individually, the rest will combine into a stochastic GW background (SGWB). In LISA data, the SGWB carries information about both astrophysical and cosmological sources, e.g. neutron star binaries, Binary Black Hole (BBH) or early Universe phenomena such as First Order Phase Transitions (FOPTs), cosmic strings and preheating after inflation. The present project aims at producing a robust, detailed parametrization of the SGWBs produced by FOPTs, taking also into account the current and forthcoming Large Hadron Collider (LHC) constraints on the underlying particle physics models. The result will be included in the LISA data analysis pipeline and, in collaboration with the LISA Data Challenge Working Group, we will produce mock data to check the detection prospects for the FOPT signal, in the presence and in the absence of the SGWB component due to stellar-origin BBHs. The crucial role of the proposed investigation is proven by the fact that the LISA consortium has already identified the “characterization of the stochastic signal” as one of the LISA priorities and already committed itself to deliver to ESA science knowledge and data analysis pipelines for this signal. In fact, a detailed but minimal parametrization of the foreseeable SGWB signals will allow the LISA collaboration to accurately fit all the resolvable and unresolvable, simultaneous signals without dangerous redundancies in the parameter space of the fit. Overall, this proposal will deliver one of the main tools enabling LISA to be a successful mission.