Exploring the dark universe with gravitational wave backgrounds

Abstract

Gravitational waves have recently emerged as a novel messenger in astronomy, providing profound insights into the evolution of the cosmos: Last year, several pulsar timing arrays (PTAs) announced the detection of a gravitational wave background (GWB) at nHz frequencies, marking a significant milestone in this field. This discovery can be interpreted as a signal emitted by the inspiral of supermassive black holes during hierarchical structure formation. More excitingly, the signal’s origin could alternatively predate recombination and Big Bang nucleosynthesis. Such an explanation would require new physics in order to explain the strong dynamics in the primordial plasma above the MeV temperature scale necessary to emit those gravitational waves. In this thesis, we explore the possibility that PTAs could have detected a cosmological phase transition within a dark sector. A better fit to the data than that for supermassive black hole binaries can be found if the dark sector hosting the transition decays before the onset of nucleosynthesis. Furthermore, it is plausible that PTAs have instead observed a GWB emitted by the inspiral of supermassive primordial black holes. Our research indicates that these primordial black holes can account for the observed signal only if they are initially clustered rather than homogeneously distributed. Looking towards future gravitational wave observatories, we examine the formidable opportunity offered by LISA, that is sensitive to mHz frequencies, to probe GWBs emitted at temperatures of the primordial plasma around the 100 GeV scale, aligning with the freeze-out epoch of a WIMP dark matter candidate. Our findings suggest that if LISA detects such a cosmological GWB, it would hint towards a dark sector phase transition producing the relic dark matter abundance, consistent with cosmological and astrophysical observations. Our results underscore the significance of gravitational wave observations in unveiling the dynamics of the early universe and provide new pathways for understanding the fundamental nature of dark matter and the cosmological history of the universe.