NEW GRAVITATIONAL INTERACTIONS IN COSMOLOGY —- FROM EFFECTIVE FIELD THEORY TO OBSERVATION

In this thesis, I present the research conducted during my Ph.D. program on new gravitational interactions in cosmology, integrating theoretical advancements with observational evidence. I begin by examining a tension between measurements of Hubble constant made using data from radically different cosmic epochs, and proposing a theoretical model that suggests an interaction between neutrinos and gravity could help alleviate this tension. I then turn to more formal considerations of Effective Field Theories (EFTs) in cosmology. In particular, I show how to generalize the EFT of the binary black hole system to extended objects, constructing the EFT for a binary cosmic string system. I further study the EFT for symmetry-breaking Conformal Field Theories (CFTs), which can be applied to the early universe. Beyond EFTs, the study of gravity becomes increasingly complex, particularly as nonlinearities become important. Dualities may provide a useful tool to make progress in this regime, and with this in mind I have also explored the massive double copy; a relation between the non-linear sigma model and the special galileon theory when there are mass scales. Finally, I describe work on the observational opportunities presented by the new era of multi-messenger cosmology. I focus on Pulsar Timing Array (PTA) systems, discussing their roles in gravitational wave detection and their potentials for testing modified gravity theories. Through these studies, this thesis aims to contribute to the ongoing dialogue between gravitational physics and cosmology.

Author dc:creator

• Liang, Qiuyue

Advisor dc:contributor.advisor

• Trodden, Mark

• Physics
• Astrophysics and Astronomy

Language dc:language.iso

en

Repository record dc:identifier.uri

https://repository.upenn.edu/handle/20.500.14332/59292

OAI identifier oai:identifier

oai:repository.upenn.edu:20.500.14332/59292