T-2, Nuclear and Particle Physics, Astrophysics and Cosmology

Cosmology beyond the standard model

David Pirtskhalava

This talk will report on recent progress in constructing effective field theories that provide a novel and qualitatively different perspective on both the very early, as well as the late-time universe. With the help of model-independent EFT techniques, I will first prove a powerful no-go theorem that puts severe constraints on how much the cosmological evolution of our universe can violate the null energy condition (NEC) of general relativity. While rather constraining, this theorem still leaves room for scalar theories, the so-called Galileons, that can lead to (a limited amount of) NEC villation. I will argue that this fact can breathe new life into non-anthropic approaches to the old cosmological constant problem, making it possible for the initially large cosmological constant to be reduced down to the tiny observed value by the pre-inflationary dynamics of the universe. The Galileon scalars have a natural embedding into a more fundamental theory: they describe the scalar component of the graviton in theories of massive gravity. The latter are well-defined (quantum) effective field theories at astrophysical and cosmological distances, in particular providing a natural candidate for dark energy at late times. An uncomfortable feature of massive gravity, however, is that it becomes quantum-mechanically strongly coupled below a macroscopic distance scale of approximately 1000 km. In the final part of the talk, I will discuss why it has been hard to find a “Higgs mechanism” for the theory and argue in favour of a particular short-distance completion, hinted by holography.


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