The Rocket Science of Bubble Walls

Abstract: Cosmological phase transitions that proceed via nucleation of bubbles are a well-motivated possibility both in minimal extensions of the Standard Model as well as in more general hidden sectors with their own dynamics. Such an event produces a stochastic background of gravitational radiation that could be observed by current and future observatories. That said, our ability to extract any detailed information about the underlying dynamics from such an observation is extremely limited, and highlights the necessity of (i) better understanding the dynamics of bubble walls in the early Universe, given their impact on the high-frequency shape of the signal, and (ii) exploring complementary probes of an observable gravitational wave background.

In this talk, I will discuss the dynamics of expanding vacuum bubbles in the presence of massive vectors (a.k.a. dark photons) that gain mass across the interface -- with a special focus on the well-motivated benchmark where the dark photons furnish the dark matter. I will argue the existence of a transient kinematic regime during which the wall behaves as an (imperfect) mirror of longitudinal -- but not transverse -- modes. Most notably, this leads to a new source of pressure on the expanding interface that can prevent the accelerated expansion of the bubble walls. Instead of accelerating the interface, the difference in vacuum energy density across the wall goes into making a fraction of the dark photons relativistic, turning them into dark radiation. The corresponding value of \Delta N_eff is determined by the strength of the phase transition and the epoch at which the transition takes place. Current \Delta N_eff constraints can rule out phase transitions with strength \alpha \simeq 0.1, whereas CMB S-4 measurements will probe scenarios down to \alpha = 0.01 for all relevant frequencies.

This talk is based on arXiv:2212.xxxxx in collaboration with Giacomo Koszegi and Rudin Petrossian-Byrne.

HEP - phenomenologyHEP - theorymathematical physics

Audience: researchers in the topic

NHETC Seminar

Series comments: Description: Weekly research seminar of the NHETC at Rutgers University

Livestream link is available on the webpage.