## Seminar series devoted to tunneling in QFT

Monthly on Thursdays, usually @ 14:00 for Central Europe (CEST in European summer and CET in winter)

Whether it be vacuum stability, phase transitions, or analogue quantum systems, tunneling is part and parcel of quantum field theory. In this seminar series we explore new developments in our understanding of these phenomena.

Please register on this Indico page to receive Zoom joining instructions plus a reminder for each talk.

### Upcoming seminars

**Patrick Draper**(Illinois University, Urbana):*Tunneling with Time Dependence*I will discuss semiclassical methods for studying bubble nucleation in models with parameters that vary slowly in time. Introducing a more general rotation of the time contour allows access to a larger set of final states, and typically a non-Euclidean rotation is necessary in order to find the most relevant tunneling solution. The method is straightforward to implement for thin wall bubble nucleation. I also describe some examples where gravitational effects are included, and give one example of an exact instanton solution in a time-dependent Kaluza-Klein cosmology.**Lorenzo Ubaldi**(Jožef Stefan Institute and Ljubljana University):

### Previous seminars

(Previous talks are uploaded at www.youtube.com/@TunnelingQFT )

**Silvia Pla Garcia**(Kings College, London): 8 February 2024*Can quantum tunnelling induce a cosmic bounce?*If we imagine rewinding the universe to early times, the scale factor shrinks and the existence of a finite spatial volume may play a role in quantum tunnelling effects in a closed universe. In this talk, I will present a novel mechanism to induce a cosmological bounce that is purely generated by quantum fluctuations without the need for exotic matter or modified gravity. The quantum-induced bounce emerges when we allow a scalar field to tunnel between two degenerate minima. I will also explain how the picture can change in the presence of anisotropies, identifying a criterium for a successful bounce in terms of the size of the closed universe and the properties of the quantum field.**Yutaro Shoji**(Jožef Stefan Institute and Ljubljana University): 14 March 2024*Gauge invariance and gauge zero modes of bubble nucleation rates*A precise computation of a vacuum decay rate requires the determination of the prefactor in front of the exponential suppression factor. When the decay is driven by charged scalar field(s), the prefactor includes the functional determinants of the gauge boson and the Faddeev-Popov fluctuation operators. They depend on the gauge fixing parameter non-trivially and it has not been clear how this dependence cancels out in the final results. We have explicitly proven that the functional determinants become independent of the gauge parameter for a general setup with multiple scalar and gauge fields.

The subtraction of the gauge zero modes is necessary to make the functional determinant finite. The path integral over these zero modes is translated into an integration over the collective coordinates, requiring a Jacobian. Due to the gauge fixing terms, the Jacobian becomes non-trivial and gauge-dependent. We have determined the correct Jacobian, which makes it possible to calculate the prefactor for general scalar and gauge fields.**Ian Moss**(Newcastle University): 11 January 2024*The hot big bang in a cold gas*False vacuum decay is a fascinating theoretical prediction of non-perturbative quantum field theory, and one that has many implications for the early universe, ranging from the formation of matter to the origin of the universe as a quantum event. I’ll explain some of the basic ideas, and discuss the first observations of thermal vacuum decay in an atomic condensate.

### Organisers

- Andreas Ekstedt (Uppsala University)
- Oliver Gould (University of Nottingham)
- Miha Nemevšek (Jožef Stefan Institute and University of Ljubljana)

Registration

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