Phase transitions and early Universe simulations

Principal Investigator: Toby Opferkuch


  • Early Universe


The evolution of the Early Universe presents a rich tapestry of phase transitions (PTs), many of which are yet to be observed. These range from the transition marking the end of inflation to the onset of radiation domination, to those well-known in the Standard Model, such as the QCD and Electroweak PTs. Tied to these transitions is a broad spectrum of non-perturbative, non-linear phenomena, demanding numerical approaches for robust theoretical predictions. The objective of this research line is the development of both novel and existing methods to use these phenomena in the early Universe as a laboratory for fundamental high-energy physics. Specifically, the research will focus on two main areas: (i) examining first-order PTs, from bubble nucleation to their expansion dynamics, and (ii) lattice simulations of the early Universe, addressing bubbles from first-order PTs, topological defects, and other phenomena like particle production, field fragmentation, and oscillons.

Status of project and perspectives:

First-order cosmological PTs are ubiquitous in extensions of the SM, examples include warped extra-dimensions, confining SU(N) gauge theories, SM electroweak sector extensions (gauge singlets or otherwise) as well as a whole host of hidden sector models. These PTs source stochastic gravitational wave backgrounds with unique spectra that feature arguably the most intimate connection with fundamental particle physics. The energy scales that we will probe range from keV scales, with existing pulsar timing array experiments, all the way to 106 TeV with future ground based experiments. The latter of which is not only far in excess of far-future collider experiments, but may also be the only means to probe sectors that are coupled only gravitationally to the Standard Model. However, the current state- of-the-art lattice simulations and theory calculations for predicting these signatures suffer from tremendous theoretical uncertainties. It is imperative that sharp theoretical predictions can be made, as without, it will prove challenging to extract PT signals from the array of predicted astrophysical foregrounds.

The second research direction covers a diverse array of phenomena, including simulations of reheating, the dynamics of axion-like particles, and large-volume simulations of cosmic string networks, bubble collisions in first-order PTs and other cosmic defects. These are violent events in the early Universe and represent the best chance for producing observable signatures. A unifying aspect of these processes is the need for precise non-perturbative calculations, usually conducted through (classical) lattice simulations. In this epoch of precision cosmology, developing dependable tools and employing them to generate robust predictions for the cosmological dynamics in various Beyond the Standard Model scenarios is crucial.