Christian Byrnes

Early universe

Constraining the small scale perturbations in our big universe

Inflation, a description of rapid expansion taking place shortly after the big bang, is the leading paradigm of the very early universe. Even quantum fluctuations are inflated by the expansion and they may become the initial seeds of all perturbations. So inflation links the very smallest (the quantum mechanical) and largest (galactic and cosmic microwave background) scales. The aim of this project is to learn more about inflation.

Today we have precision era measurements of the cosmic microwave background and information about clusters of galaxies, for example from the Planck satellite and soon from the Dark Energy Survey, Sussex University has members of both experiments (Antony Lewis and Kathy Romer respectively). These precision measurements are restricted to about 3 orders of magnitude in length scales, which may sound like a lot. However, another 20-30 orders of magnitude in length scales must also been produced by inflation on smaller scales than those which we have observed and managed to directly relate to inflation. This means that we know very little about the vast majority of the length scales on which quantum mechanical perturbations were produced during inflation. This greatly limits our ability to learn about the physics of the early universe, for example many models of inflation are still an excellent match to all of the observational data.

There are some ways to learn about the smaller scales, however they are currently less well explored. Examples include primordial black holes which can form if the density perturbations in the early universe are large enough. They have never been observed, but searches for them place tight upper bounds on their allowed abundance, which we can translate into a constraint on models of inflation. Other possible observational probes include gravitational waves, tiny ripples in space-time, which are also currently being searched for and small compact objects made up of dark matter (yet not so compact to collapse into a black hole) which might cause light rays to bend and be detected in this way.

The PhD project may involve studying these various ways of observationally constraining the small scale perturbations, comparing their effectiveness and translating the observational constraints into constraints on models of inflation. This will involve both analytical and numerical work, whether the numerical side is a big part of the project will depend on the strengths and interests of the student. There will be good opportunities for traveling to national and international conferences.

If you are interested in this project, feel free to contact me directly at You may also learn more about my work by looking at my website or my list of publications. Also study the early universe PhD project being offered by David Seery in the same group.

Testing models of the early universe

The final results from the Planck satellite were released in 2018. They have already given us our most precise view onto the very early universe, and especially the theory of inflation. This project will probe our understanding of how quantum mechanical perturbations produced during inflation seeds all structures seen in the universe today. Classes of models will be compared to observations and the period of reheating when the universe became hot after inflation ends will also be considered. The aim is to study open questions such as how many light scalar fields were present during inflation, the energy scale of inflation and how inflation ended.

 This project will be primarily analytical but numerical skills are also desirable. Knowledge of cosmology, general relativity and some quantum field theory are all desirable.