My interests range from the very fundamental questions -- such as how the Universe began and what is quantum gravity -- to very practical questions such as the nature of dark matter and dark energy.

I am particularly interested in what symmetries can teach us about the microscopic mechanism for cosmological inflation, which is the hypothetical phase of a nearly exponential expansion of the primordial universe, and is of essential importance for our understanding of the observed universe.

I spend a lot of time pondering on how to understand the origin of cosmic microwave background (CMB) temperature perturbations, as well as of seeds for Universe’s large scale structure, and what is the most effective way to unravel their origin. Even though these seeds appear classical, their properties hint to their quantum origin. I am interested in how to extract the physical information from the quantum processes that generated these seeds, and how to resolve the physical effects from spurious (gauge) artifacts.  

Dark matter is usually modeled using phenomenological approaches, which postulate the kinetic Vlasov equation for particle density (in phase space) and the Poisson equation for gravity as the starting point. While these equations are perfectly fine for studying non-relativistic systems of particles evolving in weak gravitational fields, the upcoming measurements are gaining precision and demand a more fundamental approach whose starting point are Lagrangians for matter and gravitational fields. These methods allow for both systematic study of relativistic matter and strong field (non-linear) gravitational corrections. and even genuine quantum effects such as particle pair creation in the presence of steep gradients. These studies are of particular relevance for ultra-light scalar dark matter based on axionic scalar fields, but they are also relevant for our fundamental understanding of the dynamics of Universe’s large scale structure on small (non-linear) scales.

As a member of the Dutch LISA Cosmology Group, I am particularly interested in how gravitational waves can be generated in the early Universe, and in particular at a strong first order electroweak transition. Even though transition in the standard model is a crossover, strong first order transitions are ubiquitous in its simple conformal extensions, in which the Higgs mass parameter is generated by a condensate of a (portal) scalar field via the Coleman-Weinberg mechanism. These models are theoretically appealing, as they provide a simple resolution to the gauge hierarchy problem, the origin of mass, generation of matter-anti matter asymmetry and dark matter and moreover they can be tested by the upcoming experiments.

On a couple of occasions I ventured into trying to understand the smallness of the observed cosmological constant. The more general problem is known as the gravitational hierarchy problem. If Nature exhibits conformal symmetry at large energies, then a small cosmological constant is technically natural (in ‘t Hooft’s sense). Which means that – one created – the gravitational hierarchy does not get destroyed by quantum effects.

For master students: most of my projects are hard and demanding, but also genuinely exciting and potentially highly rewarding. Moreover, during your project you will for sure learn a lot of quantum field theory and gravity that goes well beyond what you have learned in the courses. Let the adventure begin!