Research

We are working on understanding the most fundamental problems pertaining to our understanding of the Universe; the origin of the Universe, the evolution of the Universe, the matter contents and the forces which govern the Universe.

The known matter and radiation, as we know it from the periodic table and electromagnetic radiation, only makes up about 5% of the energy density of the universe. The remaining 95% is in the form of Dark Matter and Dark Energy. Currently we do not have a firm understanding of what Dark Energy and Dark Matter are, and of their origin. However, from its gravitational effect on the visible matter, we know this dark sector of Dark Energy and Dark matter exists, and makes up most of the energy density in the universe. In order to understand our Universe, we want to understand the dark sector.

Recently, an inconsistency in the measurement of the expansion rate of the universe has appeared, the Hubble tension. Using astronomical observations, like the redshift and luminosity distance relation for stars and supernovae, we can measure the expansion rate of the universe in a model independent way. On the other hand, if one assumes a Universe consisting only of the simplest form of Dark Energy (a cosmological constant Λ), the simplest form of Dark Matter (Cold Dark Matter), and then ordinary matter, i.e. if one assumes the ΛCDM model, then one can also infer the expansion rate from measurements of the Cosmic Microwave Background (CMB). But the most recent measurements indicate that the two ways of measuring the expansion rate do not agree, and this disagreement is what cosmologists call the Hubble tension. It is an indication that the simple ΛCDM model is too simple.

We have shown that a simple phase transition in the dark sector has the potential to resolve the Hubble tension. We call this proposal New Early Dark Energy (NEDE). Phase transitions happen at all energy scales in nature, and one is even involved in making your espresso. Thus, it is natural to think, that a phase transition could have taken place in the dark sector. The energy scale of the required phase transition is similar to the scale of neutrino masses, and it is possible that the NEDE phase transition is the mechanism by which the neutrinos acquired their mass, just like the remaining massive particles in the Standard Model of particles got their mass in the Electro-Weak phase transition. This is currently our main direction of research, to explore and understand NEDE as a solution to the Hubble tension.

Other directions of our research concerns inflation and quantum gravity. Three of the fundamental problems in cosmology, the black hole information problem, the quantum origin of our Universe, and the cosmological constant problem/dark energy, all involves infrared issues in quantum gravity. Assuming that this links together these problems in quantum gravity, we aim at exploring their interconnection.

Inflation is considered the most succesful paradigm for the origin of the universe from Big Bang, but the underlying theory of inflation is still poorly understood. Much of our work is also focussed on understanding the theory of inflation and minimal extensions/alternatives, such as the curvaton mechanism.

Another problem concerns the nature of dark matter. Given that we have not seen any conclusive evidence of any new physical properties of dark matter beyond the purely gravitationally interacting cold dark matter, we are developing a minimal model of only gravitationally interacting dark matter, which is the darkest and coldest possible form of dark matter, called Planckian Interacting Dark Matter (PIDM).

The full publication list of Martin S. Sloth can be found here: list of publications.