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 make 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 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.

## Hubble tension and dark energy

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 can potentially 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 occurred in the dark sector. The energy scale of the required phase transition is similar to the scale of neutrino masses, and the NEDE phase transition may be 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 (see f.ex. [1] [2] [3] [4] [5])

## Quantum gravity

Other directions of our research concern inflation and quantum gravity. Three fundamental problems in cosmology, the black hole information problem, the quantum origin of our Universe, and the cosmological constant problem/dark energy, all involve infrared issues in quantum gravity. Assuming that this links together these problems in quantum gravity, we aim to explore their interconnection (see f.ex. [6] [7] [8] [9] [10] [11]).

## Primordial inflation and the curvaton

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

In 2001 M. S. Sloth was one of the original proposers of the curvaton mechanism as the origin of the CMB fluctuations and the Large Scale Structure of the universe. With more precise CMB and Large Scale Structure observations, data are reaching a precision that will enable us to discriminate between single-field inflation models and the curvaton model. Future precise measurements of local non-Gaussianity and the tensor-to-scalar ratio will enable us to verify or exclude the curvaton model over single-field inflation. Today early-time solutions to the Hubble tension, such as NEDE, require a bluer spectrum of primordial perturbations than ΛCDM and therefore prefer the curvaton mechanism as the explanation for the origin of the universe over Starobinsky single-field inflation (see f.ex. [12] [13] [14]).

## Dark matter

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) (see f.ex. [15] [16] [17]).

The full publication list of Martin S. Sloth can be found here: list of publications HEP-INSPIRE, Google Scholar profile.