**THEORY PROGRAMME – Theoretical Cosmology**

**Project leader: Sami Nurmi **Personnel

**Project leader: Sami Nurmi**Personnel

##### Background

The goal of the project is to connect the observed properties of our universe to fundamental theories of matter and gravity. We study the origin and evolution of the universe using a complementary set of theoretical methods, opening new connections between particle physics and cosmology. The experimental context of the project is extremely topical, covering the currently most exciting enquiries of fundamental physics and cosmology. Interpretation of the data requires an extensive combination of theoretical methods: the project task groups consist of internationally recognised experts in inflationary theory, dark matter modelling, cosmological phase transitions and out-of-equilibrium quantum phenomena, structure formation simulations and gravitational wave physics.

##### Research plan

**Quantum origins of the universe**: The structure of the universe seems to originate from quantum processes during inflation, an epoch dominated by scalar matter of yet unknown microscopic nature. We adopt a bottom-up approach, asking if inflation can be realised in weakly coupled extensions of the Standard Model (SM) of particle physics which can be studied using perturbative QFT methods and an effective theory of gravity. A major question is if the Higgs could be the inflaton field: we study the strong coupling problem, observational signatures and reheating in the Higgs inflation model. Investigation of the electroweak vacuum stability during inflation has now become mainstream; we were the first to develop a consistent curved space QFT approach to the problem. We use the primordial vacuum stability to test and constrain theoretically motivated SM extensions, and study how the stability bounds depend on details of the inflationary dynamics. Our third main research line is the study of new signals of primordial physics that can be tested by future surveys of the Cosmic Microwave Background (CMB) and the Large Scale Structure (LSS) of the universe. In particular, we are developing a numerical code package to investigate the CMB spectral distortion and primordial black hole signals from quantum diffusion dominated regimes of inflation.

**Out-of-equilibrium quantum fields and dark matter theory**: Many important phenomena in the early universe involve complicated out-of-equilibrium quantum systems. We are world leaders in developing quantum transport theory methods for cosmological applications. This includes the semi-classical baryogenesis method and the coherent quasiparticle approximation which we are applying to study the electroweak baryogenesis and the reheating process after inflation. The sensitivity of dark matter (DM) direct detection surveys has already ruled out the most natural parameter space of thermal relics, suggesting that the DM particles may never have been in equilibrium with the visible matter. The DM properties can then depend on primordial physics during inflation and reheating. We study the primordial DM production in various extensions of the SM and investigate how such DM components can be tested observationally through precision surveys of structure formation.

**Large scale structure**: Future LSS surveys and 21 cm tomography will map the gravitational growth of structures over the history of the universe to unprecedented accuracy. This will launch a new epoch in testing the composition of the universe, theory of gravity, nature of the dark energy and dark matter components and details of primordial perturbations. Theoretical understanding of the high precision data requires extended numerical simulations of structure formation. We are running isolated merger simulations and simulations in a full cosmological setting that describe the formation and evolution of galaxies. These simulations will also be used to probe dark matter properties, especially at the scales relevant for low-mass dwarf galaxies. The quantitative effect of large scale inhomogeneities on the expansion rate of the universe and on the extraction of cosmological parameters from the data is not yet completely understood. In particular, we are studying if this could be the source of the tension between the local and high redshift determination of the Hubble rate.

**Gravitational waves**: The gravitational wave astronomy initiated by the LIGO interferometer ob-servations will be brought to a new frequency range by the space-based LISA survey. We are part of the LISA consortium and the LISA astrophysics and cosmology working groups. Our group is running world leading simulations of various astrophysical systems: our next goal is to calculate the gravitational wave signal from supermassive binary black holes in resolved large-scale numerical simulations. The LISA survey also opens up an entirely new window to early universe non-equilibrium phenomena, most notably electroweak scale phase transitions. We are leading experts in the study of gravitational waves from phase transitions and topological defects. On this front, our goal is to improve the modelling of the gravitational wave signal from first-order phase transitions, and establish phenomenological understanding of the particle physics theories which can give rise to observable signals.

##### Collaboration

We have several ongoing collaborations between the JyU and HU research teams. One of the key goals of the project is to further enhance this co-operation: we promote frequent visits and active researcher exchange between the two teams, and organise joint events such as the annual Finnish cosmology meeting. We are also running a regular international visitor programme and organising schools (next: Summer school in cosmology, Nuuksio 2020) and international workshops (next Finnish-Spanish cosmology workshop, Jyväskylä 2020).

We have excellent international connections through frequent collaboration and research mobility to several high-profile institutes on the field, including CERN, Nordita, University of Sussex, University of Heidelberg, McGill University, Imperial College, Kings College, University of Nottingham, University of Würzburg, APC Paris, DESY, UMass Amherst, Max Planck Institute for Astrophysics (MPA), Max Planck Institute for extra-terrestrial Physics (MPE), University of Durham, University of Cambridge, Columbia University, University of St. Andrews and Dublin City University.