Norwegian Participation in Euclid: Early Post-Launch Phase

Cosmology, the study of the Universe as a whole, has come a long way in the last twenty years, mainly because of observatories that have studied the cosmic microwave background and galaxy surveys that have mapped the distribution of galaxies in the Universe. This has given us a model, called the Standard Model of Cosmology, or LCDM, where most of the parameters are determined with a precision unthinkable 20 years ago. However, there are three major issues with the Standard Model. First, it postulates that ordinary matter, made of protons, neutrons and electrons, only make up 16% of the matter density in the Universe. The remaining 84% consists of something called Cold Dark Matter, which we don’t know what is. Secondly, it postulates that the Big Bang started with a so-called inflation phase, when the Universe expanded at an incredible rate. There are many possible theories for inflation, but so far they have few observational constrains. Most mystical at all, in the Standard Model, only 31% of the total energy density in the University is in the form of matter in any form, 69% consists of what we call Dark Energy, which is understood even less than the Dark Matter. However, it is needed to understand the accelerated expansion of the Universe. One possibility is that our gravity theory, general relativity, needs modifications. The satellite Euclid attacks all these three problems, and especially the Dark Energy problem. By studying the shapes of billions of galaxies and by measuring the velocity of about a hundred million galaxies, it maps the expansion of the Universe as a function of time with high accuracy. Euclid, which was adopted by the European Space Agency ESA in 2012, was successfully launched on the 1st of July 2023. After some months' commissioning, testing and calibration, it started its routine operation in February 2024. The first results from test observations have already been published with Norwegian participation. The cosmological results from the first year of survey observations (February 2024 - January 2025) are planned to be published in June 2026. This project financed first preparatory research that enable Norwegian scientists to have a leading role in the analysis of the data, and then to participate in the analysis of real data, which now has started. We have especially a leading role in the use of statistics of voids in the galaxy distribution to set constraints on modified gravity theories. Ph.D. student Sladana Radinovic (funded by the University of Oslo in connection with this project) was first author on a paper by the Euclid consortium with 120 co-authors, which made a study of such constraints. In April 2022 we organized the annual Euclid consortium meeting in Oslo. This consortium meeting, which was the first meeting of the Euclid consortium after the pandemic, was attended by more than 300 in-person participants and almost as many digital participants. The meeting was a great success.

One of the most outstanding problems in physics and astrophysics today is the existence of dark energy, which is inferred from the apparent accelerated expansion of the universe. Dark energy is believed to comprise about 70% of the energy content of the universe today, however, it's nature is largely unknown. The simplest model is to assume that the dark energy is in the form of a cosmological constant. There are, however, good reasons to believe that the nature of the dark energy could be more complicated than this. Euclid is an M-class mission to be launched in late 2022 or early 2023. Its primary objective is to study dark energy, secondary objectives are covering most of cosmology and astrophysics. The main observational targets will be baryonic acoustic oscillations and weak lensing measurements. Euclid was selected by ESA in 2011 as the third M mission in the Cosmic Vision programme, and it was adopted in 2012. This proposal is first of all for, coordinated by the Euclid Cosmological Simulations and Cosmological Theory Science Working Groups, studying the evolution of non-linear structures in the universe in a class of interesting cosmological models with modified gravity, and with massive neutrinos, baryonic physics and non-Gaussian initial conditions, and to compare them with the first-year Euclid data. This research project will give forecasts for how Euclid data best can give constraints, which are vital for the success of the mission. The project will continue building competence in numerical simulations of growth of structure in non-standard cosmological models. This is of utmost importance within the Euclid collaboration, since the comparison between data and theory cannot be performed only using a set of numerical and analytical models for galaxy clustering based on General Relativity and standard cold dark matter. This would limit the scientific return of Euclid and could lead to severe errors of interpretation.