# Astrophysical observations in relativistic inhomogeneous cosmological models

Navn på bevillingshaver

Sofie Marie Koksbang

Institution

I received my PhD from Aarhus University, Institute of physics and astronomy, and am applying for a reintegration fellowship for the University of Southern Denmark at their center for cosmology and particle physics phenomenology: CP3-origins.

Beløb

DKK 1,505,876

År

2019

Bevillingstype

Reintegration Fellowships

## Hvad?

The focus of the project is to develop and study inhomogeneous cosmological models, i.e. models describing the Universe as a whole, including structures on the largest scales (galaxy clusters and voids). The models will be based on exact solutions to Einstein's equation of General Relativity and will be studied in terms of their light propagation qualities. Specifically, different types of mock astrophysical observations based on the light propagation computations will be compared with the spatially averaged evolution of the cosmological models in order to understand how and under what circumstances these are related, especially in the circumstance where the model universe exhibits average accelerated expansion.

## Hvorfor?

Einstein's equation describes how a system (e.g. the Universe) changes in time. It has the special feature that the order of spatially averaging and solving the equation affects the result. Studying the dynamics of the Universe thus requires initial conditions describing the Universe in full detail at a given time. This is not practicable. Hence, the issue is usually ignored altogether by using fully spatially averaged initial conditions. The result is used for interpreting observations but it is not clear that this is valid. The possible error could solve big mysteries such as explaining dark energy and recent observations in conflict with the cosmological standard model. Whether spatial averages indeed can solve these mysteries depends on how spatial averages are related to observations.

## Hvordan?

If you have an exact solution to Einstein's equation you can directly compute how light propagates through that model universe. Almost all astrophysical observations are based on light propagation. The light propagation computations can therefore be used to generate mock observations in the model universe. The caveat is that solving Einstein's equation is extremely difficult for realistic cosmological scenarios. In order to be able to obtain useful solutions to Einstein's equation for the project, I will focus on obtaining solutions which exhibit exactly those features expected to be relevant for the relation between spatial averages and observations and which are expected to be valid in the real universe.