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Astrophysical observations in relativistic inhomogeneous cosmological models

Carlsberg Foundation Reintegration Fellowships


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.


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.


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.


My project concerns fundamental physics. This research field pushes the boundaries of human capability, with results made available for all to explore and utilize for the benefit of society. Examples of this e.g. include the World Wide Web invented at CERN, the GPS which requires relativity theory to work properly, cameras in smart phones which are based on technology e.g. developed by NASA for astrophysics observations, and the technology of gravitational waves detectors, e.g. being considered for making better earthquake detectors.

The theme of my research is directly related to the three latter examples but what technological advances my research will help drive is hard to predict. It is clear, however, that society benefits hugely from research in the field of fundamental physics.