Interview with Martin Sahlén

We continue our interview series of Oskar Klein Centre fellows. Today we meet Martin Sahlén, starting his third year around. Martin works in the CoPS, Cosmoparticle Physics Group.

When did you start working for the OKC, and how it is going so far?
I arrived at the Oskar Klein Centre in September 2009, and it has been both enjoyable and stimulating. Much of my time has been spent preparing a computer code to model the cosmological distribution of galaxy clusters in great detail, to be used for a number of projects. At present I am mainly working on utilizing the code in the different projects and preparing resulting articles, so although it’s been slow going periodically things are now coming together. Some good joint projects also appear to be coming together in the near future, which I think will be excellent.

Why did you choose the OKC for doing a postdoc?
I chose the OKC knowing the excellent facilities here, the quality/profile of the group and also the rather generous funding in the Oskar Klein Fellowship. The broad approach of the centre appealed to me and was a strong reason for applying.

What is your field of research? Can you describe the project/projects in which you are involved?

My general field is cosmology, both towards the observational and towards the theoretical. The projects I am involved in centre largely around multi-consistent analysis and testing of beyond-concordance cosmology.

One area is clusters of galaxies, where I work on cosmological constraints from the XMM Cluster Survey (XCS) and the XXL Survey. Both are X-ray surveys of galaxy clusters based on XMM-Newton images. In the cluster context, my
interests include dark energy/modified gravity, neutrinos, and non-gaussianity – the theoretical modeling of such effects and observational constraints from clusters but also based on other probes such as supernovae, the cosmic microwave background, baryon acoustic oscillations, Big Bang nucleosynthesis, etc. Other topics of interest in this vein are testing the distance-duality relation (the factor (1+z)^2 between luminosity distance and angular diameter distance), and quintessence models. My background includes a lot of work on statistical cosmological analysis, so I have an interest in the development and application of such methods also.

Dark energy is one of the central areas of research in the OKC, and one which I have been involved in. For a year I co-ordinated the activities of the Dark Energy Working Group, which was rewarding. The work of the Dark
Energy Working Group has recently been converging into a series of papers on gravitational models where gravitons gain mass, uniquely well-motivated modified gravity models. Observational tests of these models is a hot topic, and the models should particularly be testable using the growth history of cosmic structure. Work is just starting to look at the transfer functions and linear growth of structure in these models, that is, how perturbations
in the different components of the Universe evolve. We are also thinking about analytical approximations to the halo mass function, i.e. the distribution of non-linearly collapsed dark matter halos. Personally, I will particularly be looking at translating such work into constraints based on current data.

What do you find unique for your field of research about the OKC?
Most specifically it is the strong representation in supernova/observational cosmology and the leading research in non-standard gravitational theory.

What is your story before the OKC?
I am from the north of Sweden originally, but lived quite a few years in Stockholm also. I actually spent some time in the same research group as I currently am in, during my undergraduate studies, doing my Masters’ thesis on primordial black holes. After finishing, I went to England and ended up staying there more than six years. First for a year at Cambridge doing Part III Maths, and thereafter at the University of Sussex in Brighton for the DPhil (as the PhD is called there). Those were very rewarding and fun years both academically and personally, but towards the end I did miss home a bit.
Following a brief spat in mathematical biology at the Sanger Institute in Cambridge, I returned back to Sweden and cosmology at the Oskar Klein Centre.

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