PhD defense by Johan Samsing – University of Copenhagen

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PhD defense by Johan Samsing

Title: Dynamics of Stars, Dark Matter and the Universe

Supervisor: Steen Hansen (DARK)

Assessment Committee: Johan Fynbo (DARK), Adrian Jenkins (Durham University), Licia Verde (University of Oslo)

Abstract: For my PhD I have explored our Universe from its smallest scales with stars and galaxies to its largest to understand its cosmological evolution.

On the largest scales, the universe seems to be speeding up faster and faster. Our current cosmological model indicates this acceleration could be due to a component known as dark energy. From observations we find that the Universe consist of about 70% dark energy, but its fundamental properties are completely unknown. To address this question, I did a study on how well we can measure its possible evolution as a function of cosmic time. I especially focused on dark energy which could be present shortly after the big bang as this could strongly challenge our current picture of the Universe. My collaborators and I used temperature measurements of the cosmic microwave background (CMB) observed by the Planck satellite, to look for any variations in this dark component. This work was followed up by how we can measure dark energy at later times. For this we instead used redshift and angular position observations of millions of galaxies. The central problem was here how well we need to know how galaxies move and clump together during cosmic history. We explored a new model independent way of doing this which also seems promising for measuring modifications to the theory of gravity itself. On slightly smaller scales I looked into what happens when two dark matter structures merge. Numerical simulations show that a smaller fraction of the dark matter particles are kicked out during the merger process. In my work I discovered a dynamical mechanism explaining this. The mechanism is a double scattering process where a particle gains energy by undergoing two gravitational deflections during the merger. From this model I can explain recent observations of high velocity stellar systems. I further did a study on how well one can measure the 3D shape of a single dark matter structure. Dark matter structures attract a huge amount of gas during their formations which heats up and emit X-rays. I showed that one can estimate the dark matter structure shape from observation of these X-rays alone. This has implication for mass measurements which can be used for constraining the amount of matter and dark energy we have in our universe.

On even smaller scales I did an interesting study on the interaction between stars and black holes. I especially looked into the interaction where a binary stellar system is hit by a third object. In my calculations I included the possibility for the system to emit gravitational waves (GWs), these waves carry both energy and momentum. By performing millions of scatterings I discovered a new outcome where a binary stellar system quickly merge by sending out GWs.

The two stars merge with very high eccentricity which makes this outcome very unique. My collaborators and I estimated this new outcome to be the most likely way of generating high eccentric binary mergers. This has huge implications for future GW measurements.


Steen H. Hansen

Assessment committee:

Chair: Johan P.U.Fynbo
Opponent: Adrian Jenkins, Durham University
Opponent: Licia Verde, University of Barcelona

Link to the thesis