Simulations of Cosmological Structure Formation with Nonrelativistic Scalar Fields

by Benedikt Eggemeier
Doctoral thesis
Date of Examination:2022-05-23
Date of issue:2023-01-20
Advisor:Prof. Dr. Jens Niemeyer
Referee:Prof. Dr. Jens Niemeyer
Referee:Prof. Dr. David Marsh
Referee:Prof. Dr. Günter Sigl
Persistent Address: http://dx.doi.org/10.53846/goediss-9676

 

 

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Abstract

English

Nonrelativistic, self-gravitating scalar fields are of current interest in several scenarios of cosmological structure formation. This includes the gravitational clustering of dark matter in the late-time Universe which can be described by a class of nonrelativistic scalar fields, as well as the gravitational fragmentation of the inflaton field after the end of inflation. A well-motivated and promising dark matter candidate is the QCD axion. Large axion density fluctuations can be produced in the post-inflationary scenario leading to the gravitational formation of highly compact objects known as axion miniclusters. They are expected to exist as substructures within the Milky Way at the present time which is of significance for axion detection experiments. The scalar inflaton field is generally assumed to be responsible for a phase of rapid, accelerated expansion in the very early Universe. Subsequently, the Universe may have passed through a long epoch of matter-dominated expansion during which initially small inflaton density perturbations grow and eventually collapse into gravitationally-bound structures. There exists a strong analogy in the nonlinear evolution of the studied scalar field for these two different eras of structure formation. Accordingly, the same numerical methods can be applied to explore the phenomenology of gravitationally-bound objects in both scenarios. In this thesis, we make use of large numerical simulations to study the post-inflationary gravitational clustering of the inflaton field and the gravitational collapse of axion density fluctuations into axion miniclusters. Our simulation results provide the first insights into the mass distribution and morphology of axion miniclusters and reveal the first quantitative predictions of gravitationally-bound inflaton structures in the very early Universe. While the existence of axion miniclusters directly affects searches for the axion, we show that a period of structure formation in the post-inflationary Universe can lead to the production of primordial black holes and to the generation of a stochastic gravitational wave background that might be detectable by future experiments.
Keywords: cosmological simulations; gravitational structure formation; axion dark matter; post-inflationary universe
 
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