Early Universe
 

Key Publications

Synopsis

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In the first moments of its existence, the Universe was filled by a hot dense plasma of relativistic particles scattering with each other at extremely high energies. The early Universe can hence be thought of as a particle physics laboratory that allows us to ponder over the interactions of elementary particles at energies far beyond the reach of terrestrial experiments. To describe the dynamics of the Hot Big Bang and the preceding stage of cosmic inflation, it is thus necessary to follow an interdisciplinary approach combining techniques from particle physics and cosmology. This is accomplished in the discipline of particle cosmology, which aims at a better understanding of the early Universe based on the methods of quantum field theory and string theory. A central ambition of particle cosmology is, in particular, to shine more light on new physics beyond the Standard Model that may have governed the evolution of the early Universe, including phenomena such as:

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1. Cosmic inflation: the stage of exponential expansion prior to the Hot Big Bang responsible for setting the initial conditions of the subsequent cosmological evolution (homogeneity, isotropy, primordial perturbations, etc.). 

2. Preheating and reheating: the transition between cosmic inflation and the Hot Big Bang, i.e., the stage in the expansion history dominated by a thermal bath composed of relativistic radiation.

3. Baryogenesis: the dynamical generation of the primordial asymmetry between matter and antimatter in our Universe starting from matter-antimatter-symmetric initial conditions after cosmic inflation.

4. Dark matter: the mysterious form of matter that reveals its existence through gravitational effects in countless cosmological observations but which otherwise seems to lack any appreciable interaction with Standard Model particles. 

5. Dark energy: the driving force behind the accelerated expansion of the present Universe, which is probably either related to the energy density of the vacuum (i.e., a cosmological constant) or a dynamical scalar field (i.e., quintessence). 

    
While we are broadly interested in all of these topics among several others, questions related to the generation of the baryon asymmetry of the Universe play a particularly important role in our research. In this context, we study a broad range of baryogenesis scenarios, ranging from baryogenesis in grand unified theories over baryogenesis from helical hypermagnetic fields to different models of baryogenesis via leptogenesis. Here, the last scenario refers to the idea that the primordial asymmetry between matter and antimatter is in fact first created in the lepton sector and then transferred to the baryon sector by nonperturbative Standard Model processes in the early Universe. Many leptogenesis models involve heavy sterile neutrinos, which establishes an intriguing connection between early-Universe cosmology and neutrino physics.  

In our work, we explore the phenomenology of existing variants of leptogenesis, including models of thermal, nonthermal, resonant, and spontaneous leptogenesis, and we propose new scenarios for the generation of the baryon asymmtry that come with their own unique phenomenology and predictions for upcoming experiments and observations. An example for the latter would be wash-in leptogenesis, a new leptogenesis scenario that we recently proposed and that generalizes thermal leptogenesis to arbitrary chemical background configurations in the early Universe. The mechanism of wash-in leptogenesis is representative of particle interactions in the early Universe that may collectively be referred to as Big Bang chemistry, the interplay between the chemical potentials of all particle species during the Hot Big Bang. 

Big Bang chemistry is concerned with questions such as:

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1. How does one achieve a chemical composition of the primordial plasma featuring a baryon asymmetry of exactly the right size?

2. What are the phenomenological implications if individual particle species have large chemical potentials in the early Universe?

3. Can we constrain the maximally allowed size of individual chemical potentials in the early Universe based on other considerations?

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The exploration of these questions is partially still in its early stages; the further development of Big Bang chemistry is therefore guaranteed to keep us busy for many years to come.