Self-interacting Dark Matter Energy Density

We investigate cosmological implications of an energy density contribution arising by elastic dark matter self-interactions. Its scaling behaviour shows that it can be the dominant energy contribution in the early universe. Constraints from primordial nucleosynthesis give an upper limit on the self-interaction strength which allows for the same strength as standard model strong interactions. Furthermore we explore the cosmological consequences of an early self-interaction dominated universe. Chemical dark matter decoupling requires that self-interacting dark matter particles are rather light (keV range) but we find that super-weak inelastic interactions are predicted by strong elastic dark matter self-interactions. Assuming a second, collisionless cold dark matter component, its natural decoupling scale exceeds the weak scale and is in accord with the electron and positron excess observed by PAMELA and Fermi-LAT. Structure formation analysis reveals a linear growing solution during self-interaction domination, enhancing structures up to ~ 10^(-3) solar masses long before the formation of the first stars.

The Cosmological QCD Phase Transition Revisited

The QCD phase diagram might exhibit a first order phase transition for large baryochemical potentials. We explore the cosmological implications of such a QCD phase transition in the early universe. We propose that the large baryon-asymmetry is diluted by a little inflation where the universe is trapped in a false vacuum state of QCD. The little inflation is stopped by bubble nucleation which leads to primordial production of the seeds of extragalactic magnetic fields, primordial black holes and gravitational waves. In addition the power spectrum of cold dark matter can be affected up to mass scales of a billion solar masses. The imprints of the cosmological QCD phase transition on the gravitational wave background can be explored with the future gravitational wave detectors LISA and BBO and with pulsar timing.

Cosmological implications of a Dark Matter self-interaction energy density

We investigate cosmological constraints on an energy density contribution of elastic dark matter self-interactions characterized by the mass of the exchange particle and coupling constant. Because of the expansion behaviour in a Robertson-Walker metric we investigate self-interacting dark matter that is warm in the case of thermal relics. The scaling behaviour of dark matter self-interaction energy density shows that it can be the dominant contribution (only) in the very early universe. Thus its impact on primordial nucleosynthesis is used to restrict the interaction strength, which we find to be at least as strong as the strong interaction. Furthermore we explore dark matter decoupling in a self-interaction dominated universe, which is done for the self-interacting warm dark matter as well as for collisionless cold dark matter in a two component scenario. We find that strong dark matter self-interactions do not contradict super-weak inelastic interactions between self-interacting dark matter and baryonic matter and that the natural scale of collisionless cold dark matter decoupling exceeds the weak scale and depends linearly on the particle mass. Finally structure formation analysis reveals a linear growing solution during self-interaction domination; however, only non-cosmological scales are enhanced.

Strangeness in Astrophysics and Cosmology

Some recent developments concerning the role of strange quark matter for astrophysical systems and the QCD phase transition in the early universe are addressed. Causality constraints of the soft nuclear equation of state as extracted from subthreshold kaon production in heavy-ion collisions are used to derive an upper mass limit for compact stars. The interplay between the viscosity of strange quark matter and the gravitational wave emission from rotation-powered pulsars are outlined. The flux of strange quark matter nuggets in cosmic rays is put in perspective with a detailed numerical investigation of the merger of two strange stars. Finally, we discuss a novel scenario for the QCD phase transition in the early universe, which allows for a small inflationary period due to a pronounced first order phase transition at large baryochemical potential.