Beyond ΛCDM: Exploring alternatives to the standard cosmological paradigm

Abstract

The highly successful standard model of cosmology is built upon two fundamental assumptions: that structure formation proceeds hierarchically through the gravitational collapse of cold dark matter (CDM), and that the late-time expansion of the Universe is dominated by dark energy in the form of the cosmological constant, Λ. While predictions of the ΛCDM model have survived stringent tests spanning a wide range of scales, the true nature of the dark matter and dark energy remains a mystery. Here, we investigate structure formation in well-motivated, alternative scenarios. In the first half, we consider dark matter in the form of sterile neutrinos rather than CDM. We quantify the abundance, formation times and internal structure of sterile neutrino dark matter haloes, before making a detailed comparison of the properties of their substructures compared to their CDM counterparts. Using a semi-analytic model of galaxy formation, we compare observable differences between sterile neutrino and CDM cosmologies and find that future observations of the high redshift Universe and faint dwarf galaxies in the Local Group can place strong constraints on the sterile neutrino scenario. In the second half, the dark matter is assumed to be CDM, but we modify the underlying theory of gravity according to the f(R) model as an alternative theory for accelerated expansion. We test the commonly-assumed quasi-static approximation in f(R) gravity simulations, confirming its validity for a wide choice of model parameters. We then propose a new method for solving the equations of motion in f(R) gravity simulations. Using a suite of high resolution simulations, we find that the new method greatly accelerates the convergence rate of the solutions, improving the efficiency of these simulations by more than a factor of 20 compared to previous methods. This new method will bring us to a new era for precision cosmological tests of gravity.