Accuracy of the small-scale structure of the Lyman-alpha forest in cosmological hydrodynamical simulations

Confronting measurements of the high-redshift Lyman-alpha forest with cosmological hydrodynamical simulations has proved to be a source of stringent constraints on the thermal state of the intergalactic medium (IGM) as well as the small-scale matter power spectrum. In recent times, however, such investigations have led to inconsistent results. This problem will worsen with the arrival of new measurements of the forest that are expected to be accurate to within 5%. In this talk, I will discuss the robustness of current theoretical models of the Lyman-alpha forest that use hydrodynamical cosmological simulations.  I will focus on the dependence of the Lyman-alpha forest statistics in these models on the assumed initial conditions.  I will discuss the role of glass and grid-based initial conditions, the numerical gravitational coupling between gas and dark matter particles, and cosmological radiation density on the Lyman-alpha forest.  By comparing results from five of the most commonly used initial conditions codes, I will show that models that produce the correct linear theory evolution of the power difference between dark matter and baryons predict a Lyman-alpha flux power spectrum that differs from conventional models by up to 50% at k = 0.1 s/km, at redshifts z = 2--5.  The difference rapidly worsens towards smaller scales and higher redshifts. While this difference is far larger than the measurement uncertainties expected in upcoming datasets, the finding also suggests a path forward towards more accurate models of the Lyman-alpha forest.  I will end my talk by discussing implications for forest-based inferences of the mass of the dark matter particle and the thermal state of the IGM.