PI: Hayley J Macpherson

In cosmology we typically simplify the dynamics of the Universe via the assumption of a homogeneous and isotropic expansion of space-time. While such an assumption is extremely useful and necessary in most application, the dynamics are realistically more complicated. The existence of the large-scale structure of galaxies naturally results in an expansion of space-time which is dependent on both our physical location as well as which direction we observe on the sky. Theoretical descriptions of this complex expansion typically contain far too many degrees of freedom to be useful for observational constraints. These types of inhomogeneity can only be reliably studied in the context of general-relativistic simulations of cosmological structure formation.

In recent work, we explored these theoretical formalisms using numerical simulations which take into account the full complexity of the inhomogeneous expansion of space-time. Using simulations performed on DiRAC systems, we confirmed the simplified `quiet universe’ model as a good description for a realistic inhomogeneous space-time. From these models we were able to reduce the number of degrees of freedom by a factor of two, as well as identify the dominant anisotropic signatures we could hope to detect in future cosmological data.

Figure 1: Snapshots from a general-relativistic simulation of the cosmic web. Left panel shows the density distribution, normalised by the average value, lighter regions are higher density and are the regions where galaxies would reside, while darker regions represent voids. The right two panels show the shear scalar and electric Weyl curvature scalar, respectively, — both of which are exactly zero within usual simplifications in cosmology. These general-relativistic aspects of our universe are crucial for determining the validity of the `quiet universe’ models as well as predicting signatures of anisotropic expansion in cosmological data. Figure adapted from data used in Heinesen & Macpherson (2022).
Categories: 2022 Highlights