Safely simplifying redshift drift computations in inhomogeneous cosmologies: Insights from LTB Swiss-cheese models

Abstract

One of the most important discoveries in cosmology is the accelerated expansion of the Universe. Yet, the accelerated expansion has only ever been measured {\em in}directly. Redshift drift offers a direct observational probe of the Universe's expansion history, with its sign revealing whether there has been acceleration or deceleration between source and observer. Its detection will mark a major milestone in cosmology, offering the first direct measurement of an evolving expansion rate. Given its epistemic importance, it is essential to understand how its measurements can be biased. One possibility of bias comes from cosmological structures. However, theoretical estimates of such effects are difficult to obtain because computing redshift drift in general inhomogeneous cosmologies is computationally demanding, requiring the solution of 24 coupled ordinary differential equations. In this work, we use Lemaitre-Tolman-Bondi Swiss-cheese models to show that only the two dominant contributions are needed to achieve percent-level accuracy up to $z = 1$. This allows the reduction of the full system of 24 ODEs to the standard null geodesic equations, significantly simplifying the calculation. \newline\indent Although our analysis is based on idealized Swiss-cheese models with spherical structures, we expect that similar simplifications apply to more complex scenarios, including cosmological N-body simulations. Our analysis thus underpins a practical and robust framework for efficient redshift drift computations applicable to a wider range of inhomogeneous cosmologies.