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Inhomogeneous cosmological models have had significant success in explainingcosmological observations without the need for dark energy. Generally, thesemodels imply inhomogeneous matter distributions alter the observable relationsthat are taken for granted when assuming the Universe evolves according to thestandard Friedmann equations. Moreover, it has recently been shown that bothinhomogeneous matter and pressure distributions are required in both early andlate stages of cosmological evolution. These associated pressure gradients arerequired in the early Universe to sufficiently describe void formation, whilstlate-stage pressure gradients stop the appearance of anomalous singularities.In this paper we investigate the effect of pressure gradients on cosmologicalobservations by deriving the luminosity distance - redshift relations inspherically symmetric, inhomogeneous spacetimes endowed with a perfect fluid.By applying this to a specific example for the energy density distribution andusing various equations of state, we are able to explicitly show that pressuregradients may have a non-negligble effect on cosmological observations. Inparticular, we show that a non-zero pressure gradient can imply significantlydifferent residual Hubble diagrams for $z\lesssim1$ compared to when thepressure is ignored. This paper therefore highlights the need to properlyconsider pressure gradients when interpreting cosmological observations.

The effect of pressure gradients on luminosity distance - redshift relations