The Lyman alpha forest from the spectra of distant sources (e.g. quasars) allows us to measure the statistical spatial distribution of the gas on the largest scales. When modelling the ionisation of gas in the intergalactic medium after reionisation, it is standard practice to assume a uniform radiation background. This assumption is not always valid. Quasars’ beamed geometry and their inhomogeneous distribution cause the intensity of radiation to vary from place to place. The discrete distribution of sources produces local fluctuations that have an effect on the gas density.
In this talk I will describe how the radiation fluctuations affect the 3D power spectrum of the Lyman-alpha forest using an analytic model of radiative transfer – for the first time including the effect of beaming of the quasar light emission. I include a correction of the number density of sources in terms of the probability of a quasar to be detected, and also the direct effects of anisotropic emission on the radiative transfer equations. I will discuss prospects for observing these effects.