The H I Lyman [alpha] opacity at redshift 2.7 ≤ z ≤ 3.6 Most of the baryonic matter in the Universe resides in a diffuse gaseous phase in between galaxies consisting mostly of hydrogen and helium. This intergalactic medium (IGM) is distributed in large scale filaments as part of the overall cosmic web. The luminous extragalactic objects that we can observe today such as galaxies and quasars are surrounded by the IGM in the most dense regions within the cosmic web. The radiation of these objects contributes to the so called ultraviolet background (UVB) which keeps the IGM highly ionized ever since the epoch of reionization. Measuring the amount of absorption due to intergalactic neutral hydrogen (HI) against extragalactic background sources is a very useful tool to constrain the energy input of ionizing sources into the IGM. Observations suggest that the HI Lyman alpha effective optical depth τ_eff decreases with decreasing redshift which is primarily due to the expansion of the Universe. However some studies find a smaller value of the effective optical depth than expected at the sp…