Spherical top-hat collapse of a viscous unified dark fluid

In this paper, we test the spherical collapse of a viscous unified dark fluid (VUDF) which has constant adiabatic sound speed and show the nonlinear collapse for VUDF, baryons, and darkmatter, which are important in forming the large-scale structure of our Universe. By varying the values of the model parameters alpha and zeta(0), we discuss their effects on the nonlinear collapse of the VUDF model, and we compare its result to the Lambda CDM model. The results of the analysis show that, within the spherical top-hat collapse framework, larger values of a and smaller values of zeta 0 make the structure formation earlier and faster, and the other collapse curves are almost distinguished with the curve of Lambda CDM model if the bulk viscosity coefficient zeta(0) is less than 10(-3).