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 forVUDF, baryons, and dark matter, which are important in forming the large-scale structure of our Universe. By varying the values of the model parameters α and ζ0, we discuss their effects on the nonlinear collapse of the VUDF model, and we compare its result to the CDM model. The results of the analysis show that, within the spherical top-hat collapse framework, larger values of α and smaller values of ζ0 make the structure formation earlier and faster, and the other collapse curves are almost distinguished with the curve of CDM model if the bulk viscosity coefficient ζ0 is less than 10−3.