theory itself seems to be stable [@HiLi89; @Ma95].

In this article, we aim to investigate the effects that bulk viscosity has on the Early Universe. We consider a background corresponding to a FRW model filled with ultra-relativistic viscous matter, whose bulk viscosity and equation of state have been deduced from recent heavy-ion collisions experiments and lattice QCD simulations.

The present paper is organized as follows. The basic equations of the model are written down in Section \[field\]. In Section \[approx\] we present an approximate solution of the evolution equation. Section \[part1\] is devoted to one particular solution, in which we assume that $H=const.$ The results and conclusions are given in Sections \[final\] and  \[final2\], respectively.

Evolution equations {#field}
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We assume that geometry of the early Universe is filled with a bulk viscous cosmological fluid, which can be described by a spatially flat FRW type metric given by $$\label{1}
ds^{2}=dt^{2}-a^{2}\left( t\right) \left[ dr^{2}+r^{2}\left( d\theta
^{2}+\sin ^{2}\theta d\phi ^{2}\right) \right] .$$ The Einstein gravitational field equations are: $$R_{ik}-\frac{1}{2}g_{ik}R=8\pi GT_{ik}.  \label{ein}$$ In rest of this article, we take into consideration natural units, i.e., $c=1$, for instance.

The energy-momentum tensor of the bulk viscous cosmological fluid filling the very early Universe is given 