2.3. The Homogeneity Feature

The Universe today is strikingly homogeneous on large scales. Observations of the Cosmic Microwave Background (CMB) anisotropies show that the Universe was even more inhomogeneous in the distant past. Because the processes of gravitational collapse causes inhomogeneities to grow with time, an even greater degree of homogeneity must have been present at earlier epochs.

As with the flatness feature, the equations are driving the Universe into a state (very inhomogeneous) that is very different from the way it is observed today. This difference is present despite the fact that the equations have had a very long time to act. To arrange this situation, very special (highly homogeneous) initial conditions must be chosen, so that excessive inhomogeneities are not produced.

One might note that at early times the Universe was a hot relativistic plasma in local equilibrium and ask: Why can't equilibration processes smooth out any fluctuations and produce the necessary homogeneity? This idea does not work because equilibration and equilibrium are only possible on scales smaller than the ``Jeans Length'' (R_J). On larger scales gravitational collapse wins over pressure and drives the system into an inhomogeneous non-equilibrium state.

At early epochs the region of the Universe we observe today occupied many Jeans volumes. For example, at the epoch of Grand Unification the observed Universe contained around 10⁸⁰ Jeans volumes (each of which would correspond to a scale of only several cm today). So the tendency toward gravitational collapse has had a chance to have a great impact on the Universe over the course of its history. Only through extremely precisely determined initial conditions can the amplitudes of the resulting inhomogeneities be brought down to acceptable levels. At the GUT epoch, for example, initial density contrasts

(6)

that are non-zero only in the 20th, 40th or even later decimal places, depending on the scale, are required to arrange the correct degree of homogeneity. So here we have another special feature, the ``Homogeneity Feature'' which is required to make the SBB consistent with the Universe that we observe.