3.2. The horizon problem

Microwave photons emitted from opposite sides of the sky appear to be in thermal equilibrium at almost the same temperature. The most natural explanation for this is that the Universe has indeed reached a state of thermal equilibrium, through interactions between the different regions. But unfortunately in the big bang theory this is not possible. There was no time for those regions to interact before the photons were emitted, because of the finite horizon size,

(25)

This says that the distance light could travel before the microwave background was released is much smaller than the present horizon distance. In fact, any regions separated by more than about 2 degrees would be causally separated at decoupling in the hot big bang theory. In the big bang theory there is therefore no explanation of why the Universe appears so homogeneous.

In more recent years this problem has been brought into sharper focus through the improving understanding of irregularities in the Universe, as will be discussed later in this article. The same argument that prevents the smoothing of the Universe also prevents the creation of irregularities. For example, as we will see the COBE satellite observes irregularities on all accessible angular scales, from a few degrees upwards. In the simplest cosmological models, where these irregularities are intrinsic to the last scattering surface, the perturbations are on too large a scale to have been created between the big bang and the time of decoupling, because the horizon size at decoupling subtends only a degree or so. Hence these perturbations must have been part of the initial conditions. ⁽²⁾

If this is the case, then the hot big bang theory does not allow a predictive theory for the origin of structure. While there is no reason why it is required to give a predictive theory, this would be a major setback and disappointment for the study of structure formation in the Universe.

² Note though that it is not yet known for definite that there are large-angle perturbations intrinsic to the last scattering surface. For example, in a topological defect model such as cosmic strings, such perturbations could be generated as the microwave photons propagate towards us. Back.