The Kalachakra description of the universe is quite different from that presented in the other major Buddhist system of metaphysics: abhidharma, or topics of special knowledge. There are, of course, common elements in both, found in non-Buddhist Indian descriptions as well. These include multiple universes each passing through, at different times from each other, beginningless four-part cycles of formation, stabilization, disintegration and being empty, and each universe having a core mountain, Mount Meru, surrounded by continents, heavens and hells. The main differences between the two Buddhist systems concern the specifics of the four-part cycles, and the shape and size of the universe, Mount Meru and the continents.
It is significant that Buddhism offers two descriptions of the universe. Each is valid for a different purpose, and there is no contradiction in having multiple portraits. The description of any phenomenon, then, is dependent on not only the conceptual framework of the author and the audience, but also the use to which that description is put. For instance, we would certainly explain the plans to send a manned mission to Mars in a different manner to the politicians who are deciding the budget than to the engineers who are designing the machinery. Both portrayals of the mission, however, are valid, useful and necessary. Appreciating this point helps us understand voidness. Nothing exists with inherent characteristics on its own side rendering only one correct way to conventionally perceive, apprehend or describe it.
The purpose of the abhidharma picture of the universe is to help practitioners develop discriminating awareness by working with complex systems of multiple variables. The purpose of the Kalachakra version is quite different. It is to provide the Buddhist equivalent of a unified field theory that explains the structure and workings of the cosmos, atoms, the human body and the experience of rebirth in a parallel manner. The need for this unified theory is to provide a comprehensive basis, covering as much of samsara as possible, at which to aim the meditative practices of alternative Kalachakra for gaining liberation and enlightenment.
A description of the external and internal worlds in terms of their unifying parallels reveals the shared underlying basis from which both derive, namely, clear light mind. The winds of karma that provide the impulses for a particular universe to evolve come from the collective karma on the clear light minds of prior beings. These clear light minds remain present during empty eons in between universal epochs. Likewise, the winds of karma that provide the impulses for a specific rebirth to occur arise from the individual karma on the clear light mind of a particular being. That clear light mind also continues during bardo periods in between rebirths.
Meditation in analogy with the cycles through which the external and internal worlds pass and, in particular, in analogy with how each of these cycles periodically returns to its clear light basis provides a means to reach that basis. This is a unique feature of the anuttarayoga tantra technique. Once clear light mind is accessed, it is possible to make the necessary changes, namely, by focusing on voidness, to eliminate the confusion and its instincts that cloud it so that this basis no longer gives rise to the problems and sufferings associated with the external and internal cycles. This is the deepest reason why the proportions and shape of the universe, human body, and the mandala and body of the Buddhafigure Kalachakra are all the same.
From Introduction to the Kalachakra Initiation by Alexander Berzin