A cosmological model is enormously ambitious. Its aim is to understand the universe as a whole; to explain how galaxies, galaxy clusters and superclusters formed; and to offer a coherent account of their history that agrees with all observations. Cosmological models also fall into several categories. Starting from various extensions and conjectures of general relativity, each capable of defining a geometry, some models focus on the objects found in space and time. Finally, some models focus on the birth of the universe: Big Bang cosmologies. Our view of the universe is limited because observations reach us at a finite speed (via electromagnetic waves and gravitational waves, for example), which means that our field of observation can never encompass more than part of the universe we seek to describe in its entirety.
Making the universe static
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Substantial assumptions are therefore unavoidable. A cosmological model can only provide a possible description of the universe, consistent with all observations. There is no guarantee that such a model is unique. If theorists intend to use local observations—the only ones available to them!—they must also assume that the laws of physics remain constant, an assumption that is far from trivial and cannot be proved. Yet we must accept the paradox of the long-term "variation" of some of the constants used in these models. This is the case with the "standard" cosmological model. Einstein's equation (see the FOCUS in the article "The tensor: an indispensable tool"), published in 1915, had a non-static universe among its solutions, something Einstein could not accept. He introduced an additional "constant" to make the universe static. But the universe is expanding. And to describe every phase of its evolution, the "cosmological constant" must be assigned different values over time!
Within the same standard model, reality is described using two incompatible theories: general relativity for large-scale macroscopic objects, and quantum physics for the microscopic behavior of the contents of spacetime. The result is a clash between continuous and discrete physics.
