3.6 Conclusions

The Big Bang model is currently the most robust scientific framework for describing the evolution of the universe. It is consistent with a remarkable set of independent observations: the expansion of galaxies, the cosmic microwave background, the abundance of light elements, and the formation of large-scale structures.

However, an essential distinction must be maintained:

• the Big Bang describes the evolution of the universe from an extremely dense and hot state,
• but it is not sufficient, by itself, to establish the existence of an absolute beginning.

To address this question, we must turn to more general arguments. Two lines of reasoning converge:

• The Borde–Guth–Vilenkin (BGV) theorem indicates that any expanding universe cannot be extended indefinitely into the past.
• The second law of thermodynamics suggests that an eternal universe would already have reached a state of maximum equilibrium—which is not the case for ours.

Thus, even if the physical description of the earliest moments remains incomplete, the idea of a real beginning of the universe appears difficult to avoid.

It is therefore neither irrational nor scientifically unfounded to think that the universe had a beginning: a point of origin approximately 13.8 billion years ago, marked by an extremely dense and hot state. From a philosophical and mathematical perspective, the idea of a truly infinite past also raises difficulties: a genuinely infinite sequence of successive events cannot be “traversed” step by step, yet we have in fact arrived at the present. This suggests that time had a beginning and that the series of past events is not infinite.

On this basis, we will adopt as Axiom 2 the statement that the universe began to exist.

We have also examined why it is reasonable to think that “whatever begins to exist has a cause,” our Axiom 1. It rests on the widely accepted principle of causality—namely, that we do not observe realities arising without prior conditions. Both ordinary experience and scientific practice show that beginnings (births, transformations, phase transitions) occur when causes and constraints are in place. From a logical standpoint, self-causation is incoherent (what begins cannot bring itself into being), and circular causation explains nothing. Quantum phenomena do not undermine this framework: the physical “vacuum” is not nothingness, and random processes (such as decay) still operate under laws and prior states.

If we accept these two axioms, the conclusion of the Kalam cosmological argument¹ follows: the universe has a cause. This cause cannot belong to the universe, since it is the origin of it. It must therefore be immaterial, outside of time, immensely powerful, and intentional. In other words, it has the attributes of a first cause.

Finally, it is useful to recall Occam’s razor²: all else being equal, one should prefer the explanation that relies on the fewest unnecessary assumptions. It is sometimes invoked against the idea of a first cause; yet it applies just as well to cosmological theories competing with the Big Bang that multiply unverified assumptions (for example, certain bounce scenarios that posit ad hoc mechanisms for resetting entropy). As long as no robust observation requires these additions, the standard framework—Big Bang, possibly preceded by a phase of inflation and complemented by ΛCDM—as a beginning of the universe remains the most parsimonious and predictive description. If new data were to challenge it, it would naturally have to be revised: this is how knowledge progresses, through explicit hypotheses, testable predictions, and continual refinement in light of observation.

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