For much of the history of Western science, the dominant cosmological assumption was that the universe was eternal — that it had always existed, that there was no moment of beginning, and therefore no question about what caused it to begin. An eternal universe is a universe that requires no creator. This was one reason many scientists in the eighteenth and nineteenth centuries preferred it.
That assumption is no longer scientifically tenable.
The evidence for a beginning
Multiple independent lines of evidence now converge on the conclusion that the universe began at a finite point in the past. The expansion of the universe, discovered in the early twentieth century, implies that the universe was once smaller, denser, and hotter — and tracing the expansion backward leads to a state of extreme density at a finite time in the past. The cosmic microwave background radiation — the thermal afterglow of the early universe — provides direct observational confirmation of this early hot dense state. The abundances of light elements in the universe match the predictions of Big Bang nucleosynthesis.
The second law of thermodynamics provides an independent line of argument. In a closed system, entropy increases over time. If the universe were infinitely old, it would have already reached maximum entropy — a state of complete thermal equilibrium, sometimes called heat death. The universe has not reached this state. Therefore it has not existed forever. It had a beginning.
What this means philosophically
The beginning of the universe is not merely a scientific datum. It has philosophical implications that were recognised long before the Big Bang theory was formulated. If the universe began to exist, it began to exist at some point rather than at no point. Its beginning was either caused or uncaused.
An uncaused beginning of the universe — something coming from nothing, with no prior state and no causal conditions — is not merely scientifically controversial. It is philosophically baffling. “Nothing” in the technical sense is not quantum vacuum fluctuation or spacetime foam or a prior universe. It is the absence of anything at all, including space, time, and physical laws. The idea that such a nothing spontaneously produced a universe requires explanation, not mere assertion.
A caused beginning implies a cause that is external to the universe — since the universe includes all matter, energy, space, and time, its cause must be non-physical, non-spatial, and non-temporal. It must also be enormously powerful — powerful enough to bring a universe into existence. And since space and time did not exist before the universe, the cause must be capable of existing and acting without being embedded in space and time — which is what the classical theistic tradition has always claimed about God.
The reaction of physicists
The cosmological implications of a universal beginning were not welcomed by all physicists. Several prominent physicists in the mid-twentieth century actively sought alternatives — steady-state cosmology, bouncing universe models, eternal inflation — partly because they recognised that a universe with a beginning pointed in a theistic direction. Some of them said so explicitly. The steady-state theory’s developers acknowledged that their motivation was partly to avoid the theological implications of the Big Bang.
The evidence, however, continued to accumulate. The cosmic microwave background radiation, discovered in 1965, effectively ended the steady-state model as a serious option. The universe had a beginning. The philosophical implications did not go away simply because physicists preferred they did.