What Came Before the Big Bang

In general relativity, time is not a backdrop against which events happen. It is part of the structure of the universe, along with space. If you run the expansion backward, both collapse into a single point, and asking what came before that point is roughly like asking what lies north of the North Pole.

There is, however, an important caveat. The collapse into a point is what the theory predicts when pushed to the limits of its applicability. Under those conditions, it stops working: we need a theory that unifies gravity and quantum mechanics, and we do not have one yet.

One group of hypotheses says there was no point at all. Instead, there was a transition from an earlier state. In these models, the universe contracted and then began expanding, and our history is just one cycle in a long sequence.

Another says that expansion never stopped elsewhere. In this picture, regions like ours bud off from an eternally expanding background, and each has its own starting point in time. The question "before" then becomes a question of "outside."

A third approach tries to describe the beginning using quantum theory, where the very concept of time does not appear immediately. In these models, there is a boundary, but it does not resemble a moment: time emerges gradually, as a direction rather than a starting point.

The rating is low not because the hypotheses are poor. We can observe only what happened after the universe became transparent to light, and we reconstruct everything earlier indirectly. With the tools we have, we cannot distinguish between models of the beginning.

There is hope in two things: traces of very early events in the cosmic microwave background and gravitational waves that pass through plasma opaque to light. Both lines of research are advancing, but they have not yet reached the necessary sensitivity.