During the 20th century, we learned that the Milky Way is but one among countless other galaxies in our Universe. We also learned these galaxies are moving away from one another, a collective cosmic spreading-out that we interpret as the result of space expanding. If we imagine time moving backward, these galaxies will move closer and closer to each other until they end up squeezed into a tiny volume. Matter heats up and breaks down into its elementary components, the particles that make up everything in the Universe. As the squeezing continues, we approach the beginning of everything — the t = 0 of the cosmos.
Of course things are not so simple, as we have seen over the course of this special series. As matter gets squeezed into smaller volumes, we must abandon all hope that the rules of classical physics can describe what is happening. We turn at this point to quantum physics, the physics of the very small. Things now get interesting, but far more speculative.
To push physics into the very early Universe, we need to extrapolate what we currently know into realms that remain unknown to us. Of course, this is always a necessary step to advance knowledge, but there are dangers when we venture into the unknown. If we take a wrong step forward, we can end up getting lost. This is why we turn to the scientific method. It provides an important constraint, limiting viable hypotheses to those that can be tested before they are established as reliable.
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