In the first instants after birth, the universe might have been multidimensional, like a sheet of paper that suddenly gained thickness. This "thickness"—the extra dimensions — began to inflate rapidly. Such superfast growth is called inflation. It lasted a tiny fraction of a second, but in that time the new dimensions grew to sizes that today we see as our entire vast universe.
Then everything changed. Inflation ended, and the extra dimensions stopped growing. What's more, they shrank to microscopic sizes — about a thousandth of a millimeter. The authors describe this transition as a special scenario: from the end of inflation until the universe cooled enough (the so-called reheating), the extra dimensions behaved quietly. They didn't produce excess ripples in spacetime or elusive particles — hypothetical carriers of gravity that could have flooded the early universe.
Such a delicate balance is possible only if there are one or two extra dimensions. If there were more, inflation would have produced too many unwanted particles. Observations of the cosmic microwave background, ancient light that has reached us from the edges of the visible world, confirm that the spectrum of primordial fluctuations remains nearly scale-invariant. In other words, the density ripples in the universe are the same on all scales, and this agrees only with one or two extra dimensions of micron size.
Thus a coherent picture emerges: from multidimensional inflation to the familiar four-dimensional universe (three spatial plus time), where gravity is barely felt and stars and galaxies are rushing apart. The extra dimensions haven't disappeared: they've just become too small to notice, but their imprint remains woven into the very fabric of the cosmos.
If the extra dimensions were larger than a micron, gravity would behave differently at small distances — for example, two small spheres would attract each other noticeably more strongly than Newton predicts. Such experiments have already been conducted, and so far no deviations have been found.
In Terry Pratchett's novel "The Discworld," the world is a disc on the backs of elephants, but our universe could turn out to be a "bubble" in multidimensional space, where other worlds exist right next door, literally a micron away.