The Planck time is the furthest back in time that we can go before quantum gravitational effects become significant. It has been suggested that inflation may have occurred when the universe was just 10−43 seconds old. This is precisely the era when superstring theory is supposed to be important.

More significantly, at least for our discussion, is that all the spatial dimensions of the universe would have had similar sizes. We discussed some of the theoretical arguments as to why there may be higher dimensions in Chapter 6. The existence of these extra dimensions has always been viewed as a problem. We know that the universe today contains only three large space dimensions, so why have the others remained so small that they cannot be seen? In other words, why is it that only three of the dimensions have grown to cosmological sizes? What is it that prevents the universe from having a different number of large dimensions?

Inflation, by its very nature, increases the size of the universe by a huge factor in a very small amount of time. We might therefore expect it to shed some light on the question of why three of the dimensions in the universe are so much larger than the others. The problem of small dimensions would be solved if only three of them were able to inflate to large sizes.