Gravity is a remarkably weak force. A small magnet holds a paperclip against the pull of an entire planet. Physicists have long sought the reason for this weakness, and one guess leads us into hidden dimensions. Perhaps gravity 'leaks' into them, losing strength in our three-dimensional world.
In the early universe, these extra dimensions could have been tiny, but then they were inflated to sizes of about a micron, like the thickness of a bacterium. Such a scenario ties together two puzzles: the weakness of gravity and the vastness of the observable cosmos. The problem is that this beautiful picture works only for one or two extra dimensions.
Now the authors impose a new constraint — the 'Trans-Planckian censorship' hypothesis (a ban on observing quantum fluctuations born in an era when energies exceeded the Planck scale). It acts like a strict editor, crossing out most options. It turns out that the allowed parameters of the model are squeezed into a very narrow corridor. Extra dimensions, if they exist, must be exactly micron-sized and obey strict rules.
Thus, the censorship hypothesis does not reject the idea of hidden worlds but demands extreme modesty from them: they remain hidden in the microworld, without violating observable laws of cosmology.
If extra dimensions are about a micron in size, then at distances smaller than a micron, gravity should sharply increase — an effect being tested in laboratory experiments with high-precision measurements of gravitational force.
The idea of hidden dimensions is explored in Isaac Asimov's novel 'The Gods Themselves', where contact with a parallel universe occurs through the exchange of matter.