In the first instants after the Big Bang, the Universe expanded faster than light could cross it. This rapid growth, inflation, stretched tiny quantum jitters to the size of entire galaxies. But from what state did these jitters begin? Physicists usually take the simplest one — the Bunch–Davies vacuum, like a clean sheet without wrinkles. The authors of the paper test what happens if the sheet is initially crumpled: they use a more general α-vacuum state.

Calculations in the Starobinsky inflation model showed that the α-vacuum shifts the spectral index (a measure of how unevenly the primordial inhomogeneities are distributed) and its derivatives. Fresh data from the Planck satellite impose very tight constraints on these shifts — the allowed 'crumpledness' of the sheet turns out to be tiny. Furthermore, the authors found that the energy scale of the α-vacuum can be bounded from above by a value on the order of $10^{13}$ GeV (the energy at which inflation ends), and this does not conflict with experiments testing the law of gravity at distances smaller than a millimeter.

Such a result brings us back to the original image: the quantum sheet from which the Universe is cut is almost perfectly smooth. Almost — because it is precisely the slight wrinkles that became galaxies and stars. The new Planck data leave no room for strong initial crumpling, and this narrows the range of possible scenarios for the birth of our world.