... ¹

E-mail address: ansoldi@trieste.infn.it

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... Milotti²

E-mail address: milotti@trieste.infn.it

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... region³

This hypothesis is inessential at this stage: the region could have any shape and we assume a spherical shape because - in a given reference frame - it is invariant under the dynamical evolution that we will assume later on.

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... detail⁴

The interested reader can find in [7] a vey recent study of the formation of ${\mathbb{S}} ^{2} \times {\mathbb{S}} ^{2}$ bubbles in a de Sitter background. Also worth are the issues discussed in [8] about a general formulation of a quantum theory of gravity.

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... well)⁵

$n_0$ is thus a new constant that we introduce as an essential part of the assumption on the statistics of fluctuations. We discuss this constant more in detail in section 6.

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... length⁶

This definition differs from the usual one by the uninteresting factor $\sqrt{2}$.

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... about⁷

We thank the referees for raising this point, thus giving us the opportunity to spend some words about it.

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