Initial conditions leave IR imprints in quantum cosmology

Abstract

We investigate the gravitational path integral of $\mathbb{R} \times S^3$ geometries in Lorentzian signature along with the metric fluctuations which live on the $S^3$ hypersurfaces. Each metric configuration is characterised by lapse, scale factor and fluctuations, and is weighted by the exponential of the Einstein-Hilbert action. A particular boundary requirement for the scale-factor is achievable via an infinite family of covariant boundary actions, each restricting the boundary behaviour of the fluctuation field which cannot be arbitrarily chosen. The paper explores the effect of this infinite family of covariant boundary actions on the wavefunction of Universe to one-loop. These boundary actions are characterized by parameters $c$ and $ω$. Dominant contribution of initial momentum in the path-integral restricts $ω$ to be half-integer, leading to an infinite family of $c$-dependent conjugate momenta. In special case for fixed $ω$, four such $c$-dependent dominant branches of conjugate momenta emerge: two lead to deSitter, one leads to Hartle-Hawking and one to the tunnelling Universe. For all these $c$-dependent initial conjugate momenta, the lapse-saddles are relevant, however only, one of deSitter saddle and Hartle Hawking saddle have well-behaved perturbations. The one-loop wavefunction at these well-behaved saddles is computed systematically by isolating the UV-divergences, where the covariant boundary parameter $c$ acts as a UV-regulator and also helps in shifting the poles of one-loop determinant, away from the real $q_f$ axis for dS saddles, where $q_f$ is the final size of the Universe. UV-finite wavefunction for no-boundary and dS is obtained by adding suitable counterterms. The infrared behaviour of these wavefunctions show not only IR-divergences but also sensitivity toward the initial boundary choices.