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Entanglement between pair-created twin universes with opposite time arrows should leave a birthmark on the CMB spectrum
Phys. Rev. D 113, 106025 – Published 22 May, 2026
DOI: https://doi.org/10.1103/z2c7-qm66
Abstract
Why (and how) the Universe was born is one of the ultimate questions in physics. Another big puzzle is about the arrow of time: Why is there only one direction of time? Are these two issues related? One way to solve both puzzles at one stroke is to posit that our Universe was pair-created with a twin, whose time arrow is opposite to ours. If so, then the twins must naturally be quantum entangled. In Euclidean quantum gravity, this implies the existence of a Euclidean wormhole bridging the twin universes. Each universe is then in a mixed state, and the mutual entanglement shall leave signatures in the cosmic microwave background (CMB) power spectrum. Invoking the Klebanov-Susskind-Banks wormhole as a toy model for the sake of tractability, we show that the entanglement selects a novel and unique nonthermal global vacuum for the total inflaton perturbations in both universes. This is equivalent to imposing a simple harmonic oscillator boundary condition on the Euclidean wave function of the total perturbations, and it turns out that the entanglement enhances the CMB power spectrum for long-wavelength modes. Such a birthmark renders our notion refutable.
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Bunch-Davies mode refers to the analytic continuation of Eq. (15), while the analytic continuation of Eq. (16) is the complex conjugate of the Bunch-Davies mode.
