Quantum Theory: A Two-Time Success Story pp 101-112 | Cite as

# Quantum Weak Measurements and Cosmology

## Abstract

The indeterminism of quantum mechanics generally permits the independent specification of both an initial and a final condition on the state. Quantum pre- and post-selection of states opens up a new, experimentally testable, sector of quantum mechanics, when combined with statistical averages of identical weak measurements. In this paper I apply the theory of weak quantum measurements combined with pre- and post-selection to cosmology. Here, pre-selection means specifying the wave function of the universe or, in a popular semi-classical approximation, the initial quantum state of a subset of quantum fields propagating in a classical background spacetime. The novel feature is post-selection: the additional specification of a condition on the quantum state in the far future. I discuss “natural” final conditions, and show how they may lead to potentially large and observable effects at the present cosmological epoch. I also discuss how pre- and post-selected quantum fields couple to gravity via the DeWitt-Schwinger effective action prescription, in contrast to the expectation value of the stress-energy-momentum tensor, resolving a vigorous debate from the 1970s. The paper thus provides a framework for computing large-scale cosmological effects arising from this new sector of quantum mechanics. A simple experimental test is proposed. [*Editors note*: for a video of the talk given by Prof. Davies at the Aharonov-80 conference in 2012 at Chapman University, see quantum.chapman.edu/talk-13.]

## Keywords

Black Hole Weak Measurement Quantum Cosmology Bogoliubov Transformation Massless Scalar Field## Notes

### Acknowledgements

I have greatly benefited from discussions with Alonso Botero, Jeff Tollaksen and Yakir Aharonov in preparing this paper. I would like to thank Katherine Lee and Saugata Chatterjee for their help reformatting and editing the paper.

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