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arXiv:hep-th/0610032 · Journal of Physics A Mathematical and Theoretical

Astrophysical and cosmological tests of quantum theory

Antony Valentini

Published Mar 7, 2007• 53 citations
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Abstract

We discuss several proposals for astrophysical and cosmological tests of quantum theory. The tests are motivated by deterministic hidden-variables theories, and in particular by the view that quantum physics is merely an effective theory of an equilibrium state. The proposed tests involve searching for nonequilibrium violations of quantum theory in: primordial inflaton fluctuations imprinted on the cosmic microwave background, relic cosmological particles, Hawking radiation, photons with entangled partners inside black holes, neutrino oscillations and particles from very distant sources.

DOI
10.1088/1751-8113/40/12/s24

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We discuss several proposals for astrophysical and cosmological tests of quantum theory. The tests are motivated by deterministic hidden-variables theories, and in particular by the view that quantum physics is merely an effective theory of an equilibrium state. The proposed tests involve searching for nonequilibrium violations of quantum theory in: primordial inflaton fluctuations imprinted on the cosmic microwave background, relic cosmological particles, Hawking radiation, photons with entangled partners inside black holes, neutrino oscillations and particles from very distant sources.
We discuss several proposals for astrophysical and cosmological tests of quantum theory. The tests are motivated by deterministic hidden-variables theories, and in particular by the view that quantum physics is merely an effective theory of an equilibrium state. The proposed tests involve searching for nonequilibrium violations of quantum theory in: primordial inflaton fluctuations imprinted on the cosmic microwave background, relic cosmological particles, Hawking radiation, photons with entangled partners inside black holes, neutrino oscillations, and particles from very distant sources.
We show that inflationary cosmology may be used to test the statistical predictions of quantum theory at very short distances and at very early times. Hidden-variables theories, such as the pilot-wave theory of de Broglie and Bohm, allow the existence of vacuum states with nonstandard field fluctuations (``quantum nonequilibrium''). We show that inflationary expansion can transfer microscopic nonequilibrium to macroscopic scales, resulting in anomalous power spectra for the cosmic microwave background. The conclusions depend only weakly on the details of the de Broglie-Bohm dynamics. We discuss, in particular, the nonequilibrium breaking of scale invariance for the primordial (scalar) power spectrum. We also show how nonequilibrium can generate primordial perturbations with nonrandom phases and intermode correlations (primordial non-Gaussianity). We address the possibility of a low-power anomaly at large angular scales, and show how it might arise from a nonequilibrium suppression of quantum noise. Recent observations are used to set an approximate bound on violations of quantum theory in the early Universe.
The primordial fluctuations during inflation are widely believed to seed the Large Scale Structure of the universe. However, it remains unknown whether these fluctuations have a quantum nature. According to the inflation theory, the expanding universe creates particle pairs, which are entangled in their momenta due to momentum conservation. Therefore, cosmological Bell inequalities are well motivated and can be formulated using pseudo-spin operators, constructed from operators involving the momenta of the fluctuations. The violation of these Bell inequalities indicates the quantum nature of primordial fluctuations. However, the cosmological Bell test faces several obstructions, including decoherence and the fast-decaying momenta. We thus study the Bell test on scalar modes and tensor modes with decoherence from the interactions of cosmological perturbations, including the bulk and boundary interaction terms. We explore the possibility of extracting the information of the decoherence rate and the structure of primordial interactions from the Bell test. We also discuss the possibility of constructing Bell tests for the entanglement between polarizations, which may avoid the fast-decaying momenta problem and be another way, besides the tabletop experiments and large gravitational waves detectors, to test the quantum nature of gravity.
[Shortened Abstract:] This thesis explores the possibility that quantum probabilities arose thermodynamically. A chief concern is the detection of primordial `quantum nonequilibrium', since this is observably distinct from textbook quantum physics. Chapters 2, 3, 4, and 5 are adaptations of references [1,2,3], and [4] respectively. Chapter 2 proposes (information) entropy conservation as a minimal requirement for a theory to feature classical-style thermodynamic relaxation. The resulting structure is dubbed `the iRelax framework'. Both classical mechanics and de Broglie-Bohm quantum theory are shown to be special cases. Indications for a possible extension or unification of de Broglie-Bohm theory are briefly highlighted. Chapter 3 examines ways in which quantum relaxation may be prevented. The method of the drift-field is introduced. A systematic treatment of nodes is given. A category of quantum states is found for which relaxation is significantly impeded, and may not complete at all. Chapters 4 and 5 consider the possibility that primordial quantum nonequilibrium may be conserved in the statistics of a species of relic cosmological particle. Necessary factors for this to be the case are discussed and illustrative scenarios are given both in terms of nonequilibrium particles created by inflaton decay, as well as relic vacuum modes for species that decoupled close to the Planck temperature. The search for so-called `smoking-gun' spectral lines created by dark matter decay or annihilation is argued to be a particularly promising setting for the detection of quantum nonequilibrium. Unintuitive spectral effects relating to the contextuality of quantum measurements are described. If such a suspected source of quantum nonequilibrium were found, its subjection to a specifically quantum mechanical test would confirm or deny the presence of the quantum nonequilibrium conclusively.
In this review we first discuss the string theoretical motivations for induced decoherence and deviations from ordinary quantum-mechanical behaviour; this leads to intrinsic CPT violation in the context of an extended class of quantum-gravity models. We then proceed to a description ofprecision tests of CPT symmetry and quantum mechanics using mainly neutral kaons and neutrinos. We also emphasize the possibly unique role of neutral meson factories in providing specific tests of models where the quantum-mechanical CPT operator is not well-defined, leading to modifications of Einstein-Podolsky-Rosen particle correlators. Finally, we discuss experimental probes of decoherence in a cosmological context, including studies of dissipative relaxation models of dark energy in the context of non-critical (non-equilibrium) string theory and the associated modifications of the Boltzmann equation for the evolution of species abundances.
In terms of the de Broglie-Bohm pilot-wave formulation of quantum theory, we develop field-theoretical models of quantum nonequilibrium systems which could exist today as relics from the very early Universe. We consider relic excited states generated by inflaton decay, as well as relic vacuum modes, for particle species that decoupled close to the Planck temperature. Simple estimates suggest that, at least in principle, quantum nonequilibrium could survive to the present day for some relic systems. The main focus of this paper is to describe the behavior of such systems in terms of field theory, with the aim of understanding how relic quantum nonequilibrium might manifest experimentally. We show by explicit calculation that simple perturbative couplings will transfer quantum nonequilibrium from one field to another (for example from the inflaton field to its decay products). We also show that fields in a state of quantum nonequilibrium will generate anomalous spectra for standard energy measurements. Possible connections to current astrophysical observations are briefly addressed.