Nonunitarity problem in quantum gravity corrections to quantum field theory with Born-Oppenheimer approximation

Gioia, Federico Di; Maniccia, Giulia; Montani, Giovanni; Niedda, Jacopo

doi:10.1103/physrevd.103.103511

Cited by 13 publications

(15 citation statements)

References 23 publications

(121 reference statements)

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“…This feature must be taken into account when discussing the energy conditions, as done in [12], since the fluid itself is not a source for gravity at the first order. Furthermore, we show that there exists a connection between the reference fluid procedure and the kinematical action path presented previously in [35]: Indeed, when one implements only the Gaussian time condition (i.e., the reference fluid is an incoherent dust, not transporting heat), the resulting dynamics for the matter sector corresponds exactly to the results obtained via the kinematical action insertion in the homogeneous case, when the component N i of the deformation vector is set to zero.…”

Section: Introductionsupporting

confidence: 63%

“…In Sec. III, we introduce the reference fluid model and perform its expansion via the WKB procedure, showing the analogy with the kinematical action insertion proposed in [35] and demonstrating that the homogeneous (mini-super-space) case corresponds to the choice of a fluid without heat conduction. In Sec.…”

Section: Introductionmentioning

confidence: 91%

“…In [35], a detailed discussion and criticism of this type of approach was presented, and a different proposal has been inferred. In fact, instead of deriving the time evolution of the matter wave function from its parametric dependence on the classical (say better, quasiclassical) gravitational variables, there the so-called "kinematical action" was added to the dynamics introduced in [5] to covariantize a quantum field theory on a classical background and so outlining the natural constraints of the theory.…”

Section: Introductionmentioning

confidence: 99%

“…In [36], the kinematical action was also implemented in a fully quantum gravity scenario to provide a time evolution for the wave function, and it was also shown that, in the classical limit, it acquires the morphology of a physical fluid. In [35], the kinematical action does not appear in the classical limit of gravity, since it is associated with a matterlike variable in the BO approximation. This way, the kinematical action simply provides the proper time evolution of quantum field theory; the quantum gravity corrections are no longer affected by nonunitarity problems.…”

Section: Introductionmentioning

confidence: 99%

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Quantum gravity corrections to the matter dynamics in the presence of a reference fluid

Maniccia

Montani

2022

Phys. Rev. D

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We analyze the effect induced on standard quantum field theory (in a functional approach) by quantum gravity corrections to a pure classical background. In the framework of the Kuchař and Torre proposal for a gravity-matter theory constrained to a Gaussian reference frame, materialized as a fluid in the system evolution, we consider a Born-Oppenheimer separation of the system, regarding the gravity degrees of freedom as the slow varying component and the matter plus the Gaussian fluid as fast quantum coordinates. The slow gravity component obeys the Wheeler-DeWitt equation, and we consider a Wentzel-Kramer-Brillouin expansion of its quantum dynamics via a Planckian parameter. The main issue of the proposed scenario is that, on one hand, we recover a modified quantum field theory in the presence of a Hermitian Hamiltonian (not affected by nonunitarity as in other approaches) and, on the other hand, we get the Gaussian fluid as a physical clock for such an amended quantum theory (verifying the correct energy conditions). We also show its equivalence with the kinematical action method, used in a previous work, in the homogeneous setting. Then, we implement the proposed paradigm to describe the dynamics of a homogeneous free massless scalar field, living on an isotropic universe, in the presence of a cosmological constant. We completely solve the dynamics up to the first order correction in the Planckian parameter to the standard quantum field theory. We determine the explicit form of the modified scalar field wave function, due to quantum features of the cosmic scale factor evolution. Phenomenological considerations and discussions are provided.

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Section: Introductionsupporting

confidence: 63%

Section: Introductionmentioning

confidence: 91%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Quantum gravity corrections to the matter dynamics in the presence of a reference fluid

Maniccia

Montani

2022

Phys. Rev. D

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“…In ref. [20], the authors discuss the problem of time and non-unitarity emerging in an adiabatic, WKB approach concerning quantum gravity corrections to quantum field theory.…”

Section: Jhep12(2021)165mentioning

confidence: 99%

Probability distribution for the quantum universe

Kehagias¹,

Partouche

Toumbas

2021

J. High Energ. Phys.

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We determine the inner product on the Hilbert space of wavefunctions of the universe by imposing the Hermiticity of the quantum Hamiltonian in the context of the minisuperspace model. The corresponding quantum probability density reproduces successfully the classical probability distribution in the ħ → 0 limit, for closed universes filled with a perfect fluid of index w. When −1/3 < w ≤ 1, the wavefunction is normalizable and the quantum probability density becomes vanishingly small at the big bang/big crunch singularities, at least at the semiclassical level. Quantum expectation values of physical geometrical quantities, which diverge classically at the singularities, are shown to be finite.

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Unitarity, clock dependence and quantum recollapse in quantum cosmology

Gielen

Menéndez-Pidal

2022

Class. Quantum Grav.

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We continue our analysis of a quantum cosmology model describing a ﬂat Friedmann–Lemaˆıtre–Robertson–Walker universe ﬁlled with a (free) massless scalar ﬁeld and an arbitrary perfect ﬂuid. For positive energy density in the scalar and ﬂuid, each classical solution has a singularity and expands to inﬁnite volume. When quantising we view the cosmological dynamics in relational terms, using one degree of freedom as a clock for the others. Three natural candidates for this clock are the volume, a time variable conjugate to the perfect ﬂuid, and the scalar ﬁeld. We have previously shown that requiring unitary evolution in the “ﬂuid” time leads to a boundary condition at the singularity and generic singularity resolution, while in the volume time semiclassical states follow the classical singular trajectories. Here we analyse the third option of using the scalar ﬁeld as a clock, ﬁnding further dramatic diﬀerences to the previous cases: the boundary condition arising from unitarity is now at inﬁnity. Rather than singularity resolution, this theory features a quantum recollapse of the universe at large volume, as was shown in a similar context by Pawlowski and Ashtekar. We illustrate the properties of the theory analytically and numerically, showing that the ways in which the diﬀerent quantum theories do or do not depart from classical behaviour directly arise from demanding unitarity with respect to diﬀerent clocks. We argue that using a Dirac quantisation would not resolve the issue. Our results further illustrate the problem of time in quantum gravity.

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Nonunitarity problem in quantum gravity corrections to quantum field theory with Born-Oppenheimer approximation

Cited by 13 publications

References 23 publications

Quantum gravity corrections to the matter dynamics in the presence of a reference fluid

Quantum gravity corrections to the matter dynamics in the presence of a reference fluid

Probability distribution for the quantum universe

Unitarity, clock dependence and quantum recollapse in quantum cosmology

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