Quantum Theory of Cosmological Perturbations  

Quantization of Cosmological Perturbations > s.a. quantum cosmology; quantum-gravity effects on cosmology.
* Idea: In the quantum theory of cosmological perturbations one normally assumes that the pre-inflationary state of the universe was the vacuum of a (scalar) field coupled to gravity, and the observed cosmic microwave background fluctuations are then interpreted as due to quantum fluctuations turned into classical perturbations; The quantum theory itself is trivial, but it is used to give a framework for choosing as initial conditions an appropriate vacuum, like the Bunch-Davies vacuum, a local attractor (because deviations are redshifted away) which gives Gaussian fluctuations.
@ General references: Giovannini CQG(03)ht; Peter et al JCAP(05)ht [including quantum backgrounds]; Pinho & Pinto-Neto PRD(07); Campo & Parentani PRD(08)-a0805 [decoherence and entropy]; Prokopec & Rigopoulos PRD(10) [inflation, path-integral formulation]; Vitenti et al PRD(13)-a1206; Guth a1312-proc [and the multiverse]; Kanno JCAP(14)-a1405 [effect of entanglement]; Castelló et al JCAP(15)-a1503 [gauge-invariant]; Armendáriz-Picón & Şengör JCAP(16)-a1606 [BRST quantization]; Han et al PRD(20)-a2005 [from lqg].
@ Coherence: Giovannini CQG(17) + CQG+ [and Glauber theory]; Giovannini MPLA(17)-a1709.
@ Phenomenology: Mukhanov EPJC(13)-a1303 [predictions and observations]; Kuntz & da Rocha EPJC(19)-a1903 [instability due to runaway modes].
@ Types of matter: Peter et al PRD(16)-a1510 [multiple fluids].

blue bullet In theories of quantum gravity: see perturbations in quantum cosmology.

From Quantum Fluctuations to Classical Perturbations > s.a. decoherence phenomenology; quantum-to-classical transition.
* Idea: The collapse approach uses physics beyond the established paradigm, possibly associated with a quantum-gravity effect à la Penrose, in which the collapse of the state function of the inflaton field would be responsible for the emergence of the primordial inhomogeneities.
@ And observation: Green & Porto PRL(20) [distinguishing classical and quantum initial states].
@ General references: Kiefer et al IJMPD(98)gq; Kiefer & Polarski AdP(98)gq [emergence from the quantum state]; Lombardo PhD(98)gq; Martin LNP(05)ht/04; Pérez et al CQG(06)gq/05; Lyth & Seery PLB(08)ap/06 [after horizon exit]; Armendáriz-Picón et al CQG(09)-a0805 [limits]; Bojowald & Skirzewski ASL(08)-a0808 [effective theory]; Kiefer & Polarski ASL(09)-a0810 [rev]; Sudarsky IJMPD(11)-a0906, IJMPD(11); Dimopoulos JPCS(11)-a1009 [rev]; Malik a1011-conf; Berkhahn et al PRL(11); Martin & Vennin PRD(16)-a1510 [how to prove the quantum-mechanical nature of perturbations]; Ryssens a1907-MS [pilot-wave approach]; Green & Porto a2001 [distinguishing quantum and classical primordial fluctuations]; Berjon et al PRD-a2009; > s.a. early-universe cosmology; inflation.
@ And entanglement: Genovese ASL(09)-a0904; Nelson & Riedel a1704 [squeezing of the quantum state for super-horizon modes]
@ Semiclassical theory: Aslanyan et al JCAP(13)-a1301 [limits on semiclassical fluctuations]; Donà & Marcianò PRD(16)-a1605 [and Dirac fermions].
@ And de Broglie-Bohm quantum theory: Pinto-Neto et al PRD(12)-a1110, PRD(14)-a1309; Goldstein et al a1508 [structure formation and Boltzmann brains].
@ Inflation: Diez-Tejedor et al GRG(12)-a1106 [and loss of symmetries]; Martin et al PRD(12)-a1207 [and the quantum measurement problem]; Singh a1607-fs [model, evolution of the classicality parameter]; Martin Univ-a1904 [and quantum information theory].
@ Collapse of the wave function: De Unánue & Sudarsky PRD(08)-a0801; Diez-Tejedor & Sudarsky JCAP(12)-a1108 [details of approach]; Landau et al PRD(12)-a1112; Das et al PRD(13)-a1304 [continuous spontaneous localization]; León & Sudarsky JCAP(15)-a1503 [statistical characterization]; León et al PRD(15)-a1509 ["self-induced collapse" and primordial gravitational waves]; Stargen & Sreenath a1605.
@ Quantum corrections to correlations: Weinberg PRD(06); van der Meulen & Smit JCAP(07)-a0707.
@ Approaches: de Alwis a1504 [replacing the quantum vacuum with a classical statistical distribution].

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