`ISRN Mathematical PhysicsVolume 2013 (2013), Article ID 509316, 17 pageshttp://dx.doi.org/10.1155/2013/509316`
Review Article

## Conceptual Problems in Quantum Gravity and Quantum Cosmology

Institute for Theoretical Physics, University of Cologne, Zülpicher Strasse 77, 50937 Köln, Germany

Received 26 May 2013; Accepted 28 June 2013

Academic Editors: M. Montesinos and M. Sebawe Abdalla

Copyright © 2013 Claus Kiefer. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. C. W. Misner, K. S. Thorne, and J. A. Wheeler, Gravitation, Freeman, San Francisco, Calif, USA, 1973.
2. F. Gronwald and F. W. Hehl, “On the gauge aspects of gravity,” Erice 1995, Quantum Gravity, vol. 10, pp. 148–198, 1996.
3. S. Carlip, “Is quantum gravity necessary?” Classical and Quantum Gravity, vol. 25, no. 15, Article ID 154010, 6 pages, 2008.
4. D. N. Page and C. D. Geilker, “Indirect evidence for quantum gravity,” Physical Review Letters, vol. 47, no. 14, pp. 979–982, 1981.
5. D. Giulini and A. Großardt, “Gravitationally induced inhibitions of dispersion according to the Schrödinger-Newton equation,” Classical and Quantum Gravity, vol. 28, no. 10, Article ID 195026, 2011.
6. C. Kiefer, Quantum Gravity, Oxford University Press, Oxford, UK, 3rd edition, 2012.
7. C. M. DeWitt and D. Rickles, “The role of gravitation in physics,” Report from the 1957 Chapel Hill Conference, Berlin, Germany, 2011.
8. H. D. Zeh, “Feynman’s quantum theory,” European Physical Journal H, vol. 36, pp. 147–158, 2011.
9. M. Albers, C. Kiefer, and M. Reginatto, “Measurement analysis and quantum gravity,” Physical Review D, vol. 78, no. 6, Article ID 064051, 17 pages, 2008.
10. R. Penrose, “On gravity's role in quantum state reduction,” General Relativity and Gravitation, vol. 28, no. 5, pp. 581–600, 1996.
11. T. P. Singh, “Quantum mechanics without spacetime: a case for noncommutative geometry,” Bulgarian Journal of Physics, vol. 33, p. 217, 2006.
12. A. Bassi, K. Lochan, S. Satin, T. P. Singh, and H. Ulbricht, “Models of wave-function collapse, underlying theories, and experimental tests,” Reviews of Modern Physics, vol. 85, pp. 471–527, 2013.
13. T. Padmanabhan, “Thermodynamical aspects of gravity: new insights,” Reports on Progress in Physics, vol. 73, Article ID 046901, 2010.
14. S. W. Hawking and R. Penrose, The Nature of Space and Time, Princeton University Press, Princeton, NJ, USA, 1996.
15. A. D. Rendall, “The nature of spacetime singularities,” in 100 Years of Relativity—Space-Time Structure: Einstein and Beyond, A. Ashtekar, Ed., pp. 76–92, World Scientific, Singapore, 2005.
16. J. Anderson, Principles of Relativity Physics, Academic Press, New York, NY, USA, 1967.
17. H. Nicolai, “Quantum Gravity: the view from particle physics,” In press, http://arxiv.org/abs/1301.5481.
18. M. Zych, F. Costa, I. Pikovski, T. C. Ralph, and C. Brukner, “General relativistic effects in quantum interference of photons,” Classical and Quantum Gravity, vol. 29, no. 22, Article ID 224010, 2012.
19. S. W. Hawking, “Particle creation by black holes,” Communications in Mathematical Physics, vol. 43, no. 3, pp. 199–220, 1975, Erratum ibid, vol. 46, pp. 206, 1976.
20. J. H. MacGibbon, “Quark- and gluon-jet emission from primordial black holes. II. The emission over the black-hole lifetime,” Physical Review D, vol. 44, pp. 376–392, 1991.
21. B. J. Carr, “Primordial black holes as a probe of cosmology and high energy physics,” in Quantum Gravity: From Theory to Experimental Search, D. Giulini, C. Kiefer, and C. Lämmerzahl, Eds., vol. 631 of Lecture Notes in Physics, pp. 301–321, Springer, Berlin, Germany, 2003.
22. W. G. Unruh, “Notes on black-hole evaporation,” Physical Review D, vol. 14, pp. 870–892, 1976.
23. P. G. Thirolf, D. Habs, A. Henig, et al., “Signatures of the Unruh effect via highpower, short-pulse lasers,” The European Physical Journal D, vol. 55, no. 2, pp. 379–389, 2009.
24. A. Strominger, “Five problems in quantum gravity,” Nuclear Physics B, vol. 192-193, pp. 119–125, 2009.
25. S. W. Hawking, “Breakdown of predictability in gravitational collapse,” Physical Review D, vol. 14, no. 10, pp. 2460–2473, 1976.
26. D. N. Page, “Black hole information,” in Proceedings of the 5th Canadian Conference on General Relativity and Relativistic Astrophysics, R. Mann and R. McLenaghan, Eds., pp. 1–41, World Scientific, 1994.
27. D. N. Page, “Time dependence of Hawking radiation entropy,” In press, http://arxiv.org/abs/1301.4995.
28. C. Kiefer, “Hawking radiation from decoherence,” Classical and Quantum Gravity, vol. 18, no. 22, pp. L151–L154, 2001.
29. C. Kiefer, “Is there an information-loss problem for black holes?” in Decoherence and Entropy in Complex Systems, H.-T. Elze, Ed., vol. 633 of Lecture Notes in Physics, Springer, Berlin, Germany, 2004.
30. K. Freese, C. T. Hill, and M. T. Mueller, “Covariant functional Schrödinger formalism and application to the Hawking effect,” Nuclear Physics B, vol. 255, no. 3-4, pp. 693–716, 1985.
31. N. D. Birrell and P. C. W. Davies, Quantum Fields in Curved Space, vol. 7, Cambridge University Press, Cambridge, UK, 1982.
32. H. D. Zeh, “Where has all the information gone?” Physics Letters A, vol. 347, pp. 1–7, 2005.
33. A. Almheiri, D. Marolf, J. Polchinski, and J. Sully, “Black holes: complementarity or firewalls?” Journal of High Energy Physics, vol. 2, article 62, 2013.
34. C. Kiefer, J. Marto, and P. V. Moniz, “Indefinite oscillators and black-hole evaporation,” Annalen der Physik, vol. 18, no. 10-11, pp. 722–735, 2009.
35. C. J. Isham, “Quantum gravity,” in General Relativity and Gravitation, M. A. H. MacCallum, Ed., pp. 99–129, Cambridge University Press, Cambridge, UK, 1987.
36. S. Weinberg, The Quantum Theory of Fields, vol. 1, Cambridge University Press, Cambridge, UK, 1995.
37. H. W. Hamber, Quantum Gravitation—The Feynman Path Integral Approac, Springer, Berlin, Germany, 2009.
38. M. H. Goroff and A. Sagnotti, “Quantum gravity at two loops,” Physics Letters B, vol. 160, pp. 81–86, 1985.
39. Z. Bern, J. J. M. Carrasco, L. J. Dixon, H. Johansson, and R. Roiban, “Ultraviolet behavior of $N=8$ supergravity at four loops,” Physical Review Letters, vol. 103, no. 8, Article ID 081301, 2009.
40. N. E. J. Bjerrum-Bohr, J. F. Donoghue, and B. R. Holstein, “Quantum gravitational corrections to the nonrelativistic scattering potential of two masses,” Physical Review D, vol. 67, no. 8, Article ID 084033, 12 pages, 2003.
41. J. Ambjørn, A. Goerlich, J. Jurkiewicz, and R. Loll, “Quantum gravity via causal dynamical triangulations,” http://arxiv.org/abs/1302.2173.
42. A. Nink and M. Reuter, “On quantum gravity, asymptotic safety, and paramagnetic dominance,” In press, http://arxiv.org/abs/1212.4325.
43. G. Ellis, J. Murugan, and A. Weltman, Eds., Foundations of Space and Time, Cambridge University Press, 2012.
44. G. Amelino-Camelia and J. Kowalski-Glikman, Eds., Planck Scale Effects in Astrophysics and Cosmology, vol. 669 of Lecture Notes in Physics, Springer, Berlin, Germany, 2005.
45. J. M. Pons, D. C. Salisbury, and K. A. Sundermeyer, “Revisiting observables in generally covariant theories in the light of gauge fixing methods,” Physical Review D, vol. 80, Article ID 084015, 23 pages, 2009.
46. J. B. Barbour and B. Foster, “Constraints and gauge transformations: dirac’s theorem is not always valid,” In press, http://arxiv.org/abs/0808.1223.
47. B. S. DeWitt, “Quantum theory of gravity. I. The canonical theory,” Physical Review, vol. 160, pp. 1113–1148, 1967.
48. J. A. Wheeler, “Superspace and the nature of quantum geometrodynamics,” in Battelle Rencontres, C. M. DeWitt and J. A. Wheeler, Eds., pp. 242–307, Benjamin, New York, NY, USA, 1968.
49. J. R. Klauder, “An affinity for affine quantum gravity,” Proceedings of the Steklov Institute of Mathematics, vol. 272, no. 1, pp. 169–176, 2011.
50. C. Vaz, S. Gutti, C. Kiefer, T. P. Singh, and L. C. R. Wijewardhana, “Mass spectrum and statistical entropy of the BTZ black hole from canonical quantum gravity,” Physical Review D, vol. 77, no. 6, Article ID 064021, 9 pages, 2008.
51. A. Ashtekar, “New variables for classical and quantum gravity,” Physical Review Letters, vol. 57, no. 18, pp. 2244–2247, 1986.
52. I. Agullo, G. Barbero, E. F. Borja, J. Diaz-Polo, and E. J. S. Villaseñor, “Detailed black hole state counting in loop quantum gravity,” Physical Review D, vol. 82, no. 8, Article ID 084029, 31 pages, 2010.
53. C. Kiefer and G. Kolland, “Gibbs' paradox and black-hole entropy,” General Relativity and Gravitation, vol. 40, no. 6, pp. 1327–1339, 2008.
54. R. Gambini and J. Pullin, A First Course in Loop Quantum Gravity, Oxford University Press, Oxford, UK, 2011.
55. C. Rovelli, Quantum Gravity, Cambridge University Press, Cambridge,UK, 2004.
56. A. Ashtekar and J. Lewandowski, “Background independent quantum gravity: a status report,” Classical and Quantum Gravity, vol. 21, no. 15, pp. R53–R152, 2004.
57. T. Thiemann, Modern Canonical Quantum General Relativity, Cambridge University Press, Cambridge, UK, 2007.
58. C. Rovelli, “Covariant loop gravity,” in Quantum Gravity and Quantum Cosmology, G. Calcagni, L. Papantonopoulos, G. Siopsis, and N. Tsamis, Eds., vol. 863 of Lecture Notes in Physics, pp. 57–66, Springer, Berlin, Germany, 2013.
59. R. Blumenhagen, D. Lüst, and S. Theisen, Basic Concepts of String Theory, Springer, Berlin, Germany, 2013.
60. A. Strominger and C. Vafa, “Microscopic origin of the Bekenstein-Hawking entropy,” Physics Letters B, vol. 379, no. 1–4, pp. 99–104, 1996.
61. G. T. Horowitz, “Quantum states of black holes,” in Black Holes and Relativistic Stars, R. M. Wald, Ed., pp. 241–266, The University of Chicago Press, Chicago, Ill, USA, 1998.
62. M. R. Douglas, “The statistics of string/M theory vacua,” Journal of High Energy Physics, vol. 5, article 046, 2003.
63. B. J. Carr, Universe or Multiverse?Cambridge University Press, Cambridge, UK, 2007.
64. D. Oriti, Ed., Approaches to Quantum Gravity, Cambridge University Press, Cambridge, UK, 2009.
65. D. H. Coule, “Quantum cosmological models,” Classical and Quantum Gravity, vol. 22, no. 12, pp. R125–R166, 2005.
66. J. J. Halliwell, “Introductory lectures on quantum cosmology,” in Quantum Cosmology and Baby Universes, S. Coleman, J. B. Hartle, T. Piran, and S. Weinberg, Eds., pp. 159–243, World Scientific, Singapore, 1991.
67. C. Kiefer and B. Sandhoefer, “Quantum cosmology,” In press, http://arxiv.org/abs/0804.0672.
68. D. L. Wiltshire, “An introduction to quantum cosmology,” in Cosmology: The Physics of the Universe, B. Robson, N. Visvanathan, and W. S. Woolcock, Eds., pp. 473–531, World Scientific, Singapore, 1996.
69. G. Montani, M. V. Battisti, R. Benini, and G. Imponente, Primordial Cosmology, World Scientific, Singapore, 2011.
70. P. V. Moniz, Quantum Cosmology—The Supersymmetric Per-Spective, vol. 804 of Lecture Notes in Physics, Springer, Berlin, Germany, 2010.
71. M. Bojowald, Quantum Cosmology, vol. 835 of Lecture Notes in Physics, Springer, Berlin, Germany, 2011.
72. M. Bojowald, C. Kiefer, and P. V. Moniz, “Quantum cosmology for the 21st century: a debate,” In press, http://arxiv.org/abs/1005.2471.
73. C. Bastos, O. Bertolami, N. C. Dias, and J. N. Prata, “Phase-space noncommutative quantum cosmology,” Physical Review D., vol. 78, no. 2, Article ID 023516, 10 pages, 2008.
74. O. Bertolami and C. A. D. Zarro, “Hořava-Lifshitz quantum cosmology,” Physical Review D, vol. 85, no. 4, Article ID 044042, 12 pages, 2011.
75. S. P. Kim, “Massive scalar field quantum cosmology,” In press, http://arxiv.org/abs/1304.7439.
76. M. Bojowald, “Quantum cosmology: effective theory,” Classical and Quantum Gravity, vol. 29, no. 21, Article ID 213001, 58 pages, 2012.
77. A. Vilenkin, “Interpretation of the wave function of the Universe,” Physical Review D, vol. 39, pp. 1116–1122, 1989.
78. C. Kiefer, “Wave packets in minisuperspace,” Physical Review D, vol. 38, no. 6, pp. 1761–1772, 1988.
79. E. Calzetta and J. J. Gonzalez, “Chaos and semiclassical limit in quantum cosmology,” Physical Review D, vol. 51, pp. 6821–6828, 1995.
80. N. J. Cornish and E. P. S. Shellard, “Chaos in quantum cosmology,” Physical Review Letters, vol. 81, pp. 3571–3574, 1998.
81. W. H. Zurek and J. P. Paz, “Quantum chaos: a decoherent definition,” Physica D, vol. 83, pp. 300–308, 1995.
82. E. Joos, H. D. Zeh, C. Kiefer, D. Giulini, J. Kupsch, and I.-O. Stamatescu, Decoherence and the Appearance of a Classical World in Quantum Theory, Springer, Berlin, Germany, 2nd edition, 2003.
83. E. Calzetta, “Chaos, decoherence and quantum cosmology,” Classical and Quantum Gravity, vol. 29, no. 14, Article ID 143001, 30 pages, 2012.
84. E. Anderson, “Problem of time in quantum gravity,” Annalen der Physik, vol. 524, no. 12, pp. 757–786, 2012.
85. C. J. Isham, “Canonical quantum gravity and the problem of time,” in Integrable Systems, Quantum Groups, and Quantum Field Theories, L. A. Ibort and M. A. Rodríguez, Eds., vol. 409, pp. 157–287, Kluwer, Dordrecht, The Netherlands, 1993.
86. K. V. Kuchař, “Time and interpretations of quantum gravity,” in Proceedings of the 4th Canadian Conference on General Relativity and Relativistic Astrophysics, G. Kunstatter, D. Vincent, and J. Williams, Eds., pp. 211–314, World Scientific, 1992.
87. C. G. Torre, “Is general relativity an “already parametrized” theory?” Physical Review D, vol. 46, pp. 3231–324., 1993.
88. J. D. Brown and K. V. Kuchař, “Dust as a standard of space and time in canonical quantum gravity,” Physical Review D, vol. 51, no. 10, pp. 5600–5629, 1995.
89. A. Ashtekar and P. Singh, “Loop quantum cosmology: a status report,” Classical and Quantum Gravity, vol. 28, no. 21, Article ID 213001, p. 122, 2011.
90. S. Alexander, M. Bojowald, A. Marciano, and D. Simpson, “Electric time in quantum cosmology,” In press, http://arxiv.org/abs/1212.2204.
91. M. Bojowald, P. A. Höhn, and A. Tsobanjan, “An effective approach to the problem of time,” Classical and Quantum Gravity, vol. 28, no. 3, Article ID 035006, 18 pages, 2011.
92. C. Rovelli, “Time in quantum gravity: an hypothesis,” Physical Review D, vol. 43, no. 2, pp. 442–456, 1991.
93. M. Montesinos, C. Rovelli, and T. Thiemann, “$\mathrm{SL}\left(2,R\right)$ model with two Hamiltonian constraints,” Physical Review D, vol. 60, no. 4, Article ID 044009, 10 pages, 1999.
94. J. B. Barbour, T. Koslowski, and F. Mercati, “The solution to the problem of time in shape dynamics,” In press, http://arxiv.org/abs/1302.6264.
95. J. B. Barbour and B. Bertotti, “Mach's principle and the structure of dynamical theories,” Proceedings of the Royal Society A, vol. 382, no. 1783, pp. 295–306, 1982.
96. J. Barbour, The End of Time, Oxford University Press, New York, NY, USA, 2000.
97. E. Anderson, “The problem of time and quantum cosmology in the relational particle mechanics arena,” In press, http://arxiv.org/abs/1111.1472.
98. C. Kiefer and T. P. Singh, “Quantum gravitational corrections to the functional Schrödinger equation,” Physical Review D, vol. 44, no. 4, pp. 1067–1076, 1991.
99. C. Bertoni, F. Finelli, and G. Venturi, “The Born-Oppenheimer approach to the matter-gravity system and unitarity,” Classical and Quantum Gravity, vol. 13, no. 9, pp. 2375–2383, 1996.
100. A. O. Barvinsky and C. Kiefer, “Wheeler-DeWitt equation and Feynman diagrams,” Nuclear Physics B, vol. 526, no. 1–3, pp. 509–539, 1998.
101. C. Kiefer and M. Krämer, “Quantum gravitational contributions to the CMB anisotropy spectrum,” Physical Review Letters, vol. 108, no. 2, Article ID 021301, 4 pages, 2012.
102. D. Bini, G. Esposito, C. Kiefer, M. Krämer, and F. Pessina, “On the modification of the cosmic microwave background anisotropy spectrum from canonical quantum gravity,” Physical Review D, vol. 87, Article ID 104008, 2013.
103. H. D. Zeh, The Physical Basis of the Direction of Time, Springer, Berlin, Germany, 5th edition, 2007.
104. R. Penrose, “Time-asymmetry and quantum gravity,” in Quantum Gravity, C. J. Isham, R. Penrose, and D. W. Sciama, Eds., vol. 2, pp. 242–272, Clarendon Press, Oxford, UK, 1981.
105. C. Kiefer, “Can the arrow of time be understood from quantum cosmology?” The Arrows of Time. Fundamental Theories of Physics, vol. 172, pp. 191–203, 2009.
106. G. W. Gibbons and S. W. Hawking, “Cosmological event horizons, thermodynamics, and particle creation,” Physical Review D, vol. 15, no. 10, pp. 2738–2751, 1977.
107. H. D. Zeh, “Time in quantum gravity,” Physics Letters A, vol. 126, pp. 311–317, 1988.
108. J. J. Halliwell and S. W. Hawking, “Origin of structure in the Universe,” Physical Review D, vol. 31, no. 8, pp. 1777–1791, 1985.
109. V. A. Belinskii, I. M. Khalatnikov, and E. M. Lifshitz, “A general solution of the Einstein equations with a time singularity,” Advances in Physics, vol. 31, pp. 639–667, 1982.
110. C. Kiefer and H.-D. Zeh, “Arrow of time in a recollapsing quantum universe,” Physical Review D, vol. 51, no. 8, pp. 4145–4153, 1995.
111. R. Penrose, “Black holes, quantum theory and cosmology,” Journal of Physics, vol. 174, no. 1, Article ID 012001, 2009.
112. A. Vilenkin, “Arrows of time and the beginning of the universe,” In press, http://arxiv.org/abs/1305.3836.
113. J. Maldacena, “The gauge/gravity duality,” in Black Holes in Higher Dimensions, G. Horowitz, Ed., Cambridge University Press, 2012.
114. M. Blau and S. Theisen, “String theory as a theory of quantum gravity: a status report,” General Relativity and Gravitation, vol. 41, no. 4, pp. 743–755, 2009.
115. J. Maldacena, “The illusion of gravity,” Scientific American, pp. 56–63, 2005.
116. C. Rovelli, “Statistical mechanics of gravity and the thermodynamical origin of time,” Classical and Quantum Gravity, vol. 10, no. 8, pp. 1549–1566, 1993.
117. M. Montesinos and C. Rovelli, “Statistical mechanics of generally covariant quantum theories: a Boltzmann-like approach,” Classical and Quantum Gravity, vol. 18, no. 3, pp. 555–569, 2001.
118. A. Y. Kamenshchik, C. Kiefer, and B. Sandhöfer, “Quantum cosmology with a big-brake singularity,” Physical Review D, vol. 76, no. 6, Article ID 064032, 13 pages, 2007.
119. M. Bouhmadi-Lopez, C. Kiefer, B. Sandhoefer, and P. V. Moniz, “Quantum fate of singularities in a dark-energy dominated universe,” Physical Review D, vol. 79, no. 12, Article ID 124035, 16 pages, 2009.
120. A. Y. Kamenshchik and S. Manti, “Classical and quantum big brake cosmology for scalar field and tachyonic models,” Physical Review D, vol. 85, Article ID 123518, 11 pages, 2012.
121. M. P. Dąbrowski, C. Kiefer, and B. Sandhöfer, “Quantum phantom cosmology,” Physical Review D, vol. 74, no. 2, Article ID 044022, 12 pages, 2006.
122. A. Kleinschmidt, M. Koehn, and H. Nicolai, “Supersymmetric quantum cosmological billiards,” Physical Review D, vol. 80, no. 6, Article ID 061701, 5 pages, 2009.
123. N. Pinto-Neto, F. T. Falciano, R. Pereira, and E. Sergio Santini, “Wheeler-DeWitt quantization can solve the singularity problem,” Physical Review D, vol. 86, no. 6, Article ID 063504, 12 pages, 2012.
124. H. Bergeron, A. Dapor, J. P. Gazeau, and P. Malkiewicz, “Wavelet quantum cosmology,” In press, http://arxiv.org/abs/1305.0653.
125. P. S. Joshi, “The rainbows of gravity,” In press, http://arxiv.org/abs/1305.1005.
126. S. W. Hawking, “The boundary conditions of the universe,” Pon-Tificia Academiae Scientarium Scripta Varia, vol. 48, pp. 563–574, 1982.
127. J. B. Hartle and S. W. Hawking, “Wave function of the universe,” Physical Review D, vol. 28, no. 12, pp. 2960–2975, 1983.
128. S. W. Hawking, “The quantum state of the universe,” Nuclear Physics B, vol. 239, no. 1, pp. 257–276, 1984.
129. J. J. Halliwell and J. Louko, “Steepest-descent contours in the path-integral approach to quantum cosmology. III. A general method with applications to anisotropic minisuperspace models,” Physical Review D, vol. 42, no. 12, pp. 3997–4031, 1990.
130. C. Kiefer, “On the meaning of path integrals in quantum cosmology,” Annals of Physics, vol. 207, no. 1, pp. 53–70, 1991.
131. A. Vilenkin, “Quantum cosmology and eternal inflation,” in The Future of the Theoretical Physics and Cosmology, pp. 649–666, Cambridge University Press, Cambridge, UK, 2003.
132. H. D. Conradi and H. D. Zeh, “Quantum cosmology as an initial value problem,” Physics Letters A, vol. 154, pp. 321–326, 1991.
133. A. O. Barvinsky, A. Y. Kamenshchik, C. Kiefer, and C. Steinwachs, “Tunneling cosmological state revisited: origin of inflation with a nonminimally coupled standard model Higgs inflaton,” Physical Review D, vol. 81, Article ID 043530, 2010.
134. J. B. Hartle, S. W. Hawking, and T. Hertog, “Local observation in eternal inflation,” Physical Review Letters, vol. 106, no. 14, Article ID 141302, 4 pages, 2011.
135. H. Everett, III, ““Relative state” formulation of quantum mechanics,” Reviews of Modern Physics, vol. 29, pp. 454–462, 1957.
136. H. D. Zeh, “Emergence of classical time from a universal wavefunction,” Physics Letters A, vol. 116, no. 1, pp. 9–12, 1986.
137. C. Kiefer, “Continuous measurement of mini-superspace variables by higher multipoles,” Classical and Quantum Gravity, vol. 4, no. 5, pp. 1369–1382, 1987.
138. A. O. Barvinsky, A. Y. Kamenshchik, C. Kiefer, and I. V. Mishakov, “Decoherence in quantum cosmology at the onset of inflation,” Nuclear Physics B, vol. 551, no. 1-2, pp. 374–396, 1999.
139. C. Kiefer and C. Schell, “Interpretation of the triad orientations in loop quantum cosmology,” Classical and Quantum Gravity, vol. 30, no. 3, Article ID 035008, 9 pages, 2013.
140. C. Kiefer and D. Polarski, “Why do cosmological perturbations look classical to us?” Advanced Science Letters, vol. 2, pp. 164–173, 2009.
141. C. Kiefer, I. Lohmar, D. Polarski, and A. A. Starobinsky, “Pointer states for primordial fluctuations in inflationary cosmology,” Classical and Quantum Gravity, vol. 24, no. 7, pp. 1699–1718, 2007.
142. A. Vilenkin, “Global structure of the multiverse and the measure problem,” in AIP Conference Proceedings, vol. 1514, pp. 7–13, Szczecin, Poland, 2012.
143. S. Landau, C. G. Scóccola, and D. Sudarsky, “Cosmological constraints on nonstandard inflationary quantum collapse models,” Physical Review D, vol. 85, Article ID 123001, 2012.