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1. Report on the session Loop Quantum Gravity: Cosmology and Black Holes of the 16th Marcel Grossmann Meeting

   Parampreet Singh, Jorge Pullin (October 2022, Proceedings of the Marcel Grossmann Meeting)

   Remo Ruffini (Ed.)

   We summarize the main results of 19 talks presented at the QG3 session (loop quantum gravity: cosmology and black holes) of the 16th Marcel Grossmann Meeting held online from July 5th-10th, 2021. 

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2. Critical Evaluation of Common Claims in Loop Quantum Cosmology

   https://doi.org/10.3390/universe6030036  

   Bojowald, Martin (March 2020, Universe)

   A large number of models have been analyzed in loop quantum cosmology, using mainly minisuperspace constructions and perturbations. At the same time, general physics principles from effective field theory and covariance have often been ignored. A consistent introduction of these ingredients requires substantial modifications of existing scenarios. As a consequence, none of the broader claims made mainly by the Ashtekar school—such as the genericness of bounces with astonishingly semiclassical dynamics, robustness with respect to quantization ambiguities, the realization of covariance, and the relevance of certain technical results for potential observations—hold up to scrutiny. Several useful lessons for a sustainable version of quantum cosmology can be drawn from this evaluation. 

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3. Cosmic Tango Between the Very Small and the Very Large: Addressing CMB Anomalies Through Loop Quantum Cosmology

   https://doi.org/doi: 10.3389/fspas.2021.685288  

   Ashtekar, A; Gupt, B; Jeong, D; Sreenath, V. (June 2021, Frontiers in astronomy and space sciences)

   While the standard, six-parameter, spatially flat ΛCDM model has been highly successful, certain anomalies in the cosmic microwave background bring out a tension between this model and observations. The statistical significance of any one anomaly is small. However, taken together, the presence of two or more of them imply that according to standard inflationary theories we live in quite an exceptional Universe. We revisit the analysis of the PLANCK collaboration using loop quantum cosmology, where an unforeseen interplay between the ultraviolet and the infrared makes the primordial power spectrum scale dependent at very small k. Consequently, we are led to a somewhat different ΛCDM Universe in which anomalies associated with large scale power suppression and the lensing amplitude are both alleviated. The analysis also leads to new predictions for future observations. This article is addressed both to cosmology and loop quantum gravity communities, and we have attempted to make it self-contained. 

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4. Quantum geometry and black holes

   Gambini, Rodolfo; Olmedo, Javier; Pullin, Jorge (January 2023, Handbook of Quantum Gravity",)

   Bambi, Cosimo; Modesto, Leonardo; Shapiro, Ilya (Ed.)

   We summarize our work on spherically symmetric midi-superspaces in loop quantum gravity. Our approach is based on using inhomogeneous slicings that may penetrate the horizon in case there is one and on a redefinition of the constraints so the Hamiltonian has an Abelian algebra with itself. We discuss basic and improved quantizations as is done in loop quantum cosmology. We discuss the use of parameterized Dirac observables to define operators associated with kinematical variables in the physical space of states, as a first step to introduce an operator associated with the space-time metric. We analyze the elimination of singularities and how they are replaced by extensions of the space-times. We discuss the charged case and potential observational consequences in quasinormal modes. We also analyze the covariance of the approach. Finally, we comment on other recent approaches of quantum black holes, including mini-superspaces motivated by loop quantum gravity. 

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5. Alleviating the Tension in the Cosmic Microwave Background Using Planck-Scale Physics

   https://doi.org/https://doi.org/10.1103/PhysRevLett.125.051302  

   Ashtekar, A; Gupt, B; Sreenath, V. (July 2020, Physical review and Physical review letters index)

   Certain anomalies in the CMB bring out a tension between the six-parameter flat ΛCDM model and the CMB data. We revisit the PLANCK analysis with loop quantum cosmology (LQC) predictions and show that LQC alleviates both the large-scale power anomaly and the tension in the lensing amplitude. These differences arise because, in LQC, the primordial power spectrum is scale dependent for small k, with a specific power suppression. We conclude with a prediction of larger optical depth and power suppression in the B-mode polarization power spectrum on large scales. 

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