Variational minimization of the squared Hamiltonian constraint in a truncated one-vertex loop gravity model yields three classes of near-kernel states; one factorized branch matches reduced Thiemann coherent states with high fidelity.
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Background Independent Quantum Gravity: A Status Report
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abstract
The goal of this article is to present an introduction to loop quantum gravity -a background independent, non-perturbative approach to the problem of unification of general relativity and quantum physics, based on a quantum theory of geometry. Our presentation is pedagogical. Thus, in addition to providing a bird's eye view of the present status of the subject, the article should also serve as a vehicle to enter the field and explore it in detail. To aid non-experts, very little is assumed beyond elements of general relativity, gauge theories and quantum field theory. While the article is essentially self-contained, the emphasis is on communicating the underlying ideas and the significance of results rather than on presenting systematic derivations and detailed proofs. (These can be found in the listed references.) The subject can be approached in different ways. We have chosen one which is deeply rooted in well established physics and also has sufficient mathematical precision to ensure that there are no hidden infinities. In order to keep the article to a reasonable size, and to avoid overwhelming non-experts, we have had to leave out several interesting topics, results and viewpoints; this is meant to be an introduction to the subject rather than an exhaustive review of it.
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Loop quantum cosmology models harbor physical singularities or inconsistent space-time structures, with a new effective Friedmann equation revealing a sub-Planckian bounce after a singularity at infinite scale factor resembling a time-reversed big rip.
A minimum purification time for evaporating black holes is derived as scaling with M0^4/hbar^{3/2}, becoming exponential in initial area under a metastability assumption for Planck-scale holes, implying white-hole remnants.
SU(∞)-QGR yields an Einstein-like energy-momentum constraint that includes spin-1 gravitons and treats inflation and accelerating expansion as order parameters tracking the evolution of the universe's quantum states under Hilbert-space fragmentation.
In a path-integral model of timeless quantum systems, time evolution arises when a clock is prepared in a semiclassical state, showing that the cosine problem in quantum gravity follows from time-reversal invariance and neutral boundary conditions.
EHT observations of Sgr A* constrain deviations from GR black hole solutions including regular BHs, string-inspired spacetimes, and BH mimickers, with some limits exceeding cosmological bounds.
Nonlocal gravity shrinks the ISCO radius, boosts QPO frequencies, and constrains α/M ≤ 0.452 with M ≲ 43.6 M_⊙ for observed high-frequency QPOs under resonance models.
Nonlocal black holes remain consistent with general relativity at the 1.13-sigma level after joint lensing and quasinormal-mode constraints.
The work calculates scalar quasinormal mode spectra for a rotating quantum-corrected black hole and constructs a methodological pipeline to infer the quantum correction parameter from gravitational-wave ringdown data using informative priors.
Quantum corrections in rotating black holes produce detectable but spin-suppressed gravitational wave phase shifts in LISA EMRIs.
citing papers explorer
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Emergent Thiemann coherent states in the near-kernel sector of quantum reduced loop gravity
Variational minimization of the squared Hamiltonian constraint in a truncated one-vertex loop gravity model yields three classes of near-kernel states; one factorized branch matches reduced Thiemann coherent states with high fidelity.
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Singularities in loop quantum cosmology
Loop quantum cosmology models harbor physical singularities or inconsistent space-time structures, with a new effective Friedmann equation revealing a sub-Planckian bounce after a singularity at infinite scale factor resembling a time-reversed big rip.
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Minimum lifetime of a black hole
A minimum purification time for evaporating black holes is derived as scaling with M0^4/hbar^{3/2}, becoming exponential in initial area under a metastability assumption for Planck-scale holes, implying white-hole remnants.
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Energy-momentum and dark energy in $\boldsymbol{SU(\infty)}$-QGR quantum gravity
SU(∞)-QGR yields an Einstein-like energy-momentum constraint that includes spin-1 gravitons and treats inflation and accelerating expansion as order parameters tracking the evolution of the universe's quantum states under Hilbert-space fragmentation.
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The problem of time: a path integral view
In a path-integral model of timeless quantum systems, time evolution arises when a clock is prepared in a semiclassical state, showing that the cosine problem in quantum gravity follows from time-reversal invariance and neutral boundary conditions.
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Horizon-scale tests of gravity theories and fundamental physics from the Event Horizon Telescope image of Sagittarius A$^*$
EHT observations of Sgr A* constrain deviations from GR black hole solutions including regular BHs, string-inspired spacetimes, and BH mimickers, with some limits exceeding cosmological bounds.
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The properties and predictions of quasi-periodic oscillations around a black hole in nonlocal gravity
Nonlocal gravity shrinks the ISCO radius, boosts QPO frequencies, and constrains α/M ≤ 0.452 with M ≲ 43.6 M_⊙ for observed high-frequency QPOs under resonance models.
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Observational constraints on nonlocal black holes via gravitational lensing
Nonlocal black holes remain consistent with general relativity at the 1.13-sigma level after joint lensing and quasinormal-mode constraints.
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The quasinormal modes of the rotating quantum corrected black holes
The work calculates scalar quasinormal mode spectra for a rotating quantum-corrected black hole and constructs a methodological pipeline to infer the quantum correction parameter from gravitational-wave ringdown data using informative priors.
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Assessing EMRI Detectability of the Rotating Quantum Oppenheimer-Snyder Black Hole
Quantum corrections in rotating black holes produce detectable but spin-suppressed gravitational wave phase shifts in LISA EMRIs.