Bouncing Cosmology and Quantum Gravity
Summary
Bouncing cosmology offers an alternative to the classical Big Bang paradigm by postulating that the Universe undergoes a contraction phase followed by a non-singular bounce and then re-expands. This approach aims to resolve the initial singularity problem by invoking mechanisms from quantum gravity that violate or evade the classical energy conditions. In leading proposals, quantum corrections to the gravitational action—whether arising in loop quantum cosmology, string-inspired models or asymptotically safe gravity—produce a repulsive force at Planckian densities, driving a turnaround instead of a singularity. Such models often incorporate phases of ekpyrotic (ultra-slow) contraction to suppress anisotropies, followed by a short bounce mediated by ghost-free scalar fields or higher-order curvature terms. The resulting dynamics can generate nearly scale-invariant spectra of primordial perturbations, while predicting distinctive imprints in the tensor sector. Ongoing efforts focus on establishing robust criteria for stability across the bounce, embedding scenarios in a full quantum-gravitational framework and extracting observational signatures—from the cosmic microwave background to stochastic gravitational-wave backgrounds—that could distinguish bouncing models from inflationary cosmologies. By combining theoretical consistency with growing observational sensitivity, bouncing cosmology lies at the nexus of fundamental physics and precision cosmology, offering a global programme to probe the earliest epochs of our Universe.
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Bouncing Cosmology and Quantum Gravity publication trend
The graph below shows the total number of articles in bouncing cosmology and quantum gravity across all publications each year (not limited to Nature Index journals).
Technical terms
Bouncing cosmology: A scenario in which the Universe undergoes contraction followed by a non-singular reversal (bounce) instead of a singular Big Bang.
Quantum gravity: A theoretical framework that unifies general relativity and quantum mechanics, predicting modifications at Planckian scales.
Loop quantum cosmology: A quantisation of cosmological spacetimes based on loop quantum gravity, yielding discrete geometry and natural bounces.
Null energy condition (NEC): A classical requirement on stress-energy that, when violated or modified, permits non-singular bounces.
Ekpyrotic phase: An ultra-slow contraction epoch driven by a steep scalar potential, used to suppress anisotropies before a bounce.
Primordial gravitational waves: Tensor perturbations generated in the early Universe, carrying information about high-energy physics and bounce dynamics.
References
- Pulsar timing array observations as possible hints for nonsingular cosmology. European Physical Journal C (2023).
- A new kind of cyclic universe. Physics Letters B (2019).
- Bouncing cosmological solutions from f(R,T) gravity. European Physical Journal C (2018).
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