Big bounce cosmology
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Big Bounce Cosmology: Replacing the Big Bang Singularity
Big Bounce Cosmology Overview and Key Concepts
Big bounce cosmology proposes that the universe did not begin with a singular big bang, but instead underwent a prior contraction phase, reached a minimum size, and then "bounced" into the current expansion. This model avoids the initial singularity problem of the standard big bang theory and provides a continuous evolution of the universe through the bounce point Cubero2019Popławski2011Battisti2010+2 MORE.
Theoretical Foundations: Einstein–Cartan Theory and Loop Quantum Cosmology
Several theoretical frameworks support the big bounce scenario. In the Einstein–Cartan theory, the intrinsic spin of fermions generates spacetime torsion, which prevents the formation of singularities and leads to a nonsingular bounce at a finite minimum scale factor Cubero2019Popławski2011. This mechanism also helps explain why the universe appears flat, homogeneous, and isotropic on large scales .
Loop quantum cosmology (LQC) offers another approach, where quantum gravity effects modify the early universe's dynamics. In LQC, the big bang singularity is replaced by a deterministic big bounce, and this result holds even when considering inhomogeneities or anisotropies in the universe Battisti2010Moriconi2017Brizuela2009+1 MORE. The bounce is generic and does not depend on the details of the matter content or quantization ambiguities .
Dynamics and Features of the Big Bounce
In big bounce models, the universe contracts to a minimum size before expanding again. Some models predict a "double bounce," where the scale factor decreases, increases, decreases, and then increases again as the universe passes through high-density phases . In five-dimensional cosmological models, the bounce is characterized by finite scale factor and mass density, with pressure undergoing a sudden transition, and the universe may have existed for an infinitely long time before the bounce .
Thermodynamic Arrow of Time and Entropy
A key debate in big bounce cosmology concerns the behavior of entropy and the arrow of time at the bounce. Some interpretations suggest that the bounce represents the birth of two expanding universes ("double bang"), but others defend the orthodox view that features of one universe explain those of the other, with a continuous thermodynamic arrow of time even if entropy is "reset" at the bounce .
Gravitational Waves and Observational Constraints
Primordial gravitational waves generated during the bounce phase are a key prediction of big bounce cosmology. These waves contribute to the stochastic gravitational wave background (SGWB), which can be probed by current and future gravitational wave detectors Li2024Li2025. Recent studies provide analytical relations between the bounce energy scale and the SGWB spectrum, allowing for constraints on big bounce models using data from detectors like Planck, LIGO/Virgo, and pulsar timing arrays . Only certain contraction scenarios, such as matter-dominated contraction, remain viable given current observational limits .
Inhomogeneities, Anisotropies, and Quantum Effects
Big bounce models have been extended to include inhomogeneities and anisotropies. In loop quantum cosmology, the bounce persists even in the presence of gravitational waves and anisotropic features, with the universe's anisotropies remaining finite across the bounce Moriconi2017Brizuela2009. The statistical properties of inhomogeneities can be amplified or remain unchanged depending on the regime of the bounce .
Conclusion
Big bounce cosmology offers a compelling alternative to the traditional big bang, resolving the initial singularity and providing a continuous, nonsingular evolution of the universe. Supported by various theoretical frameworks and increasingly constrained by gravitational wave observations, the big bounce scenario remains a vibrant area of research, with ongoing debates about entropy, the arrow of time, and the detailed dynamics of the bounce itself Linford2020Cubero2019Popławski2011+7 MORE.
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