Cycles Without End: An Ekpyrotic Braneworld Bounce with a Kinetically Coupled Entropic Mechanism
Abstract
Cyclic cosmologies replace inflation's single creation event with an eternal sequence of smooth turnarounds, but three problems have historically stood in their way: singular bounces, the blue tilt generic to single-field ekpyrotic contraction, and Tolman's observation that entropy production should make successive cycles longer and larger without bound. We resolve all three within one explicit model. A Randall-Sundrum brane with a timelike extra dimension regularizes the bounce without violating the null energy condition, and a single scalar field's potential carries the universe from a thawing dark-energy plateau through a sign-changing turnaround into a BKL-safe ekpyrotic contraction and back. We show that no correction internal to the brane's Friedmann equation, and no straight two-field trajectory, can rescue the spectral index from the generic ekpyrotic result ns≈3. We then build the minimal completion that can: a kinetically coupled entropy field, following the general mechanism of Ijjas, Lehners, and Steinhardt, whose coupling steepness has a closed-form, exact-scale-invariance value for this model's ekpyrotic potential, with only a percent-level offset needed to match the measured tilt ns=0.9649. This fixes the local non-Gaussianity (fNL≈−0.02) and tensor-to-scalar ratio (r≈7.6×10−26). Matching the observed amplitude fixes the conversion coupling and brane tension to unremarkable values (λ≈5.7×10−3, ρc1/4=1013GeV). Finally, because the brane construction is spatially flat, Tolman's entropy argument does not bound the number of cycles here: we derive NDE∼70-80 e-folds of accelerated expansion needed each cycle to dilute the entropy produced back to a negligible density before the next contraction.
Related
- Effects of the Ekpyrotic Mechanism on Inflationary Phase in mLQC-II
- Gauge-Invariant Brout--Englert--Higgs Mechanism without Spontaneous Breaking of Local Gauge Symmetry: A Quantum-Reference-Frame Formulation
- The alternative scalar field dark sector of the Universe: Cosmological inflation, quintessence and ekpyrotic model in the framework of scalar-tensor cosmology
- Torsion, Frames and Quantum Bounces: Inflation and Ekpyrotic Contraction in Quantum Trajectory Cosmology with a Non-Minimally Coupled Scalar
- Transport Signatures of a Kinetically Self-Trapped Nagaoka Polaron
BibTeX
@misc{sengupta2026cycles,
title = {{Cycles Without End: An Ekpyrotic Braneworld Bounce with a Kinetically Coupled Entropic Mechanism}},
author = {Rikpratik Sengupta},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.06485},
eprint = {2609.06485},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.06485},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Cycles Without End: An Ekpyrotic Braneworld Bounce with a Kinetically Coupled Entropic Mechanism
AU - Rikpratik Sengupta
PY - 2026
DA - 2026/09/06
PB - arXiv
DO - 10.48550/arxiv.2609.06485
UR - https://arxiv.org/abs/2609.06485
AB - Cyclic cosmologies replace inflation's single creation event with an eternal sequence of smooth turnarounds, but three problems have historically stood in their way: singular bounces, the blue tilt generic to single-field ekpyrotic contraction, and Tolman's observation that entropy production should make successive cycles longer and larger without bound. We resolve all three within one explicit model. A Randall-Sundrum brane with a timelike extra dimension regularizes the bounce without violating the null energy condition, and a single scalar field's potential carries the universe from a thawing dark-energy plateau through a sign-changing turnaround into a BKL-safe ekpyrotic contraction and back. We show that no correction internal to the brane's Friedmann equation, and no straight two-field trajectory, can rescue the spectral index from the generic ekpyrotic result n_s≈3. We then build the minimal completion that can: a kinetically coupled entropy field, following the general mechanism of Ijjas, Lehners, and Steinhardt, whose coupling steepness has a closed-form, exact-scale-invariance value for this model's ekpyrotic potential, with only a percent-level offset needed to match the measured tilt n_s=0.9649. This fixes the local non-Gaussianity (f_NL≈−0.02) and tensor-to-scalar ratio (r≈7.6×10^−26). Matching the observed amplitude fixes the conversion coupling and brane tension to unremarkable values (λ≈5.7×10^−3, ρ_c^1/4=10^13GeV). Finally, because the brane construction is spatially flat, Tolman's entropy argument does not bound the number of cycles here: we derive N_DE∼70-80 e-folds of accelerated expansion needed each cycle to dilute the entropy produced back to a negligible density before the next contraction.
ER -