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The early Universe was like a tightly coiled spring. In the first moments, it expanded at an unimaginable rate — a phase called inflation. Usually, this is attributed to a special field, but there's another idea: spacetime itself twists like a spring, and this twisting (in physics, torsion) becomes the engine of expansion.

When inflation ends, the energy of torsion must be transferred to ordinary particles: this is how the Universe reheats. But how exactly? The authors calculated that the inflaton (the torsion particle) hardly produces fermion pairs directly. Instead, through quantum anomalies, it actively creates gauge bosons and triples of particles involving the Higgs boson. This process is like a compressed spring, as it uncoils, brushing against neighboring parts and transferring energy to them.

This unusual decay gives a natural lower limit on the reheating temperature. This is important: if the temperature is too low, the first stars cannot ignite. The calculations show that spacetime torsion leaves a noticeable imprint on the early Universe, and this imprint can be sought in observational data.

The spring uncoiled, transferring energy to particles. The Universe became hot, and then began to cool, forming galaxies and stars. The torsion that drove inflation has vanished, but its consequences are still visible in the cosmic microwave background.

Original abstract

as written by the authors arXiv:2607.29618v1

In gravity theories with torsion, inflation can be driven by the dynamical torsion. Since it naturally has a derivative coupling to the axial current consisting of Standard Model particles, the reheating dynamics should be much different from conventional inflation models like Starobinsky inflation. We calculate the inflaton decay rate in detail and find that, although the 2 body decay into the fermion pair is vanishing in the massless fermion limit, the anomaly-induced 2 body decay into the gauge bosons and 3 body decay involving the Higgs boson are non-vanishing and they give sizable contributions to the total decay width of the inflaton. This gives a natural lower bound on the reheating temperature in the inflation model with dynamical torsion.