Ancient Space Thermometers Challenge Twist in Cosmic History

Cosmic background light from the early universe appears roughly 7 to 8.5 percent colder than standard theoretical models predict, according to a theoretical study investigating the geometric properties of spacetime. The research addresses a long-standing ambiguity in modified gravity theories regarding how energy flows between expanding geometry and various matter components.

How Spacetime Torsion Allocates Energy

When gravitational frameworks incorporate spacetime torsion, a geometric property describing the twisting of the fabric of the universe alongside its curvature, matter and radiation exchange energy dynamically. Cosmologists have debated how this geometric source term gets distributed among individual fluids in the universe. Writing on the arXiv preprint server, physicists Fernando Izaurieta, Samuel Lepe, and Cristian Quinzacara analyzed the mathematical identities governing these interactions.

The authors established that torsion talks exclusively to intrinsic spin. Each matter sector exchanges energy with the geometry in direct proportion to its own spin properties, meaning spinless sectors remain strictly conserved under the Levi-Civita derivative regardless of how much torsion is present. Electromagnetic radiation, such as visible light and photons from the cosmic microwave background, possesses no vector spin under standard minimal coupling. Consequently, photons should be immune to direct energy transfer from background torsion.

Why Universal Coupling Fails Observational Tests

Recent cosmological proposals have suggested universal coupling models where every fluid, including radiation, shares energy exchanges with the geometry. Under such universal allocation rules, cosmic microwave background photons would absorb nearly a third of their present energy after recombination. However, precise measurements from instruments like the Far Infrared Absolute Spectrophotometer place extremely tight limits on any deviations from a pristine blackbody spectrum.

If photons were absorbing energy universally, the resulting heating would distort the background spectrum far beyond allowed thresholds. To remain compatible with observational data, the background temperature would drop significantly below standard scaling predictions. At a redshift of 0.886, toward the quasar absorber PKS 1830-211, the model predicts a temperature of 4.78 Kelvin instead of the standard 5.14 Kelvin expectation.

Testing Predictions Against Ancient Thermometers

The researchers tested these theoretical calculations against molecular absorption lines acting as natural thermometers left behind in ancient galaxies billions of years ago. Analysis of carbon monoxide and molecular hydrogen absorption systems across multiple redshifts up to about 3 generally favors standard temperature scaling over universal allocation models. Furthermore, Sunyaev-Zel’dovich effect measurements across Planck cluster bins strongly reject the universal coupling hypothesis at roughly 4 standard deviations.

The study indicates that modifying the recipient of the geometric energy exchange can resolve these observational tensions. If the spin is reassigned entirely to an exotic dark sector while baryons and photons carry none, standard radiation history is restored, and low-redshift cosmological data remains unaffected. However, this fix requires dark matter to possess substantial vector spin, presenting a fresh theoretical challenge for identifying particle candidates capable of providing such properties in the early universe.

The study was posted to the arXiv preprint server on November 5, 2026.