Breaking model degeneracies with next-generation observations

Determine how forthcoming observations from LiteBIRD, CMB-S4, and LISA can break current degeneracies among modified gravity inflationary models by exploiting measurements of the tensor tilt n_t, isocurvature modes, and precise bounds on the tensor-to-scalar ratio r.

Background

Many modified gravity scenarios produce similar scalar spectral indices and low tensor amplitudes, complicating model selection with current data. The review points to tensor properties, isocurvature, and precision B-mode measurements as key discriminators.

Establishing concrete forecasted criteria would guide both experimental design and theoretical model development.

References

We highlight several key open problems: How will future missions such as LiteBIRD, CMB-S4, and LISA break degeneracies between different modified gravity inflationary models? Can tensor tilt n_t, isocurvature modes, or precision measurements of r distinguish among models that are currently statistically equivalent?

— From geometry to cosmology: a pedagogical review of inflation in curvature, torsion, and extended gravity theories (2509.14306 - Momeni, 17 Sep 2025) in Section: Open Questions and Future Directions