Multimessenger and Supernova Constraints on Disformal Curvature Screening in Closed Linear-Coasting Cosmology

A closed Friedmann–Lemaître–Robertson–Walker universe whose curvature radius grows linearly, R(t) = vₑₓₚ t, predicts H(t) = 1/t and q = 0, and, if that radius belongs to the metric to which matter couples, also Ω_k = -β⁻² with β ≡ vₑₓₚ/c. This last identity is difficult to reconcile with the observed near-flatness of the Universe, and it has been suggested that the conflict may be avoided by relating an ontic metric g_μν to a matter metric g̃_μν through a Bekenstein-type disformal transformation with constant effective lapse N, which replaces the above by Ω̃_k = -N²/β². We assess this proposal against two independent observations. If tensor modes propagate on g_μν while photons follow g̃_μν, the locally inferred tensor speed is c_g/c = 1/N, and the GW170817/GRB 170817A timing bound then restricts N to within one part in 10¹⁵ of unity, leaving the curvature fraction essentially unscreened. If instead the two causal cones coincide, gravitational-wave timing is uninformative, but we show that Type Ia supernovae then constrain the single combination b ≡ β/N, and that Ω̃_k = -1/b² exactly, so that the Hubble diagram measures the physical-frame curvature directly and independently of the sound horizon. Fitting 1580 Pantheon+ supernovae with the full statistical-plus-systematic covariance gives -Ω̃_k < 0.054 at 95% confidence. The value β = 1/√3, singled out in the original proposal by a series cancellation in the comoving angle and implying Ω̃_k = -3, is disfavoured by Δχ² ≃ 10³ relative to flat ΛCDM. We have not been able to identify a configuration in which this form of curvature screening both operates appreciably and remains observationally permitted; what survives is the spatially flat coasting cosmology, which our fit disfavours by ΔAIC ≃ +104.