Testing cosmic acceleration from thermogravity without vacuum energy

Abstract: We present a first background-level observational test of a thermogravity theory in which the Einstein equations are clipped to a trace-free version, the cosmological constant $Λ$ does not gravitate, and controlled violations of energy conservation obstruct the usual reinstatement of $Λ$ as an integration constant. We therefore set $Λ=0$ and ask whether late-time acceleration can instead be generated solely by energy non-conservation, with no extra background parameter relative to flat $Λ\mathrm{CDM}$. We contrast a minimal universal implementation with a model in which only CDM partakes in non-conservation. The universal model is strongly disfavored when combining Supernovae and BAO distances, with the inclusion of DESI BAO worsening the fit by $Δχ^2\simeq 51$ relative to $Λ\mathrm{CDM}$, because matter creation ties the intermediate-redshift normalization of $H(z)$ too rigidly to the present acceleration. This conclusion, however, should be interpreted with caution, since universal non-conservation would modify the observational dictionary itself. By contrast, restricting non-conservation to CDM leaves baryonic and photon observables unaffected at the background level and therefore allows a self-contained analysis. The resulting $ξ\mathrm{CDM}$ model, which can be thought of as a one parameter extension of $Λ\mathrm{CDM}$, provides an improved fit relative to $Λ\mathrm{CDM}$ for the background dataset combinations considered, with improvements reaching $Δχ<sup>2=-4.85$ and a maximum Bayesian preference of $\ln\mathcal{B}=2.77$. However, when the BAO ruler is calibrated using BBN or CMB information, the reduced effective early-time CDM density increases the sound horizon and drives $H_0$ towards lower values, thereby increasing rather than alleviating the tension with the distance-ladder calibration.