Cosmothermodynamics (CTD): A Covariant Effective-Fluid Framework Motivated by Horizon Thermodynamics and Infrared Memory Bookkeeping

Late-time expansion inferences exhibit heightened sensitivity to variations in dataset combi-nations and systematic treatments, motivating disciplined exploration of flexible backgroundextensions beyond rigid late-time parameterizations. Public DESI communications emphasizethat apparent preferences for evolving dark energy arise in combined analyses (e.g. BAO withexternal priors and supernova data), and that the quoted strength depends on the probe com-bination and systematics handling rather than constituting a single “BAO-alone” statement[1– 5]. In parallel, JWST-era observations have expanded the inventory of high-redshift compactactive sources, including lensed systems with unusually high inferred black-hole-to-host massratios at z ∼ 7–8 [ 6, 7 ]. This manuscript defines Cosmothermodynamics (CTD) as a disciplinedphenomenological response: CTD is an additional homogeneous component in the Friedmannbackground treated as a self-conserved effective fluid, with information–thermodynamic languageused strictly for dimensional bookkeeping and hypothesis construction. No parameter values,detections, or forecasts are reported. Instead we provide: (i) a dimensionally explicit definitionof ρCTD(t), (ii) minimal GR-consistency conditions (continuity and Bianchi compliance), (iii)an explicit toy closure yielding a closed background ODE system, (iv) limiting-case analysesyielding inequalities (bounds, not fitted numbers), and (v) an empirical program and falsifiersfor confronting CTD with distance data and early compact-object demographics.