Cosmological test of a length-preserving biconnection gravity

Abstract We investigate the cosmological implications of an extended gravitational framework based on biconnection gravity, constructed from the Schr $$\ddot{\textrm{o}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mover> <mml:mtext>o</mml:mtext> <mml:mo>¨</mml:mo> </mml:mover> </mml:math> dinger connection and its dual. In this approach, the difference between the two connections defines the mutual curvature, which encodes the non-Riemannian geometric degrees of freedom, while their symmetric combination reduces to the Levi-Civita connection and hence reproduces general relativity at the background level. Within this setting, we derive the generalized Friedmann equations for a spatially flat Friedmann–Lemaître–Robertson–Walker Universe. The resulting equations contain additional geometric contributions that may naturally encode an effective dark energy sector induced by the biconnection degrees of freedom. We explore this extra dark energy by adopting five commonly used parametrizations, namely B $$\Lambda $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>Λ</mml:mi> </mml:math> CDM, $$\omega $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>ω</mml:mi> </mml:math> CDM, Chevallier–Polarski–Linder, Barboza–Alcaniz, and a logarithmic equations of state. These considerations are confronted with recent observational data, including DESI DR2, Pantheon $$^+$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mo>+</mml:mo> </mml:mmultiscripts> </mml:math> , and CC observations. Our analysis shows that the four parameterizations enter the acceleration phase at almost the same redshifts and share the same current value of the Hubble rate. Furthermore, the statistical comparison based on the Akaike, Bayesian, and Deviance Information Criterion shows that Barboza–Alcaniz, and logarithmic parameterizations have strong evidence and are competitive with $$\Lambda $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>Λ</mml:mi> </mml:math> CDM. To classify this biconnection gravity in the plethora theoretical models describing the current cosmic acceleration, we examine its implications through cosmographic tools, including the deceleration, jerk, and snap parameters, as well as through the Statefinder analysis and Om ( z ) diagnostic. These diagnostics indicate that the geometric sector generates a dynamical dark energy component that remains observationally close to $$\Lambda $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>Λ</mml:mi> </mml:math> CDM at the background level. Indeed, the effective evolution of the biconnection gravity begins initially from a quintessence-like regime and subsequently evolves toward a phantom-like regime. All parameterizations under consideration face this transition in the recent past, except for the $$\omega $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>ω</mml:mi> </mml:math> CDM model, which enters the phantom-like regime in the future.