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Observational constraints on viscous cosmology in $f(T,L_m)$ gravity

M. Koussour, Alnadhief H. A. Alfedeel, S. Muminov, J. Rayimbaev

Abstract

We investigate the late-time cosmic acceleration within the framework of viscous $f(T,L_m)$ gravity, where the gravitational action depends on both the torsion scalar $T$ and the matter Lagrangian $L_m$. In this context, the Universe is modeled as a bulk viscous fluid, allowing for dissipative effects that generate an effective negative pressure capable of driving acceleration without invoking a cosmological constant. We adopt a simple linear model $f(T,L_m) = αT + βL_m$ and assume a constant bulk viscosity coefficient $ζ= ζ_0 > 0$. The model parameters are constrained using a joint analysis of recent observational datasets, including 31 Hubble parameter measurements, the Pantheon+ sample of 1701 Type Ia Supernovae, and the latest baryon acoustic oscillation data from DESI, employing a Markov Chain Monte Carlo (MCMC) approach. The best-fit results, $H_0 = 68.16 \pm 0.65$, $α= 1.53^{+0.49}_{-0.61}$, $β= 0.40 \pm 0.96$, and $ζ_0 = 2.15^{+0.69}_{-0.81}$, are consistent with current cosmological observations and indicate that bulk viscosity plays a significant role in the late-time dynamics. The deceleration parameter $q_0 = -0.33 \pm 0.41$ confirms the current accelerated expansion, while the effective equation of state (EoS) evolves from a matter-like regime at high redshift toward a quintessence phase at late times. The $Om(z)$ diagnostic further supports this behavior, suggesting a mild deviation from $Λ$CDM toward a dynamical dark energy component. Although information criteria ($Δ\mathrm{AIC} = 2.2$, $Δ\mathrm{BIC} = 13.13$) slightly favor the simpler $Λ$CDM model, the viscous $f(T,L_m)$ framework remains a viable and physically motivated alternative capable of explaining cosmic acceleration through the combined effects of torsion-matter coupling and viscosity.

Observational constraints on viscous cosmology in $f(T,L_m)$ gravity

Abstract

We investigate the late-time cosmic acceleration within the framework of viscous gravity, where the gravitational action depends on both the torsion scalar and the matter Lagrangian . In this context, the Universe is modeled as a bulk viscous fluid, allowing for dissipative effects that generate an effective negative pressure capable of driving acceleration without invoking a cosmological constant. We adopt a simple linear model and assume a constant bulk viscosity coefficient . The model parameters are constrained using a joint analysis of recent observational datasets, including 31 Hubble parameter measurements, the Pantheon+ sample of 1701 Type Ia Supernovae, and the latest baryon acoustic oscillation data from DESI, employing a Markov Chain Monte Carlo (MCMC) approach. The best-fit results, , , , and , are consistent with current cosmological observations and indicate that bulk viscosity plays a significant role in the late-time dynamics. The deceleration parameter confirms the current accelerated expansion, while the effective equation of state (EoS) evolves from a matter-like regime at high redshift toward a quintessence phase at late times. The diagnostic further supports this behavior, suggesting a mild deviation from CDM toward a dynamical dark energy component. Although information criteria (, ) slightly favor the simpler CDM model, the viscous framework remains a viable and physically motivated alternative capable of explaining cosmic acceleration through the combined effects of torsion-matter coupling and viscosity.
Paper Structure (10 sections, 33 equations, 5 figures, 1 table)

This paper contains 10 sections, 33 equations, 5 figures, 1 table.

Table of Contents

  1. Introduction
  2. Basic formalism of $f(T,L_m)$ gravity
  3. Viscous fluid dynamics
  4. Observational constraints
  5. $H(z)$, Pantheon+, and DESI datasets
  6. MCMC estimation of $H_0$, $\alpha$, $\beta$, and $\zeta_0$
  7. Observational comparison with $\Lambda$CDM
  8. Statistical comparison with $\Lambda$CDM (AIC and BIC)
  9. Cosmological evolution and observables
  10. Conclusion

Figures (5)

  • Figure 1: Joint likelihood contours for the viscous $f(T, L_m)$ and $\Lambda$CDM models using the combined $H(z)$+SNe+DESI datasets. The contours correspond to the $1\sigma$ and $2\sigma$ confidence regions for the free parameters.
  • Figure 2: Comparison of the viscous $f(T,L_m)$ model with $\Lambda$CDM: (top-left) Hubble parameter $H(z)$, (top-right) distance modulus $\mu(z)$, (bottom-left) residual differences $\Delta H(z)$, and (bottom-right) residual differences $\Delta \mu(z)$ as functions of redshift.
  • Figure 3: Evolution of the deceleration parameter $q(z)$ for the viscous $f(T, L_m)$ model compared with $\Lambda$CDM using the combined $H(z)$+SNe+DESI datasets.
  • Figure 4: Evolution of the effective viscous EoS parameter $\omega_v(z)$ for the viscous $f(T, L_m)$ model compared with $\Lambda$CDM using the combined $H(z)$+SNe+DESI datasets.
  • Figure 5: Evolution of the $Om(z)$ diagnostic for the viscous $f(T, L_m)$ model compared with $\Lambda$CDM using the combined $H(z)$+SNe+DESI datasets.