Modified Gravitational Theory as an Alternative to Dark Energy and Dark Matter

TL;DR

J. W. Moffat proposes nonsymmetric gravitational theory (NGT) as an extension of GR to address cosmic acceleration and dark matter problems without resorting to a cosmological constant or exotic dark matter. The theory endows the metric with a skew sector $g_{[\mu\nu]}$ carrying six extra dof and yields a modified Friedmann equation through the quantities $Q_0$ and $Y_0$ derived from the skew field, enabling late-time acceleration in a flat FRW-like background with $H^2 = \frac{8\pi G}{3}\rho_M + Q_0$ and present-day fractions $\Omega^0_M \sim 0.3$, $\Omega^0_{f_0} \sim 0.7$. The work demonstrates that if $Y_0 > 4\pi G\rho_M$, the effective equation of state satisfies $w_{\rm eff} < -\tfrac{1}{3}$ and acceleration occurs with $\Lambda=0$, while preserving standard early-Universe evolution, and notes that galaxy rotation curves can also be explained without CDM. The author emphasizes that numerical solutions are needed to assess compatibility with BBN, CMB, LSS, and galaxy formation, and that the theory is falsifiable, potentially allowing a non-negligible baryonic/neutrino dark component.

Abstract

The problem of explaining the acceleration of the expansion of the universe and the observational and theoretical difficulties associated with dark matter and dark energy are discussed. The possibility that Einstein gravity does not correctly describe the large-scale structure of the universe is considered and an alternative gravity theory is proposed as a possible resolution to the problems.