We propose a simple , nonlocal modification to general relativity ( GR ) on large scales , which provides a model of late-time cosmic acceleration in the absence of the cosmological constant and with the same number of free parameters as in standard cosmology .
The model is motivated by adding to the gravity sector an extra spin-2 field interacting nonlocally with the physical metric coupled to matter .
The form of the nonlocal interaction is inspired by the simplest form of the Deser-Woodard ( DW ) model , \alpha R \frac { 1 } { \Box } R , with one of the Ricci scalars being replaced by a constant m ^ { 2 } , and gravity is therefore modified in the infrared by adding a simple term of the form m ^ { 2 } \frac { 1 } { \Box } R to the Einstein-Hilbert term .
We study cosmic expansion histories , and demonstrate that the new model can provide background expansions consistent with observations if m is of the order of the Hubble expansion rate today , in contrast to the simple DW model with no viable cosmology .
The model is best fit by w _ { 0 } \sim - 1.075 and w _ { a } \sim 0.045 .
We also compare the cosmology of the model to that of Maggiore and Mancarella ( MM ) , m ^ { 2 } R \frac { 1 } { \Box ^ { 2 } } R , and demonstrate that the viable cosmic histories follow the standard-model evolution more closely compared to the MM model .
We further demonstrate that the proposed model possesses the same number of physical degrees of freedom as in GR .
Finally , we discuss the appearance of ghosts in the local formulation of the model , and argue that they are unphysical and harmless to the theory , keeping the physical degrees of freedom healthy .