In this work , we use recent data on the Hubble expansion rate H ( z ) , the quantity f \sigma _ { 8 } ( z ) from redshift space distortions and the statistic E _ { g } from clustering and lensing observables to constrain in a model-independent way the linear anisotropic stress parameter \eta .
This estimate is free of assumptions about initial conditions , bias , the abundance of dark matter and the background expansion .
We denote this observable estimator as \eta _ { { obs } } .
If \eta _ { { obs } } turns out to be different from unity , it would imply either a modification of gravity or a non-perfect fluid form of dark energy clustering at sub-horizon scales .
Using three different methods to reconstruct the underlying model from data , we report the value of \eta _ { { obs } } at three redshift values , z = 0.29 , 0.58 , 0.86 .
Using the method of polynomial regression , we find \eta _ { { obs } } = 0.57 \pm 1.05 , \eta _ { { obs } } = 0.48 \pm 0.96 , and \eta _ { { obs } } = -0.11 \pm 3.21 , respectively .
Assuming a constant \eta _ { { obs } } in this range , we find \eta _ { { obs } } = 0.49 \pm 0.69 .
We consider this method as our fiducial result , for reasons clarified in the text .
The other two methods give for a constant anisotropic stress \eta _ { { obs } } = 0.15 \pm 0.27 ( binning ) and \eta _ { { obs } } = 0.53 \pm 0.19 ( Gaussian Process ) .
We find that all three estimates are compatible with each other within their 1 \sigma error bars .
While the polynomial regression method is compatible with standard gravity , the other two methods are in tension with it .