Model-independent reconstruction of the linear anisotropic stress η

Abstract

In this work, we use recent data on the Hubble expansion rate H(z), the quantity f σ 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 η. 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 η obs . If η 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 η obs at three redshift values, z = 0.29, 0.58, 0.86. Using the method of polynomial regression, we find η obs = 0.57 ± 1.05, η obs = 0.48 ± 0.96, and η obs = −0.11 ± 3.21, respectively. Assuming a constant η obs in this range, we find η obs = 0.49 ± 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 η obs = 0.15 ± 0.27 (binning) and η obs = 0.53 ± 0.19 (Gaussian Process). We find that all three estimates are compatible with each other within their 1σ error bars. While the polynomial regression method is compatible with standard gravity, the other two methods are in tension with it.

Tension of the EG statistic and redshift space distortion data with the Planck - ΛCDM model and implications for weakening gravity

“…This equation assumes that the redshift of the lens galaxies can be approximated by a single value while E G corresponds to average value along the line of sight [114].…”

Null test for interactions in the dark sector

“…Here, we will describe how to reconstruct the cosmological functions, and also their derivatives, using the Linear Model formalism (LM); see, also, [25,39,40].…”

General modified gravity with 21cm intensity mapping: simulations and forecast

“…By the time of reionisation around redshifts z = 6 − 10, the difference in the growth factor has reached about 2 − 3% for our models, whereas for fσ 8 plotted here a difference of 1% in Y translates to a 7 − 8% difference. For comparison we show the evolution down to z = 0 together with data points compiled from RSD measurements [44], where both models are still allowed within the current precision. We stress that our 21cm measurements are sensitive to the integrated effect down from the CMB redshift, but not to post-reionisation redshifts.…”