# Forecasting Cosmological Bias due to Local Gravitational Redshift

## TL;DR

This paper forecasts the impact of local gravitational redshift on cosmological measurements, concluding that with additional CMB or redshift survey data, the bias remains negligible for future supernovae surveys.

## Contribution

It provides a forecast showing that local gravitational redshift effects are negligible when combined with CMB or redshift survey data in future cosmological measurements.

## Key findings

- Local gravitational redshift effect is negligible with additional data.
- Adding CMB data reduces parameter degeneracy and bias.
- Forecast applies to future supernovae surveys like WFIRST.

## Abstract

When photons from distant galaxies and stars pass through our neighboring environment, the wavelengths of the photons would be shifted by our local gravitational potential. This local gravitational redshift effect can potentially have an impact on the measurement of cosmological distance-redshift relation. Using available supernovae data, Wojtak et al [1] found seemingly large biases of cosmological parameters for some extended models (non-flat $\Lambda$CDM, $w$CDM, etc.). Huang [2] pointed out that, however, the biases can be reduced to a negligible level if cosmic microwave background (CMB) data are added to break the strong degeneracy between parameters in the extended models. In this article we forecast the cosmological bias due to local gravitational redshifts for a future WFIRST-like supernovae survey. We find that the local gravitational redshift effect remains negligible, provided that CMB data or some future redshift survey data are added to break the degeneracy between parameters.