Differentiating dark energy and modified gravity with galaxy redshift surveys

Understand

The observed cosmic acceleration today could be due to an unknown energy component (dark energy), or a modification to general relativity (modified gravity).

• If dark energy models and modified gravity models are required to predict the same cosmic expansion history H(z), they will predict different growth rate for cosmic large scale structure, f_g(z)=d\ln \delta/d\ln a (\delta=(\rho_m-\bar{\rho_m})/\bar{\rho_m}), a is the cosmic scale factor).
• If gravity is not modified, the measured H(z) leads to a unique prediction for f_g(z), f_g^H(z).
• Comparing f_g^H(z) with the measured f_g(z) provides a transparent and straightforward test of gravity.