One Gravitational Potential or Two? Forecasts and Tests

Understand

The metric of a perturbed Robertson-Walker spacetime is characterized by three functions: a scale-factor giving the expansion history and two potentials which generalize the single potential of Newtonian gravity.

• The Newtonian potential induces peculiar velocities and, from these, the growth of matter fluctuations.
• Massless particles respond equally to the Newtonian potential and to a curvature potential.
• The difference of the two potentials, called the gravitational slip, is predicted to be very small in general relativity but can be substantial in modified gravity theories.