# How does pressure relate to cosmological expansion?

In my cosmology class, we've been talking about pressure in the Friedmann Equation for acceleration:

$$\frac{\ddot{a}}{a}=-\frac{4 \pi G}{3} \left(\rho+\frac{3p}{c^2} \right)$$

In particular, I've been a bit hung up on this passage in the textbook:

Notice that if the material has any pressure, this increases the gravitational force, and so further decelerates the expansion. I remind you that there are no forces associated with pressure in an isotropic Universe, as there are no pressure gradients.

How, physically, does pressure do work decelerating expansion if there is no pressure gradient or forces involved? How can pressure affect any of the properties of spacetime?