uncommon questions - acceleration or not?

Taking the trace of the Einstein field equation would imply that the Ricci-scalar \(R\) is proportional to the trace of the energy momentum tensor \(T\), \(R = -T\). Now, the Ricci-scalar describes the acceleration \(q\) in a FLRW-universe: if the Universe is empty, \(T=0\) and the expansion velocity is constant, \(\dot{a}=0\), but if the Universe is critical and radiation dominated, \(T\) is likewise zero, yet there is deceleration. Why is this the case?