A new strategy for the reconstruction of f (R) gravity models have been attempted using Raychaudhuri equation. Two examples, one for an eternally accelerating universe and the other for one that mimics a ΛCDM expansion history have been worked out. For both the cases, the relevant f = f (R) could be found out analytically. In the first case, f (R) is found to be a combination of power-law terms and in the expression for the second case involves hypergeometric functions. The evolution history of the universe, given as specific values of the kinematical quantities like the jerk or the deceleration parameter, serve as the input. It is found that the corresponding f (R) gravity models, in both the examples, are not viable options.

Implications of the Raychaudhuri equation in focusing of geodesic congruences are studied in the framework of scalar–tensor theory of gravity. Specifically, we investigate the Brans–Dicke theory and Bekenstein’s scalar field theory. In both of these theories, we deal with a static spherically symmetric distribution and a spatially homogeneous and isotropic cosmological model as specific examples. We find that it is possible to violate the convergence condition under reasonable physical assumptions. This leads to the possibility of avoiding a singularity.

A recent attempt to arrive at a quantum version of Raychaudhuri’s equation is looked at critically. It is shown that the method, and even the idea, has some inherent problems. The issues are pointed out here. We have also shown that it is possible to salvage the method in some limited domain of applicability. Although no generality can be claimed, a quantum version of the equation should be useful in the context of ascertaining the existence of a singularity in the quantum regime. The equation presented in the present work holds for arbitrary $$n+1$$ n + 1 dimensions. An important feature of the Hamiltonian in the operator form is that it admits a self-adjoint extension quite generally. Thus, the conservation of probability is ensured.

The role of the Raychaudhuri equation in studying gravitational collapse is discussed. A self-similar distribution of a scalar field along with an imperfect fluid in a conformally flat spacetime is considered for the purpose. The general focusing condition is found out and verified against the available exact solutions. The connection between the Raychaudhuri equation and the critical phenomena is also explored.

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