Constraints on Fourth Order Gravity from Binary Pulsars and Gravitational waves

The recent direct detection of gravitational wave by LIGO collaboration has
once again confirmed the accuracy of General Theory of Relativity (GR) in describing the nature of
space-time. Despite the tremendous success of GR, there are still questions which cannot satisfactorily 
be answered with this theory without invoking new constituents in the universe (dark matter and dark energy), 
for e.g. the accelerated expansion of the universe and the flattening of galaxy rotation curves. Many works 
have been done to explain these observations without dark energy and/or dark matter by modifying GR. In this talk, 
I will focus on one such modified gravity theory containing fourth order modifications in the field equations
(Fourth Order Gravity). This model has one free length parameter that depends on the scale of the system under 
study, and another parameter ε appearing as an integration constant in the weak field limit. 
I will describe how these parameters can be constrained with (indirectly) observed gravitational radiation 
from binary pulsars. I will show that the energy loss predicted by this model is modified with respect to GR predictions, 
with an additional dependence on the fourth time derivative of the quadrupole moment. The parameters are constrained using
two observations from binary pulsars: periastron advance and orbital period decay due to gravitational wave emission. 
I will show, using three different binary pulsar systems (PSR B1913+16, PSR J0737-3039, PSR J1518+4904), 
that for a suitable choice of parameters, the model is consistent with the observations.