In this work , we analyze the implications of graviton to photon conversion in the presence of large scale magnetic fields .
We consider the magnetic fields associated with galaxy clusters , filaments in the large scale structure , as well as primordial magnetic fields .
We analyze the interaction of these magnetic fields with an exogenous high-frequency gravitational wave ( HFGW ) background which may exist in the Universe .
We show that , in the presence of the magnetic fields , a sufficiently strong HFGW background would lead to an observable signature in the frequency spectrum of the Cosmic Microwave Background ( CMB ) .
The sensitivity of current day CMB experiments allows to place significant constraints on the strength of HFGW background , \Omega _ { GW } \lesssim 1 .
These limits are about 25 orders of magnitude stronger than currently existing direct constraints in this frequency region .