Abstract.Astrophysical sources of high frequency gravitational radiation are considered in association with a new interest to very sensitive HFGW receivers required for the laboratory GW Hertz experiment. A special attention is paid to the phenomenon of primordial black holes evaporation. They act like black body to all kinds of radiation, including gravitons, and, therefore, emit an equilibrium spectrum of gravitons during its evaporation. Limit on the density of high frequency gravitons in the Universe is obtained, and possibilities of their detection are briefly discussed.

We present an approach to testing the gravitational redshift effect using the RadioAstron satellite. The experiment is based on a modification of the Gravity Probe A scheme of nonrelativistic Doppler compensation and benefits from the highly eccentric orbit and ultra-stable atomic hydrogen maser frequency standard of the RadioAstron satellite. Using the presented techniques we expect to reach an accuracy of the gravitational redshift test of order 10 −5 , a magnitude better than that of Gravity Probe A. Data processing is ongoing, our preliminary results agree with the validity of the Einstein Equivalence Principle.

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