Test of gravitational redshift and galactic and extragalactic compact radio sources with the technique of very long baseline interferometry

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Quantum theory and general relativity are the two pillars of modern physics. However, they are incompatible. While quantum theory has been confirmed at an extremely high level of accuracy, general relativity has been confirmed at several orders of magnitude less accuracy -and is speculated to need modification. Tests of general relativity are therefore of primary interest. We will use the space-very-long-baseline interferometry (space VLBI) mission RadioAstron for the most accurate test of general relativity. The RadioAstron spacecraft is on an eccentric orbit around Earth with an apogee of ~350,000 km and a perigee of ~20,000 km, and has an extremely stable frequency standard on board, a hydrogen maser. The hydrogen maser signal will experience the varying gravitational redshift in its orbit when compared with a signal from hydrogen masers from the ground. Measuring the gravitational redshift is a test of the equivalence principle and considered one of the three classical tests of general relativity. We will measure the gravitational redshift with a relative uncertainty as small as 2x10