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 2x10-5, the smallest of any such tests of general relativity yet.***We will further use RadioAstron together with a VLBI network of antennas on the ground to image compact radio sources. We will observe pulsars and probe the size and distance of the scattering material in the interstellar medium in a way never possible before. We will measure active galactic nuclei (AGNs) of radio galaxies and quasars and probe our understanding of the physics of radio emission near supermassive black holes. We will focus on M81*, the core of the nearby spiral galaxy M81 which harbours, together with Cen A, the nearest extragalactic AGN and can therefore be studied with unsurpassed linear resolution. ***We will continue to investigate massive core-collapse supernovae (SNe). We are part of a team to study through a multi-wavelength approach the connection of long-duration gamma-ray-bursts (GRBs) and SNe of Type Ibc (stripped envelope). We will lead VLBI observations of such SNe if they are sufficiently strong in the radio. GRB's are associated with relativistic expansion and VLBI-determined expansion rates are therefore important for the study of the GRB - SN Ibc connection. We will continue to study SNe Type II and determine the expansion and deceleration to obtain information on density profiles of the ejecta and the shocked media and properties of the emission region. Combining the radio transverse expansion with the optical radial expansion of the SNe will allow a geometric determination of the distance to the host galaxy and contribute to a measurement of H0. We will image the SN shell with high resolution and search for emission from the environment of the corpse of the stellar death, a young neutron star or a black hole. The sequence of the radio images will be used for our video series: "A movie of an exploding star."**