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Beacons in the Dark

Final Report Summary - BEACON (Beacons in the Dark)

Beacon performed an ambitious multi-disciplinary (optical, radio astronomy and theoretical physics) study that enabled a significantly improved understanding of gravitation and spacetime.
For almost a century Einstein's general relativity has been the last word on gravity. However, superstring theory predicts new gravitational phenomena beyond relativity, in particular a violation of the strong equivalence principle (SEP). This would cause several subtle effects on the orbits of binary systems with a pulsars and a white dwarf. In this project I have attempted to detect these phenomena with a precision better than state-of-the-art attempts. A successful detection would falsify general relativity and take physics beyond its current understanding of the Universe. No detection was made, this allowed us to exclude many alternative theories of gravity and fundamentally constrain the nature of gravitational waves.
In order to further these goals, we have built the ultimate pulsar observing system. By taking advantage of recent technological advances in microwave engineering (particularly sensitive ultra-wide band receivers), digital electronics (fast analogue-to-digital converters and digital spectrometers) and computing, me and my team have made great strides towards an improved precision for pulsar timing experiments. Some issues persist (mostly related to very strong radio frequency interference at the Effelsberg site), but we hope these will be finally addressed with continued support from the Max Planck Society.

By the end of the project, our studies of the group of systems being targeted has resulted in:
1. The best limits ever on the existence of dipolar gravitational waves, a phenomenon predicted by many alternative theories of gravity. This comes from the timing of PSR J1738+0333, optical observations of its white dwarf companion, and careful analysis and interpretation by our team and collaborators.
2. The best ever constraints on a parameter (α1) that describes local Lorentz invariance of gravity. Einstein’s general relativity predicts this parameter to be zero, as observed in the timing of PSR J1738+0333;
3. Most massive neutron star ever with a precise mass measurement, PSR J0348+0432. This seriously constrains the properties of super-dense matter at the center of neutron stars;
4. Largest neutron star birth mass (for PSR J2222-0137);
5. These systems (PSR J2222-0137 and PSR J0348+0432) have also allowed the first ever tests of the properties of gravity in a new regime of extreme gravitational fields.

Furthermore, BEACON members have led:
1. Discovery of asymmetric double neutron star systems, PSR J0453+1559 and PSR J1913+1102. The latter will very likely be the best-known laboratory for testing scalar-tensor theories of gravity.
2. Discovery and characterization of a repeating fast radio burst, a mysterious phenomenon of unknown origin.

On the technical front, we have finished:
1. Construction of the most sensitive broadband receiver ever (as measured in the labs);
2. Addressed most, but not all, issues related to the interference;
3. Design of the most advanced and capable broadband back-end ever, which will be capable, for the first time, of doing coherent dedispersion for a bandwidth larger than 1 GHz. This is useful for use with other radio receivers.