BEACON is an ambitious multi-disciplinary (optical, radio astronomy, and theoretical physics) study to enable a fundamentally improved understanding of gravitation and space-time. It has been funded by the European Research Council through a consolidator grant (n. 279702).

For almost a century Einstein's general relativity (GR) has been the last word on gravity. However, we know it cannot be the ultimate theory of gravity, since it fails at the centres of black holes, where it predicts infinite densities and fields. Furthermore it appears to be incompatible with quantum mechanics. It is not clear at what energy scale GR ceases to be a good description of gravity. However, there are many alternative theories of gravity that predict deviations from GR at the sort of energy scales that prevail in the current Universe (i.e., much below the Planck scale). These theories tend to predict several subtle effects on the orbits of binary pulsars, particularly pulsars in asymmetric binaries (e.g., pulsars with white dwarf companions):

  • Emission of dipolar gravitational waves. This would be detectable as an orbital decay larger than that expected from the emission of quadrupolar gravitational waves predicted by GR. So far, the experiments most sensitive to this phenomenon  have not detected it.
  • Violation of the Strong Equivalence Principle. This would be detectable as a small change in the orbital eccentricity of a binary pulsar with a wide orbit.
  • Violation of Local Lorentz Invariance. This would be detectable as a small change in the orbital eccentricity of a binary pulsar with a tight orbit, and/or as free precession in isolated pulsars. The best limits are consistent with GR.

The objective of this proposal is to search for these phenomena with much improved sensitivity compared to any previous attempts. To achieve this, we need better pulsar timing precision. A detection of these effects would disprove general relativity and take physics beyond its current understanding of the Universe. If we detect no deviations from general relativity, the improved limits will rule out several proposed alternative theories of gravity.

As an example, combining the results on PSR J1738+0333 with those of the double pulsar system (J0737-3039) we might be able to exclude the tensor-vector-scalar theory of gravity, the proposed relativistic formulation of Modified Newtonian Dynamics (MoND). This latter theory has been proposed as an alternative to Dark Matter. This demonstrates that testing general relativity is not only important for our understanding of gravity and the laws of physics, but also for our knowledge of the constitution of the Universe.

What are we doing about improving timing precision?

The precision of pulsar timing is presently limited by two factors:

  • Statistics: pulsars are exceedingly faint radio sources, requiring the use of the world's largest radio telescopes.
  • Systematics: as the Earth and the pulsar move, their line of sight is always sampling different regions of the interstellar medium which always have slightly differing electron densities.These unpredictable variations introduce an extra source of uncertainty in the measurement of the times of arrival of the radio pulses.

The solution to both problems is the use of an ultra broadband receiver (UBB), with corresponding broadband back-ends. This will be installed at the 100-m Effelsberg telescope. This uses an innovative horn with an innovative quadrature ridge design, which makes it sensitive from 0.6 to 3 GHz.

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