White Paper #1: Fundamental Physics

Abstract

The Standard Model (SM) of particle physics and General Relativity (GR) are the two pillars of our current understanding of Nature. Both theories have been probed individually with ever increasing precision and are consistent with nearly all experimental observations. However, they fail to explain dark matter, dark energy, or the imbalance between matter and anti-matter in the universe. Yet, dark matter and dark energy represent 95% of the energy content of our universe while known matter (atoms, molecules) amounts to only 5%. Today, dark matter and dark energy have an unknown origin and there is a great deal of experimental and theoretical activity to solve this puzzle. In summary, the clustering of large-scale structure and the accelerated behaviour of cosmic fluid could be addressed whether finding out new (unknown) forms of matter or assuming that gravity behaves in different ways at infrared scales. Furthermore, the lack of a self-consistent theory of Quantum Gravity prevents the unification of SM and GR at ultraviolet scales. This is one of the biggest challenges that theoretical physics is facing today. String theory or loop quantum gravity are good candidates to solve this puzzle and interestingly both of them foresee violations of the Einstein's Equivalence Principle. With that respect the Einstein's Equivalence Principle assumes a central role in the search for a quantum theory of gravity. The open problems in fundamental physics investigated in this white paper are:

(i) Validity of the Einstein's Equivalence Principle;

(ii) Origin and nature of dark matter and dark energy;

(iii) Decoherence and collapse models in quantum mechanics;

(iv) Quantum many-body physics.

They will be addressed from different research corners and with different experimental methods:

(i) Ultracold atoms;

(ii) High stability and accuracy atomic clocks;

(iii) Matter-wave interferometry;

(iv) Classical and quantum links.

The cosmos is a particularly attractive laboratory as it provides particles (cosmic rays) or objects (black holes, neutron stars) which are not produced in manmade laboratories. Space is also an excellent environment for high precision physics as the absence of atmosphere or drag-free satellites provide unique observation opportunities. For instance the MICROSCOPE mission has taken advantage of extremely long free-fall conditions in Earth orbit to set the record in testing the Equivalence Principle beyond what has been possible on Earth. Large velocity, velocity variations and large variation of the gravitational potential are accessible on board a spacecraft, thus providing wide signals for testing GR. Finally, the huge free propagation distances available in space provide very long baselines to test the spacetime metric with high performance links both classical and quantum.