Probing scalar particles and forces with compact objects

In the highly relativistic regime around compact objects, signatures of new physics may be unravelled. For example, dark matter and dark energy problems may be alleviated by new scalar degrees of freedom. If light scalars are coupled to matter, in relation to the dark energy problem, they generally mediate a fifth force, which could in turn contribute to gravitational phenomenology. In order for this phenomenology to be consistent with existing constraints, it must be suppressed close to matter sources, e.g. through a non linear screening mechanism. In regard to the dark matter problem, some compact objects may in fact be constructed from beyond Standard Model matter. In particular, boson stars are useful toy models for exotic compact objects that could be produced in the Early Universe or indeed for understanding the behavior of matter under extreme conditions. First, we consider a two-body problem in shift-symmetric scalar-tensor theories that exhibit kinetic screening. The highly non-linear nature of the theory doesn’t allow for an analytical solution away from spherical symmetry. We will present an approximate scheme that allows for a qualitative insight and, in most of the parameter space, very good quantitative agreement with the full numerical results. We will further discuss the partial breakdown of the screening in such systems that could allow for further constraints of these theories. Second, we consider the most compact boson stars that have a false vacuum in their potential (soliton boson stars), in isolation and in binaries. In the former case, we derive the analytic solutions in spherical symmetry and compare it with the fully numerical ones. In the high-compactness limit we find that these objects present an e⇥ectively linear equation of state, thus saturating the Buchdahl limit with the causality constraint. Far from that limit, these objects behave either as flat space-time Q-balls or (in the low-compactness limit) as mini boson stars stabilized by quantum pressure. We establish the robustness of this picture by analyzing a variety of potentials (including cosine, quartic and sextic ones). Finally, we study the coalescence of two boson stars via numerical evolution of the fully relativistic Einstein-Klein-Gordon equations. Owing to the steep mass-radius diagram, we can study the dynamics and gravitational radiation from unequal-mass binary boson stars with mass ratios up to q ≈ 23 without the di⌅culties encountered when evolving binary black holes with large mass ratios. Similar to the previously-studied equal-mass case, our numerical evolutions of the merger produce either a non spinning boson star or a spinning black hole, depending on the initial masses and on the binary angular momentum. Interestingly, in contrast to the equal-mass case, one of the mechanisms to dissipate angular momentum is now asymmetric, and leads to large kick velocities (up to a few 104 km/s) which could produce wandering remnant boson stars. We also compare the gravitational wave signals predicted from boson star binaries with those from black hole binaries, and comment on the detectability of the di⇥erences with ground interferometers.