Opzioni
Probing scalar particles and forces with compact objects
Boskovic, Mateja
2023-09-11
Abstract
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.
Diritti
open access
Soggetti