Skip to main content

Fundamental fields and compact objects: theory and astrophysical phenomenology

Periodic Reporting for period 1 - FunFiCO (Fundamental fields and compact objects: theory and astrophysical phenomenology)

Reporting period: 2017-12-01 to 2019-11-30

Epoch-making observational, theoretical and computational developments make this an exceptional time for the
understanding of gravity in the strong field regime. The recent detection of gravitational waves (GWs) and the first imaging of a black hole, together with other ongoing experiments; the discovery of unexpected compact objects in General Relativity (GR); and the breakthroughs in the field of Numerical Relativity (NR), make strong gravity a central player in the scientific scene. Exploring different scenarios in this context is timely and synergies are mandatory, for such an endeavor.

The Fundamental Fields and Compact Objects (FunFiCO) RISE project is a Marie Curie Research and Innovation Staff Exchange (RISE) partnership between Aveiro University (Portugal, coordinator), Sheffield University (UK), UFPa (Brazil), UNAM (Mexico) and Valencia University (Spain). This project focuses on the construction, theory, phenomenology and dynamics of new families of black holes (BHs) and compact objects that are being discovered in GR in the presence of fundamental fields and alternative theories of gravity. Particular focus is given to astrophysical phenomenology, including GW signals.
This has been an extremely successful project so far in terms of the scientific goals achieved. With over 100 papers already produced within the project (full list can be found in most of which already published including 7 papers in the prestigious Physical Review Letters. The latter can be considered some of the main results achieved so far. Here is a short description of a selection of them.

1) [Phys. Rev. Lett. 119, 261101] We show that certain hairy black holes can emerge dynamically from a phenomenon called superradiance. This is an example of a different type of black hole that is dynamically viable.

2) [Phys. Rev. Lett. 120, 221101] We present the first very long-term simulations (extending up to ∼140  ms after merger) of binary neutron star mergers with piecewise polytropic equations of state and in full general relativity. Our simulations reveal that, at a time of 30–50 ms after merger, parts of the star become convectively unstable, which triggers the excitation of inertial modes. The excited inertial modes are sustained up to several tens of milliseconds and are potentially observable by the planned third-generation gravitational-wave detectors at frequencies of a few kilohertz. Since inertial modes depend on the rotation rate of the star and they are triggered by a convective instability in the postmerger remnant, their detection in gravitational waves will provide a unique opportunity to probe the rotational and thermal state of the merger remnant.

3) [Phys. Rev. Lett. 121, 101102] Extended scalar-tensor Gauss-Bonnet (ESTGB) gravity has been recently argued to exhibit spontaneous scalarization of vacuum black holes (BHs). A similar phenomenon can be expected in a larger class of models, which includes, e.g. Einstein-Maxwell scalar (EMS) models, where spontaneous scalarization of electrovacuum BHs should occur. EMS models have no higher curvature corrections, a technical simplification over ESTGB models that allows us to investigate, fully nonlinearly, BH scalarization in two novel directions. First, numerical simulations in spherical symmetry show, dynamically, that Reissner-Nordström (RN) BHs evolve into a perturbatively stable scalarized BH. Second, we compute the nonspherical sector of static scalarized BH solutions bifurcating from the RN BH trunk. Scalarized BHs form an infinite (countable) number of branches and possess a large freedom in their multipole structure. Unlike the case of electrovacuum, the EMS model admits static, asymptotically flat, regular on and outside the horizon BHs without spherical symmetry and even without any spatial isometries, which are thermodynamically preferred over the electrovacuum state.

4) [Phys. Rev. Lett. 123, 011101] We constructed new types of black holes in theories that go beyond Einstein's General Relativity by including higher curvature corrections. These black holes can emerge dynamically through the phenomenon of spontaneous scalarisation. We studied their phenomenology, including their shadows, unveiling a new spin selection effect.

5) [Phys. Rev. Lett. 123, 221101] We perform numerical evolutions of the fully nonlinear Einstein (complex, massive) Klein-Gordon and Einstein (complex) Proca systems, to assess the formation and stability of spinning bosonic stars. In the scalar (vector) case these are known as boson (Proca) stars. Firstly, we consider the formation scenario. Starting with constraint-obeying initial data, describing a dilute, axisymmetric cloud of spinning scalar or Proca field, gravitational collapse toward a spinning star occurs, via gravitational cooling. In the scalar case the formation is transient, even for a nonperturbed initial cloud; a nonaxisymmetric instability always develops ejecting all the angular momentum from the scalar star. In the Proca case, by contrast, no instability is observed and the evolutions are compatible with the formation of a spinning Proca star. Secondly, we address the stability of an existing star, a stationary solution of the field equations. In the scalar case, a nonaxisymmetric perturbation develops, collapsing the star to a spinning black hole. No such instability is found in the Proca case, where the star survives large amplitude perturbations; moreover, some excited Proca stars decay to, and remain as, fundamental states. Our analysis suggests bosonic stars have different stability properties in the scalar (vector) case, which we tentatively relate to its toroidal (spheroidal) morphology.
The remaining goals of the project remain in line with the progress done so far and the planned research in the proposal. We have already considerably pushed the boundaries of knowledge in our specific research topic, as mentioned above and illustrated in the above highlights.
The progress beyond the state of the art includes:

- The construction of many new types of black hole solutions including new fundamental fields;

- The analysis of their phenomenological properties;

- The analysis of their dynamical features using numerical relativity techniques.

The expected results until the end of the project will be further developing the three work packages we have proposed in our application, building upon the results we have already obtained. Some examples include:

- Organized libraries of hairy solutions based on the synchronization mechanism, including information of basic physical properties, to be made publicly available by the end of this proposal.

- Building templates of gravitational wave signals of perturbed hairy black holes and compact objects made up of Fundamental Bosonic Fields.

- Constructing Waveform catalogs for boson star binaries, for Proca star binaries, for hairy BH binaries (with scalar or Proca hair) and for binaries of BHs with internal structure.

We believe fundamental science such as the one we are producing has a central role in attracting young people to science. Topics as black holes and gravitational waves are more fashionable than ever and the society benefits from our work
from our outreach activities like outreach talks and press releases on our results in the media.