Helvi and Paolo visiting Toronto during the International Conference on Black Holes at the Fields Institute last year.
Helvi is Research Fellow in the School of Mathematical Sciences at the University of Nottingham. Paolo is Assistant Professor at Sapienza University of Rome and Research Scientist at the Instituto Superior Técnico in Lisbon.

We are proud to present the completed Focus Issue on “Black holes and Fundamental Fields” one year after its first contribution has been published online.

This issue appears serendipitously at the same time as LIGO’s historic detection of gravitational waves which, simultaneously, provided us with the first direct observational evidence for the existence of black holes (BHs). We wish to take this opportunity to congratulate the LIGO/VIRGO Scientific Collaboration and everyone involved on  their breakthrough discovery!

The true excitement around this discovery arises from the fact that it marks the beginning of the long-sought-for era of gravitational-wave astronomy. As Kip Thorne recently put it, “Recording a gravitational wave […] has never been a big motivation for LIGO, the motivation has always been to open a new window to the Universe”. The outstanding observation of a BH binary coalescence — and the expectation of more events to come in the near future —  is only the very first step towards a new kind of science in which BHs will play a prominent role.

In addition to LIGO’s main target — providing plentiful insight into the formation and evolution of BHs and potentially uncovering the ingredients that neutron stars are made off — these new observations pave the way to turn BHs into probes for beyond-standard model physics. The idea to collect the latest advances on this topic in a Focus Issue on “Black holes as laboratories for fundamental physics” was born out of the workshop “Testing General Relativity with Present and Future Astrophysical Observations” held at the University of Mississippi in January 2014. Despite “severe” weather obstructions — many of the participants were snowed in or stranded half-way to the conference — the event kickstarted a successful collective effort within the strong-gravity community which culminated in the CQG Topical Review on (strong-field) tests of gravity.

Already while writing up the Topical Review it became evident that an outstanding question remained unanswered: can we turn compact objects such as BHs and neutron stars into precision laboratories to test fundamental physics in extreme regimes? These days this question sounds almost prophetical, given the remarkable success of General Relativity (GR) to describe gravity at astrophysical scales, and the amazing agreement between the gravitational waveform detected by LIGO and that predicted by GR for the cosmic dance of two massive, coalescing BHs.

The short answer to the above question is: yes, and the underlying reason can be traced back to the BH no-hair and uniqueness theorems. This explains why BHs are essentially like every experimentalist’s dream laboratory in which no external disturbances can interfere with the phenomena under investigation. Indeed, a BH is nothing short off such a perfect lab:  an enormous deposit of gravitational energy which, nevertheless, is beautifully described by just three numbers:  its mass, angular momentum and charge.

In recent years it has been realized that, thanks to their remarkable simplicity, BHs provide the ideal playground to search for “smoking-gun” effects of beyond-standard model physics. For instance, the superradiant or “black-hole bomb” mechanism renders BHs extremely sensitive to ultralight bosonic fields, thus providing a way to search for these particles with BH observations.

In this Focus Issue we have collected a number of contributions exploring this thriving new topic that bridges between gravity and particle physics. They will take the reader on a journey with such exciting stations as:

• black-hole based tests of GR,
• the interplay between compact objects and fundamental fields arising either in beyond-standard model physics or in strong-curvature extensions of GR,
• potentially observable signatures of this interplay, such as characteristic, monochromatic gravitational waves and the lack of highly spinning BHs,
• fundamental properties of BHs in these scenarios, such as no-hair theorems and violations thereof,
• novel, non-linear BH solutions and state-of-the-art simulations of gravity in extreme regimes,

and many more.

In a nutshell, the interplay between precision observations of compact objects, state-of-the-art theoretical models, and gravitational-wave measurements can be employed to constrain particle physics — for example by putting more stringent bounds on axion models, dark-matter candidates or massive gravitons – and to test extensions of GR to unprecedented level. We believe that – in the newborn era of gravitational-wave astronomy — upcoming observations will greatly expand our understanding of strong-gravity interactions between compact objects and fundamental fields, thus offering a novel tool to gain insight into beyond-standard model physics.

Read the focus issue in Classical and Quantum Gravity

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