The theoretical framework of our current understanding of the Universe rests on two main pillars, the Standard Model (SM) of particle physics and Einstein’s theory of General Relativity (GR). This SM+GR framework provides us with an incredible power to explain and predict a plethora of phenomena in observations and experiment, ranging from the quantum behaviour of computers, high-energy particle collisions at colliders like CERN to the gravitational-wave (GW) symphony of neutron stars and black holes, and the evolution of the Universe as a whole. In spite of this incredible success, there exist gaps in this beautiful picture of theoretical physics. Observations of galactic dynamics and the cosmic microwave background cannot be explained in terms of the expected gravitational effects of visible matter. We either need to modify the laws of gravity on extreme length scales or assume a form of dark matter that we cannot satisfactorily explain with the standard model of particle physics. Likewise, the accelerated expansion of the Universe calls for an exotic substance dubbed dark energy or the introduction of a cosmological constant with a value many orders of magnitude below the zero-point energy estimates of quantum field theory. Further challenges to the SM+GR model of the Universe arise in the form of the hierarchy problem (the extraordinary feebleness of gravity) and the lack of renormalization of GR in a quantum theory sense. It is tempting to compare the current conundrum about gravity with irregularities observed in the 19th century in the orbital motion of Uranus and Mercury. The former led to the discovery of a previously unknown or dark matter source, namely Neptune, while the latter was eventually explained in terms of a modified theory of gravity — Einstein’s GR instead of Newtonian gravity. Clearly the lesson is to remain open to all possibilities and search for observational signatures that discriminate between the scenarios.
In our search for explaining the present anomalies in terms of dark matter or modified laws of gravity (or both), we have an unprecedented new tool available, the Nobel Prize winning detection of Gravitational Waves by LIGO. This new channel to observing the universe is tailor-made to probe dark matter or possible modifications of the laws of gravity – both of which are fundamentally gravitational (rather than electromagnetic) phenomena. In our work on the “Gravitational Afterglow of Boson Stars”, we explore the first hypothesis, dark matter in the form of a light scalar field akin to the type of fields predicted by candidate theories for quantum gravity. Through the gravitational effect of their own energy, these light scalar fields can form highly compact star-like configurations known as Boson Stars. These hypothetical stars do not emit any form of light or other electromagnetic radiation, but as binary systems, they will generate GWs analogous to the black-hole binaries routinely observed with the LIGO-Virgo-KAGRA GW detector network.
The search for dark matter in future GW observations crucially hinges on our ability to distinguish between the GW signals from black-hole binaries and those from boson stars. In our work, we have identified a key feature that arises in boson-star collisions but not in the merger of black holes: a long-lived GW afterglow. Black holes, in contrast, have a comparatively short post-merger phase known as the quasi-normal ringdown. The boson-star afterglow arises from the complicated shape of the scalar field matter after merger (see figure). The most surprising result of our simulations is the exceptional longevity of this GW afterglow, which significantly improves our chances to either detect boson stars or rule them out as dark matter candidates in future observations.