Probing Fundamental Physics with Gravitational-Wave Observations

When the Laser Interferometer Gravitational Wave Observatory (LIGO) made the first ever detection of a gravitational-wave event, GW150914, in September 2015, yet another remarkable prediction by Einstein's theory of General Relativity was confirmed. Gravitational waves are ripples in spacetime generated by compact objects, like black holes, that travel through the Universe like ocean waves or sound wave propagate in water or the atmosphere. Careful analysis of the LIGO result demonstrated that the origin of GW150914 was the inspiral and merger of two black holes, each with mass about thirty times that of the sun. These waves traveled for 1.2 billion years almost unimpeded through the universe until they hit the two LIGO detectors in 2015, ushering in a new era of physics.

LIGO has now been joined by the European Virgo and the Japanese KAGRA detectors, resulting in a global network of observatories. This network has detected dozens of events, including the neutron star merger GW170817 also observed across the entire electromagnetic spectrum, and is expected to make hundreds of new detections in the upcoming fourth observation run O4. Gravitational-wave observations are a tailor made tool in our quest for answers to the most pressing questions in contemporary physics. What is the nature of the enigmatic dark matter and dark energy that make up over 90% of the mass-energy of our universe? Do we need to extend Einstein's theory as suggested by its incompatibility with the laws of quantum physics? How did our universe look like in the early stages of its evolution? What is the behaviour of matter at densities above that of nuclear matter? Do more exotic compact objects, such as wormholes, exist? These are all questions at the heart of STFC's strategic vision and the exploitation of gravitational-wave observations in the quest for answers is the central theme of our project.

The determination of the source of a gravitational-wave signal and its properties proceeds in a manner similar to the analysis of finger prints on a crime scene. A finger print (the signal) is compared with a data bank of possible culprits (a gravitational-wave template bank) and, in case of a match, identifies the source. In contrast to the finger-print analogy where an individual source is identified among a finite and discrete set of candidates, however, we are dealing with an infinite number of possible sources in gravitational-wave physics that are characterized by a number of parameters, each of which can vary continuously over a wide range. For example, a black-hole binary is characterized (among other parameters) by the two hole's masses which can take on essentially any positive value. Our results therefore do not emerge in the form of a single exact answer, but a probability distribution over a parameter space. It is critical, for this purpose, to have theoretical predictions of high precision across the spectrum of possible sources. We will generate a wealth of new predictions of this type and analyse them with present and future detector data.

More specifically, we will extend the tests of general relativity by modeling black holes for specific modifications to Einstein's theory of gravity and look for hints of these modifications in the observed data. We will improve existing models of large populations of black holes that provide us with a statistical way to measure the expansion of the universe. We will search for gravitational waves from so-called cosmic strings, which are predicted to form in the early universe. The nature of matter at extreme densities can be probed through tidal deformations in neutron stars akin to the tides in the Earth-Moon system, and the modifications they induce in the gravitational-wave signals. Finally, we will search for highly characteristic signatures of more exotic compact objects, such as gravitational-wave echoes which may arise from wormholes.