The two-body problem in General Relativity

The two-body problem in General Relativity investigates the gravitational interaction between two massive objects, such as stars or black holes. At CoG, we aim to understand the complexities due to the curvature of spacetime governed by the Einstein field equations that lead to phenomena like gravitational waves and orbital precession. Examples of solutions to the problem include the Schwarzschild and Kerr metrics, which describe the spacetime around spherical and rotating bodies, respectively. Studying the two-body problem in General Relativity enhances our comprehension of interactions in the Universe by providing insight into the fundamental nature of gravity and spacetime. A recent approach to the problem, studied at the CoG, is leveraging recent breakthroughs in quantum field theory that allow an equivalent but alternative perturbative description of gravitational two-body dynamics.

High-energy physics and quantum approaches to strong-field gravity

Via the particle/wave duality, a profound insight of quantum mechanics, the detection of gravitational waves has reinvigorated the belief that there is a quantum constituent of space-time and gravity out of which the macroscopic description emerges. Members of CoG are leading efforts in unravelling some of the most fundamental questions regarding time, space and gravity and their interplay with quantum physics: What is a black hole made of?  How does spacetime manifest itself at the quantum level? Does this involve a lower-dimensional holographic realization? How does gravity behave in the strong coupling regime? Will we have access to smoking gun detections of quantum gravity from astrophysical observations to state-of-the-art table-top experiments? What are the consequences for black hole spectroscopy and gravitational wave physics?

Black hole spectroscopy

Black holes have a characteristic “sound”, just like a bell or a piano. They emit gravitational waves when they relax, some of which fall onto the black hole, forever lost. But some waves travel over large distances, where we can study them, to probe black holes and Einstein’s theory. This is called black hole spectroscopy. The CoG members pioneered many tools and concepts in the field, and are now pushing the frontiers of spectroscopy: what happens if black holes don’t exist after all? With what precision can we study General Relativity? If time stops at the black hole horizon, what implications does it have for their sound? Can we access black hole interiors? The next decade promises to give us a wonderful concert with black holes of all sizes. Stay tuned.

Gravitational waves across the cosmos

When two black holes merge, they produce ripples in spacetime that travel for billions of years before arriving at Earth. Despite the long trip, gravitational waves travel unaltered except for the stretching due to the Universe's expansion and their gravitational interaction with cosmic structures. Just as a magnifying glass bends light, any clump of matter may act as a giant lens to magnify far away gravitational waves. Gravitational waves are thus perfect cosmic messengers to probe the elusive dark matter holding galaxies together and unveil the nature of the dark energy driving the accelerated expansion of the Universe. At CoG, we are leading world efforts to discover gravitational wave lensing, explore the properties of the most distant black holes ever observed, and unlock the mysteries of the Universe's fundamental components.

Black holes in environments

Black holes in close orbits generate gravitational waves that travel at the speed-of-light throughout the Universe. The tune and fine modulations of these waves carry imprints of the environment the orbiting black holes live in, similar to how a string that vibrates on a Guitar, a Violin, or a Japanese Shamisen will sound different. Members of the CoG are pioneering new techniques to link these wave modulations to outstanding questions: How and where do binary black holes merge? Can one use black hole binaries to detect the presence of other black holes? Is Einstein's theory correct at all scales? Do new states of particles cluster around black holes? What is the nature of dark matter? We are training our ears to learn from the sound of gravity.

Precision Gravitational Wave Observation

To extract science from gravitational wave observations, instrumental data is confronted with theoretical predictions known as gravitational waveforms. The more faithfully these theoretical waveforms represent reality, the more precise are the scientific inferences from the observations. Conversely, gravitational wave observations are mostly deaf to effects not included in the waveforms. To integrate all the newest predictions from the CoG with waveform models used in on-going observations, the CoG partners with the Max Planck Institute for Gravitational Physics in Potsdam, Germany. Together, we will develop waveforms that will allow us to distinguish new physics from the effects of astrophysical environments or gravitational lensing.