Listening to the 'ringing' produced by black holes after they collide and merge could allow scientists to test Einstein's theory of General Relativity under the most extreme conditions in the Universe whilst unlocking the secrets of these mysterious objects.
Leading a major international review with the Institute of Physics, astrophysicists at the University of Birmingham, Johns Hopkins University and Intituto Superior Tecnico of Lisbon showcase how black hole 'spectroscopy' is rapidly evolving from a theoretical concept into powerful experimental science.
During the 'ringdown' phase following collision and merger, a newly formed black hole emits characteristic gravitational-wave vibrations known as 'quasinormal modes'. By measuring these frequencies, scientists can determine the black hole's mass and how fast it is spinning, as well as investigating whether Einstein's theory is correct.
Since the first detection of gravitational waves in 2015, the LIGO-Virgo-KAGRA collaboration has observed hundreds of black hole mergers and measured tens of black hole ringing down according to their characteristic tones.
So far, every observed ringdown agrees with general relativity, but current detectors are limited. Future observatories - including the European-led Einstein Telescope, the US Cosmic Explorer and the space mission LISA - may find fresh evidence for new physics.
Review co-lead Dr Gregorio Carullo, from the University of Birmingham, said: "By listening to the ringing of newly formed black holes, we are turning gravitational waves into a tool for exploring some of the deepest questions in physics, from the nature of gravity itself to the possibility of discovering entirely new forms of matter and energy."
Black hole collisions generate intense gravitational fields that cannot be recreated in laboratories on Earth. Researchers have discovered:
Multiple ringing overtones, analogous to harmonics in musical instruments,
in LIGO data.
Mode interactions, where vibrations influence one another.
Dynamical modes excitations.
Exceptional points, where modes merge and behave in unusual ways.
"Tails" of emission, amplified by mergers in crowded astrophysical environments.
The review identifies black hole ringdowns as potential ways of testing phenomena beyond the Standard Model of particle physics, including:
Beyond-Einstein gravity theories
Dark matter
Quantum-scale effects near black hole horizons
The review brings together more than 70 experts from institutions across the UK, Europe, North America, Asia and South America to provide the most comprehensive assessment yet of the field and was spurred by the largest international workshop dedicated to the topic, hosted by the Danish Architectural Center, Copenhagen, in 2024.
The next generation of detectors is expected to transform the field, giving scientists instruments that should detect many more black hole mergers and measure multiple vibration modes routinely. These future observatories should allow astrophysicists to uncover black hole formation mechanisms challenging current models, test Einstein's theory far more precisely and search for new particles and forces.
Reflecting on these upcoming advancements, Carullo said: "As gravitational-wave detectors become more sensitive, black hole spectroscopy promises to transform black holes from mysterious objects into precision laboratories to study challenging astrophysical processes and uncover new fundamental physics phenomena."
